1. Before you proceed further ensure again,  you are in the correct chapter:
    • Before you proceed further ensure again, you are in the correct chapter:

    • The longest and biggest sleeve is a spiral wrap. If so, you can proceed according to this chapter.

    • If the longest and biggest sleeve is from textile, please use this chapter instead: 5. E-axis assembly (textile sleeve)

    Add Comment

  2. Needle-nose pliers for zip tie trimming.
    • Needle-nose pliers for zip tie trimming.

    • 2.5mm Allen key for M3 screws

    • 2mm Allen key for nut alignment

    • 1.5mm Allen key for tightening the pulley

    Can’t figure out how to let author know it took me 4 hours 45 minutes to complete. I’m probably a bit on the slow side and made a few slight errors along the way, so consider this a upper bound? Jakub Dolezal feel free to delete this once it gets read

    timcdoc - Reply

    I'm pretty quick and it took me 1 hour 15 minutes. Definately clean all holes on the printed parts before assembling using the smallest Allen key to speed things up. There were quite a lot of strings on the overhangs that break off easily with a tool but not with a nut. Also I found the hole for the bottom of the heatsink was not quite round enough, a touch of sandpaper brought that in nicely.

    Chris Vahi - Reply

    • For the following steps, please prepare:

    • Extruder-idler (1x)

    • Bondtech pulley WITHOUT the lock screw (1x)

    • Pulley bearing (2x)

    • M3nS nut (2x)

    • Pulley shaft (1x)

    • Note there are two types of the Bondtech pulley in the package (one with and without lock screw). Make sure you are using the correct one.

    For those who have a shocking moment here as it looks like one pulley bearing is missing… have a look inside the bondtech pulleys ;)

    Stefan Lutz - Reply

    ^ What he said :) ^

    Danzanzio@yahoo.com -

    Thank you!!!

    Rick -

    Thank you both of the had hidden themselves in my kit!

    Mustrum Ridcully -


    One bearing missing and it is nowhere to be found :/

    I looked everywhere. I guess I have to hope it won’t affect the performance of the printer…

    Hardi -

    Hi Hardi, you need both. Please contact our support at info@prusa3d.com

    Jakub Dolezal -

    Yuup!!! Same thing in my bag.

    Douglas Shelfoon - Reply

    When sorting through the pieces it appears I have an extra grub screw … the bondtech has one in place, and another is loose. Is it just an extra?

    Rich - Reply

    Same with the grub screw, extra?

    Mark Moore - Reply

    My packet label only references 1 Bondtech and doesn’t show the pulley bearings at all. It’s v1.9. All the parts are there though, including the extra grub screw.

    Shaun Collins - Reply

    Ditto. Mine too.

    Jeremy Mayes -

    English hint: it is called a ‘hobbed gear’. Pulleys are for belts and gears have teeth to mesh with other gears. The ‘hobbed’ portion is where the small teeth have been added to engage the filament.

    Timothy W. Skloss, Ph.D. - Reply

    Hi Timothy, thanks for the explanation. I intended to rename them to the drivegears in the next update to comply with the manufacturer’s description.

    Jakub Dolezal -

    The pulley shaft seems kind of on the short side. It is barely long enough for me.

    Mark - Reply

    I would add to the first picture the Bondtech pulley with the lock screw, and add a big red X on it.

    Gabriel Anzziani - Reply

    Hi there Gabriel, thank you for you input. For now we will leave the WITHOUT as it is. However, for the future we will try to improve this photo.


    Tomáš -

    The bag containing the bond tech pulleys has 2 of these which look identical, what am I to look for?

    Suzan Kowalski - Reply

    Hello! He/she who has 2 identical Bondtech pulleys without the lock screw, shall contact us without hesitation at info@prusa3d.com and provide a photo. We’ll provide a replacement in exchange. : )

    Martin Lexa -

    • Press the nuts to the slots on both sides of the idler.

    • Insert both bearings in the pulley. Be aware that bearings can slip out during assembly.

    • Layers might be visible in this printed part due to the geometry. The functionality and strength remain unaffected.

    There is no printed part here. Perhaps the comment should be on the next picture

    Steve Worcester - Reply

    Hi Steve, the black idler (first picture) is 3D printed.

    Jakub Dolezal -

    I am unable to get the right nut in far enough to put a screw through. Probing with an Allen key it feels like there is a blockage in one corner. Is there a good process to clean this out?

    Frank Riddle - Reply

    Hi Frank, to clean the slot I use the smallest Allen Key, it works pretty well.

    Jakub Dolezal -

    The slots for these two captive nuts isn’t especially tight. They fall out whenever you move/touch the part. I suggest that either the slots should be tighter, or the step with the nuts should be moved to step 15.

    Duane Pinkerton - Reply

    Completely agre with ++Duane Pinkerton++ same happened to me should be step 15

    PaulHarris - Reply

    My M3nS nuts kept falling out.

    Suggest you use x2 M3 x 10 screws and engage into the nuts, with no pressure. This will hold the M3nS nuts from falling out. These 2 screws can be removed later, at step 15.

    Mike Norman - Reply

    yes, slots are too big and nuts fall out. I used screws to temporarily hold them in place as well, and then forgot i did that in later steps. Eventually i remembered I added them!

    jon varteresian -

    Unfortunately I only got one bearing.

    Mike Morales - Reply

    Are you sure about that? Try to double check if it isnt somewhere in the pack :) as they are pretty small, one can easily miss their presence.

    Tomáš -

    BROKE IT! I broke the friggin’ extruder idler. Got the idler shaft stuck in it, and when I went to pull it out, and broke the crap out of it. How do I proceed?

    morris.ron1@gmail.com - Reply

    Hi there Ron, I am sorry to hear that. You can use super glue in a short run and subsequently you can re-print this part once you have your printer assembled. Or possibly you can contact our Live chat at shop.prusa3d.com, our team will help you out.


    Tomáš -

    Once again thin scotch magic tape worked a treat to keep the two nuts in their rather loose pockets until needed. then its easy to remove.

    Mustrum Ridcully - Reply

    • Ensure both bearings are inside the pulley!

    • Insert the pulley in idler as shown in the picture.

    • Slide the shaft through the idler and pulley. Use reasonable force or you will BREAK the printed part.

    • Place your finger on the bearing and ensure it can rotate freely.

    Any suggestions on how to get the shaft in? It’s impossible with my parts. Might have to try and ream out the hole.

    Alex Tramiel - Reply

    Since the shaft is 3mm - I´d suggest using a 3mm drill bit and widen up the first hole - by hand preferably.

    Richard Rohan -

    put the champfer-side into the hole (direction of the orange arrow is good) then take a wood or plastic on your workbench.

    hold the printed part vertically and slightly push downwards.

    Heiko Schultz -

    Hi Alex, did you manage to press the shaft in?

    Jakub Dolezal -

    Richard’s suggestion wound up saving me. Thank you!

    Daniel Kilboy -

    For me, the holes in the printed part were too small. I enlarged it with a small round file.

    Michal - Reply

    The hole can not be too big so that the shaft does not fall out.

    Michal - Reply

    No way to get this in by hand, but a pair of channel locks worked as sort of arbor press, and popped right in.

    Scott - Reply

    Channel locks worked like a charm and were smooth so I didn’t feel like I was going to break anything.

    Cliff Miller -

    This worked for me also. It doesn’t require much compression with the channel locks so you really don’t feel like you are going to break anything. As soon as I started to use them, the shaft started going in very easily.

    John Haro -

    +1 for a pair of channel locks . Worked great for me too. and yes, the shaft could be 2 mm longer

    alin@aandea.com -

    Channel locks worked like a charm!!!!

    Russell McCampbell -

    I drilled mine out and epoxied in the shaft. It was so tight I felt sure I would break the part before getting it in. I would rather have pressed it in, but it was just too tight!

    Steven Underwood - Reply

    Guys, feedback noted and production team alerted. Thank you :)

    Jakub Dolezal - Reply

    Needs to fixed in the M2K->MK3 upgrade files as well. I’ve got the same problem right now.

    Eric Davies -

    also, the shaft is about 3.6mm too short. I inserted it (sadly I read about the necessary drilling too late ;(( ) an now it’s flush on one side and still 3.6mm to go on the other side.

    Guntor H - Reply

    Hi Guntor, can you please share a picture? (http://manual.prusa3d.com/Answers) Thanks

    Jakub Dolezal -

    I hand-drilled the entry hole to widen it slightly but left the other hole as-is. It seems to have worked well for now.

    Brandon Oprendek - Reply

    I did the same as Brandon - it worked ok but the system needs improving here

    Chris Tipney - Reply

    Hi Chris, it is already fixed, but due to the amount of already shipped printers it will take some time. Sorry about that.

    Jakub Dolezal -

    Shaft is indeed a bit short but I used one of the allen wrenches to just push it in a little further so it has a bit of ‘flesh’ on both sides. Guess shaft could do with an additional 2mm in length (at least).

    Jeroen - Reply

    Same problems over here: Holes to tight and shaft to short

    Another thing:

    On the bag is only one part mentioned (1x Bontech). So at first I did not realised that there are two different parts

    Gerhard Kunz - Reply

    Hi Gerhard, it is written in the first step to use Bondtech pulley without tightening screw.

    Jakub Dolezal -

    Same as @kunzgbr - I had two similar looking parts in E-Axis bag which was labeled 1xBondtech. I notice now one of them has an allen lock screw, so I used the one without the lock screw for this step. Not sure if the other is needed or not yet.

    Graham McIntyre - Reply

    Hi Graham, yes for this step use the without the lock screw. It is written in the first step of this chapter.

    Jakub Dolezal -

    Too short by about the thickness of one of the hangers

    Steve Worcester - Reply

    Rubber mallet here, went just fine without drilling, but shaft could be 2 or 3 mm longer. I managed to make it equally spaced

    Giacomo - Reply

    Note: before leaving this step I would recommend you tap on the extruder and verify that the pin does not shift out of place. if the pin walks out, then you will not be able to generate good prints. It may be possible to add just a touch of super glue to the end cap to prevent the pin from walking out. Another option is to use a hot glue gun to secure the part in place. A longer pin would make this part more reliable.

    Bradley Moreavek - Reply

    Slight touch up with a small, round file…tight friction fit…equally spaced and then a drop of super-glue seems to have worked for me.

    Matt Plummer - Reply

    I used a big clamp very carefully to get it in, then I took the included screwdriver and drove the pin farther in by smacking the screwdriver with a metal wrench. Worked ok.

    Mr Cookie - Reply

    I just used the included pliers to press it in. Make sure the shaft is as straight/perpendicular as possible and use the pliers to press it in. One tip of the pliers is on the end of the shaft and the other is on the back side of the near tab (ie. the first one the shaft goes through) Once the shaft is flush with the inside wall or sticking out just a half mm or so you can put the gear in and press the rest of the way. This way the pliers support the printed part while pushing the shaft in. This minimizes the chance you will break the printed part. If you drill it out or file it, it will be easier to push in by hand but also easier for the shaft to work loose.

    Also the shaft seems too short. Should be a mm or two longer as others have stated.

    Bryan Jackson - Reply

    It’s way easier to insert once you realize the shaft has one chamfered side you need to insert first. Then press the entire part down on the shaft, with the shaft resting on a wood surface. I pushed it in a little further using the philips head, that solved the length issue as well. Wouldn’t have known this without @predatorjr mentioning the chamfered edge on the shaft.

    Bas - Reply

    Good information Bas, Thank you. No need to drill. Just press the entire part on a wood surface.

    Jacques Bourdouxhe -

    Great Information Bas. Inserting the chamfered side first and a little tap-tap with pliers, and it when right in.

    Paul Bowers -

    Thanks Bas! Per your valuable input, I first confirmed the pin chamfer and insertion direction, then pressed on the table top. First only a small start depth without the idler (to get started), then adding the idler gear once the pin was snug in the first post. (Pushing down on the inside of the post with your finger both minimizes stress on the post, and allows you to sense when the pin starts to protrude inside.) After the idler is inserted, finish the press. The pin fit is tight but not too difficult. For the final depth adjustment, I placed the part flat on it’s base up against a sturdy back-stop, then used a Philips screwdriver to press the pin in the last 1~2mm. The bevel on the Philips prevented too deep a drive, yielding a balanced depth on each end. Nice fit, no drilling, no glue. The pin could be a bit longer, but this works.

    michael_farady -

    Success with this process as well. I tapped the shaft in slightly more than flush so that I had equal part engagement in both sides. I do'n’t recommend reaming the hole as the shaft may walk out later.

    jon varteresian -

    As others have said, the shaft is not long enough

    Richard - Reply

    Yes, knowing that one side of the shaft has slightly rounded edges (chamfered) compared to the other more square-cut end definitely made a difference. No issue pressing it in by hand. That said, as others have noted, 1-2mm of extra length would help the situation. Took a little careful back and forth to get the shaft centered between the two mounts.

    Kalani - Reply

    I could not get the shaft in. Slightly pressed it like others have set and broke one side. I’ll try and get it in and super glue it. Then try and print that part again for a replacement once the printer is working. This has been the hard part of the build for me.

    Michael Aldrich - Reply

    The shaft on my pre-assembled mk3 is indeed too short. After around 200 hours of use , I noticed my filament slipping no matter how much I tightened the idler. The shaft had slipped to the side allowing the idler to lose contact with the filament.

    Robert Klein - Reply

    Same experience here. I centered the shaft as best I could during assembly but it slipped to one side after 240 hours of printing. The part is about 4mm too short.

    Jim Lombardo -

    This is still way too hard to get it and its from 3/14/18

    Alex Paradis - Reply

    Just press it against a table before putting the idler in. Keep your thumb over the hole. Once you feel the shaft start to come through, then put the idler in and press the rest of the way.

    phr0ze -

    Before you put the idler in, use your thumb to press the shaft into the first part. Just press the shaft against the table and you will feel it press in pretty easy. Now add the idler with the bearings and press the shaft the rest of the way on the table. However the shaft will still not be in enough. For the next step take a M3x10, add a nut to it and screw it in to expose 1-2mm of screw. This nut is a stopper. Balance the pin on the top of the screw and press it in so the pin goes far enough but not too far. If you have problems balancing the M3x10 you can use the hole in the center of the X Axis cover to hold the screw upright.

    phr0ze - Reply

    the shaft really should be a couple mm longer. It’s hard to center in the shaft and make sure it’s not going to wobble. I didn’t have any issues inserting, a little force got it done.

    Jesse Brockmann - Reply

    Agree with everyone else on this one. Shaft is too short by a couple mm and the hole is too tight to get the shaft into without a bit of modification. Highly recommend corrections to the design.

    Dave S - Reply

    The holes for the shaft were extremely tight, so I did not dare apply a strong force that would tend to break off one of the holders. I live in the US, and I don’t have a 3mm drill. So I drilled out one side only to 1/8”, which is 3.175mm, but I left the other side unchanged. This allowed me to easily insert the shaft through the modified holder, and through the idler pulley bearings. Then I could rest the other side on the edge of my work table and push the shaft into that side without risk of breaking the side off. As others have noted, the shaft is too short, so I used an M3 bolt as a “drift pin” to push the shaft further into the far side support, ending up with the shaft equally “indented” from each end.

    Jay Sinnett - Reply

    This was very difficult and time-consuming.

    Taylor Mills - Reply

    +1 for channel locks. Slid right in. Followed up with some light tapping with a screw driver to drive the shaft a little deeper.

    Vince - Reply

    Please don’t drill the hole out any bigger! I just pushed it in a little bit, just so it wouldn’t move, by pressing it against the metal part of the pliers. Then I used a small hammer to push it in as far as I could and then used the flat head screw driver provided, placed it on the pin, and hammered it in so that it was centered. Hope this helps :)

    Danzanzio@yahoo.com - Reply

    I had the same issue with the length of the shaft. I replaced it with a new 3mm shaft 2.35mm long. All the alternatives seem less than satisfactory since one relies on the short shaft not moving over time. There is at least one YouTuber showing how this happened to him and it took him forever to find the problem.

    john.geaney@bluewin.ch - Reply

    Those of us that have done the Prusa upgrades in the past have a 3mm drill bit with a hand knob to clean the hole out. This works great. Take your time. Never drill the holes oversized or the shaft will move and one side or the other will get cocked and you will have a tough time troubleshooting the problem some time in the future! Someone posted a youtube video with that exact problem.

    Keep in mind that you want a nice friction fit. You can use a vice to press the shaft assembly together.

    The shaft does not have to be full length…Press the shaft in from the narrower (furthest out from center) support side and press it in just a bit past flush.

    Kelly Anderson - Reply

    This was a very tight fit. I used my work space top to press against which got the shaft flush with the top face. It did take a bit of force pushing down on the assembly. Like everyone else, I also noted that the shaft when flush with the top face does not seat into the lower support frame. I used a pin punch and hammer to tap it more, so that both supports had some of the shaft. I can upload the couple of photos I took if that would be helpful.

    David Jerrell - Reply

    The pulley shaft does appear to be a few millimeters too short. when the trailing end of it was fully inserted, flush with the plastic surface of the idler, I was concerned that the leading end might not be extending far enough into the hole at the opposite end of the pulley.

    My solution was to brace the idler vertically against my work table and use a light hammer and nail set. A few light taps drove the pulley shaft a few more millimeters, so now the shaft faces are recessed a millimeter or two from the ends of the holes on both sides. Nothing cracked, and the pulley spins freely.

    Charles - Reply

    I agree, the shaft is a really tight fit and was hard to get it in place, and as has been said, a little short to know that it is in place correctly, I used the pliers to grab the end and gently wiggled it into the hole, then finished off with a gentle tap to ensure the pin was located in the opposite hole, but was hard to determine if it was far enough in to be mechanically sound for printing, with a longer shaft the ends could line up at both ends of the printed part.

    Martin Wright - Reply

    The hole for the shaft is way to small. I broke this part. I have to print it. So many people with this issue, and the design still not changed.

    Claude Serquet - Reply

    Hi Claude, the hole must be narrow in order to keep the shaft in place. We've made some adjustments and we will do more.

    Jakub Dolezal -

    I used an included screw to “file” the hole slighly bigger. Then I placed the Bondtech between the two plastic petrusions and pushed the pin in by placing the part against the table.

    When the axle did not stick out from the printed part any more, I pushed in the rest with an Allan key (it will slip back and forth a few times, try gentle hits to center the axle.)

    Tor Andre - Reply

    Applied PTFE grease to bearings and shaft before installing, and gear area afterwards (I would have used white Lithium grease, but discovered most brands don’t contain any Lithium despite the label, and use Zinc Oxide instead).

    Instructions should state to insert the shaft chamfer-end first! Then, press it against flat face of pliers (to ensure a hard surface, and avoid tabletop damage). Stop applying force when shaft moves, and repeat until shaft is fully seated.

    Shaft went in after about 5 pressings. With an extra final pressing to make sure the shaft end was perfectly flush with the surface.

    The shaft is slightly too short. It doesn’t fully penetrate the thicker support. This could be by design(?) to ensure that the shaft never protrudes from either side.

    Andrew E. Mileski - Reply

    Hi Andrew, we are working on a new design for this part, as soon as it is ready the instructions will follow. Thanks for the feedback and the tips ;)

    Jakub Dolezal -

    Channel lock pliers seem to work the best! No hole drilling, filing, etc.

    Christian Horner - Reply

    Maybe improvements were made since the previous reviews, but I’m astonished how this step is causing difficulty for so many people. I used a fully-fledged hammer like a neanderthal, as in the same kind that houses are built with. Just place the part sideways on the edge of a hard worktable; with everything hanging over the edge except the “shaft holders” which makes them flush against the surface. Gorilla-grip the entire part to secure it if you want to be extra safe. But a few taps was all it took, and it wasn’t going in “by hand” prior to trying this either. Plus breaking it was never even the slightest worry. The shaft is still short, but once again just use an Allen wrench as like a hole punch and tap it in until centered if it bothers you that much.

    David - Reply

    Widened the hole a tiny, tiny bit with a round file to enable using a reasonable amount of force and lessen the chance of breaking the printed part. Also used the very nice tip from @phr0ze to use a screw and nut to help center the pin!

    Christian Kullander - Reply

    I used a small C-clamp to push the shaft in easily.

    Doug Smith - Reply

    I assembled this without drilling or anything. Just make sure you can press down the assembly - with the shaft straight - on a flat surface. To get it all the way in my little plastic bag included a small screw I cannot identify. A bit like the fastening screw on the bondotech pully. I laid that on a flat surface to push the assembly on it and push the shaft further in. Wiggle it out carefully afterwards with sa small screw driver.

    Alain Geenrits - Reply

    I used one of the M3x30 bolts. Just thread it in and pull put gently, using the threads to ream it out slightly. Do that until you can push the bolt in and out without too much effort. Still a tight enough fit, but you can press the shaft in with your thumb on the inside and the other end of the shaft on something hard. After the pulley is in place just tap it in with something small like a jewelers hammer, supporting the opposite side on a block of wood. Once it’s flush, use the M3 bolt to tap it down so that it’s even on both sides.

    Eric Bass - Reply

    Part was too tight. I used a bit held in pliers to slowly remove just a little plastic at a time, until I could shove the shaft in one end and could barely press it in the other. I then placed the pulley and pressed the shaft in. Was so happy it worked… until I realized that in all the fiddling with bearings I’d put the pulley in backwards :-( Pushed the shaft out with an allen wrench, and turned it around, checked both bearings were still inside, pushed the shaft in - voila! I then used an allen wrench as a depth gauge to check that the shaft is in far enough on both sides.

    Please replace the shaft with a slightly longer one! Also, suggest in the manual using a 3mm hand drill, because doing that seems necessary.

    Brad Needham - Reply

    Same as the other commenters, I needed to 1) clear out the hole with a 3mm drill bit, and 2) use a screw and a vice to push the rod into the part. The rod is at least 2mm too short to completely fill both supports; I’m hoping that it doesn’t matter too much that it isn’t central (I don’t want to push any harder to shift it again).

    Arthur Tombs - Reply

    I think one of the oozing threads got caught in the bearings… It snags occasionally. The more I spun the bearing, the less it began to snag. I’d recommend tapping the rod slowly with a screwdriver as opposed to laying it on a table and pushing down on the printed part: the friction gave way and caused me to think that I broke the bearings.

    Zerg620 - Reply

    I would agree with you, this worked for me pretty well too. :)

    Tomáš -

    My shaft was a tad tight, rather than use any drilling, or large amounts of force, i rubbed the rod with a candle and it fit with a slight amount of force.

    Russell Peake - Reply

    Channel locks work perfectly

    stuck a allen key between the channel locks and shaft to straighten things up

    so smooth I thought they were loose until I tried pushing myself and couldn’t move it at all

    tyler - Reply

    Yes, Channel locks are your friend all throughout this build! Perhaps, a small 3d printed tool similar to a channel lock would be extremely helpful. I haven’t needed to drill out or file down anything so far. Channel locks really help to press in those stubborn nuts too.

    Rick -

    Building in Sept 2018 and while channel lock pliers worked to press the pin in it was a very tight fit. Using one of the allen keys, I pushed the pin in with substantial force to try to centre it in both ends. If the pin was 3.2mm longer it would be flush on the outside of the mounting fixtures.

    Darin White - Reply

    Assembling the MK3 in September 2018:

    - The verb “slide” in “slide the shaft in” is really not not reflecting what you want us to do. The shaft was not entering at all.

    - I used a 3mm drill by end to enlarge the 2 holes.

    - As the shaft would still not enter the hole, I used a channel lock and that went fine (althought this was quite stressfull).

    - Shaft should be as wide as the Extruder-idler. As it is not the case and I wanted it centered (at least a bit), I placed a M3 screw (that has a concave end) against the shaft and use the channel locker to push the shaft further in the hole.

    I think this part should be at least rephrased in the manual.

    Pierre Hilson - Reply

    Thank you for your feedback Pierre, just recently we have updated this plastic part, in order to be more durable and easier to assemble. Changes should be seen in upcoming plastic parts update.


    Tomáš -

    Freaking part broke on me. I had to keep applying more and more force because the shaft would not go into the hole and it finally gave out. Argh!

    Kyle Olson - Reply

    Hi there Kyle, I am sorry to hear that. You can use super glue as a short run and subsequently you can re-print this part once you have your printer assembled. Or possibly you can contact our Live chat at shop.prusa3d.com, our team will help you out.


    Tomáš -

    Perhaps add a note that the shaft is about 2mm shorter than shaft space air might just ease any worries on the part of the builder that might not be reading these extra notes

    Mustrum Ridcully - Reply

    Hi Mustrum, good point, we are currently discussing the shaft’s length.

    Jakub Dolezal -

    I first did a dry run putting the pin through the first hole using channel locks. I then pulled the pin out… so I knew it fit. I then assembled the unit and used the channel locks and it went in very easily. Finally, I seated it using the screwdriver and the channel locks as a hammer.

    Robert Eden - Reply

    How can i buy a new extruder ideler i broke mine while assembling

    Jax b - Reply

    Hi Jax, using our website shop.prusa3d.com (don't forget to log in), then there is a section with spare parts.

    Jakub Dolezal -

    Nearly a year on this thread and still people are breaking idlers due to missing instructions, a tight fit, and a short shaft. The proper insertion and the channel locks are both important solutions (worked for me) that should be in the instructions.

    Scott - Reply

    Hi Scott, thank you for the feedback! As Jakub stated, we are discussing the the tolerances and the design of this part.

    Martin Lexa -


    I'm in step 5. E-axis assembly (spiral wrap), step 5.

    The item says slide the shaft through the tensioner and pulley, but the shaft does not slide nor with hammer, I'm afraid to break printed part.

    What do I do, please?

    Marcos Aurelio Binoti - Reply

    First of all, read the comments. :)

    Second, use a 3mm drill bit and widen carefully widen both holes.

    Martin Lexa -

    • ATTENTION: 3D printed extruder parts were improved in order to achieve better cooling. More information including direct comparison can be found on our Prusa Research forum.

    • The assembly of the printed parts is mostly the same. You will be informed in case extra attention is needed. Before you proceed to the next step, let's learn how to recognize your parts:

    • Previous design has a grill on one side of the extruder body. This iteration has a label B6 (printed by us) or R2 (available on GitHub).

    • The latest design has NO grill to improve airflow. This iteration has a label B7 (printed by us) or R3 (available on GitHub).

    • In case you are using this guide to upgrade your MK2/S to MK3 and have already printed the B6/R2 design you can proceed or if possible reprint the latest parts B7/R3.

    If it’s better airflow, and just the grill, could you use Diagonal pliers and just snip out the grill?

    IPT - Reply

    Hi, there are more improvements to the parts, see the linked article. The missing grill is just the easiest way to recognize old and new design. Cutting out the grill might bring some improvement, but I recommend printing the new parts.

    Jakub Dolezal -

    What a lovely idea :D Very curious what the answer will be :P

    Artem - Reply

    Hi Artem, see my answer above.

    Jakub Dolezal -

    And why still send the old parts (week ago)?

    JdeL - Reply

    Hi JdeL, we had a stock of MK3 kits in our warehouse with B6 parts. If you want to upgrade to the latest design, I suggest downloading them here: https://www.prusa3d.com/prusa-i3-printab... and please use black PETG or ABS.

    Jakub Dolezal -

    I just received my kit yesterday, and it has the older (B6) part. Should I complete the build with it, or try to get hold of an upgraded part before continuing?

    Arthur Tombs - Reply

    Same here,

    My MK3 is 3 days old, and I found out it is outdated :)

    kchilingarashvili -

    Hi Arthur, version B6 is completely fine and proven by hundreds of hours of prints. You can upgrade to B7 later on. However, if you have another printer and PETG/ABS, then you can print the latest parts.

    Jakub Dolezal -

    If I was a company I would ship free replacement

    kchilingarashvili - Reply

    I know some people get bummed because their shipment didn’t contain ‘the latest part’. I for one am glad that Prusa and others are continuing to make improvements, constantly, and that in all probability I can look forward to other future improvements I can print myself. I don’t have any other products that can build improved versions of themselves.

    Rich Rector - Reply

    Hi Rich, that is the spirit! Thank you :) All the new parts are available for download so you can reprint them any time and also learn, how the printer assembled.

    Jakub Dolezal -

    • BE CAREFUL with the filament sensor, do not touch the black PCB nor the chips on it.

    • For the following steps, please prepare:

    • Extruder-body (1x)

    • Filament sensor (1x)

    • The filament sensor is in the box labeled " SUP".

    • M3x10 screw (1x)

    • M3n nut (2x)

    The step 5 / PACKaging is not correctly labled — I have 2 large and a small package. One contains screws, the other the plastic parts, the small package contains the parts from step 2. I’m missing the filament sensor from this step.

    PEB - Reply

    corection - found it ….it is in the carton package. That should be mentioned in the description

    PEB - Reply

    Thanks for the suggestion, I'm working on something already ;)

    Jakub Dolezal -

    Definitely agree that you should mention that the filament sensor is found in the “ SUP” box, and also mention that it’s in a static-protection envelope.

    Bill Magley - Reply

    Hi Bill, description was updated. Thanks for the feedback ;)

    Jakub Dolezal -

    It says not to touch the black PCB or the chips on it. What should we touch? Is it safe to hold by the edges or should we grasp the metal prongs?

    Jason Coleman - Reply

    Hi Jason, please hold the sensor on the edges.

    Jakub Dolezal -

    Its static sensitive ,only a problem on really dry days. you could ground your self ?

    Phillip Kearney - Reply

    Hi Philip, I suggest ground yourself first (touch some metal) and then hold the sensor only on the edges not on the sides with the chips.

    Jakub Dolezal -

    Current body version is B6 if you want to mention it’s different than the pictures.

    Evan - Reply

    Hi Evan,

    No matter the version of the printed part the assembly process should be the same. In case our new version of the printed part changes significantly, the manual gets updated accordingly. Thanks for the suggestion ;)

    Jakub Dolezal -

    The warning about not touching the sensor should appear BEFORE you are asked to prepare it. Not everybody read the whole instruction before doing the step-by-step operations.

    Anders Roer Jensen - Reply

    Hi Anders, order changed, however I strongly advise to read all the instructions for any step before doing it.

    Jakub Dolezal -

    I don’t know if anything can be done but from this point on, once the PCB is in place then you have the 4 connector prongs sticking out of the head. Whilst constructing the rest of the head you need to be careful not to put the piece down with these on the bottom and you also need to be aware of these whilst handing the head whilst building the other components. Maybe some sort of protective cover could be introduced which will reduce the fragility of these prongs. I did end up slightly bending one of them but it was only slight and didn’t cause any problem.

    It might be better to move the step so that the board is fitted after the hot end is in place in order to minimise the potential problem.

    Stephen Boyd - Reply

    • Before we continue with the assembly, we need to insert nuts in the Extruder-body. Take the extra time and effort to place them in properly. You won't be able to reach them later.

    • Rotate the front part towards you and insert the M3 nut in the slot, all the way in.

    • Rotate the rear part towards you and insert the M3 nut in the slot, all the way in.

    • If you can't push the nuts in, use a longer screw from the other side and tighten it until you "pull" the nut in.

    hint how to easily place nuts , just use one of M3 nuts you have from E package, screw nut with 1 turn on M3, place nut in the hole, check it is perpendicular, turn screw 1 turn back so nut is released, piece of cake, in the nut has to be deeper use longer M3 screw, you can use your thumb to press it in space

    ufon - Reply

    On my Part B3, the hole for the 2nd nut (the blue one) was oversized and the nut spun freely inside the part. I was able to use the flathead screwdriver to hold the nut in place while screwing it doen from the other side. When it was tight to the bottom of the hole the nut gripped the part.

    Chris Pardy - Reply

    I think this step can be moved to the end. I disagree with the note that you can’t add the nuts until the end because you can’t reach them later. The nuts kept falling out of the printed parts, so I left it until after step 18 IIRC with no problems.

    Jon Le - Reply

    Hi Jon, thanks for the suggestion. The reason for this order of steps is because of the filament sensor, which can be damaged while inserting the front (yellow) nut and forgetting there are pins on the other side. I’ve tested the assembly several times and these nuts were OK, I will recheck it.

    Jakub Dolezal -

    Step 6 M3 nut in front can’t be pulled down by M10 bolt, it isn’t obvious that this is ok? We don’t want anyone crushing the plastic trying to make this happen.

    timcdoc - Reply

    I noticed that if you push the nuts down all the way it is a snug fit and they don’t fall out. I tapped mine down with the phillips screwriver.

    Larry Kmiecik - Reply

    • Carefully insert the filament sensor in the slot, do not use force or you might damage the PCB!

    Is the sensor PCB supposed to be loose in the slot?

    Aubry Thonon - Reply

    For now yes, you will tighten it in the next step.

    Jakub Dolezal -

    • Turn the extruder-body like in the picture. The pins of the filament sensor must be facing up.

    • Locate the opening for the M3 screw.

    • Take the M3x10 screw and tighten the sensor in place. No nut is needed, the screw will self-tap into the plastic.

    • The sensor must be tightened completely to prevent its movement (screw head touching the board) BUT BE CAREFUL during tightening to avoid damage to the sensor.

    • Layers might be visible in this printed part due to the geometry. The functionality and strength remain unaffected.

    Does the head of the screw touch the sensor? I seem to be screwing into plastic and I’m not sure how far to go with it.

    Mark Abrams - Reply

    Hi Mark, the head of the screw must touch the PCB of the sensor. Be careful during tightening ;)

    Jakub Dolezal -

    There is an issue. No filet in the pcb and not enough space between the pcb and the plastic part to put a nut. So you must carefully make a hole with a drill (in screw direction) to have enough space to put a nut. But Prusa has to modify the manual and the parts.

    Xavier - Reply

    There is no nut involved when fixing the sensor, the M3x10 bolt is self-tapping into the plastic.

    Robert Hunt -

    Hi Xavier, as Robert said, there is no need for a nut, the screw will self-tap into the printed part.

    Jakub Dolezal -

    I was able to secure it fully by continuing to screw into the plastic until the screw head presses snuggly against the sensor. I think it would be useful to other makers to show the result from the left side (based on the current photos as “front facing”) to show what to look for.

    Mark Abrams - Reply

    Hi Mark, thanks for the feedback. I will consider extra picture to show properly tightened sensor.

    Jakub Dolezal -

    Agreed, a left side picture would make it much clearer. I was not comfortable tightening it down that far until I read the comments.

    moacg -

    Hi. I noticed the senzor itself (sensory part) is looking dirty. Probably I should no attemp to clean… Or?

    Juraj Mr. - Reply

    Hi Juraj, the sensor shouldn't be dirty otherwise it won't recognise the filament correctly. Best is to use compressed air, in other words avoid direct contact with the sensor.

    Jakub Dolezal -

    I was also a bit confused that there was no nut trap/nut for this assembly. It might be wise to include a comment in the instructions when you have screws going directly into plastic. There are a few other screws that are used this way. It would be good to have a heads up. Also since M3 screws in plastic can easily strip, a warning not to over-tighten would be good.

    Bryan Jackson - Reply

    I agree. I was trying to figure out where I missed a step about installing a nut, but then I saw the comments. A note that it screws into the plastic would be nice.

    Guido Kimble -

    Wich type of nut we have to add and were please

    Sébastien ROUSSELLE - Reply

    Hi Sébastien, no nut need for this step.

    Jakub Dolezal -

    I also agree - it would be good to mention that the screw is going into plastic. After the screw failed to bite a few times, I removed the filament sensor and got the screw started with a few threads, then I removed the screw and reinstalled the filament sensor. The screw was much easier to start after that (still being careful since it’s going into plastic). I’m not extremely confident that I have the screw tightened enough, yet not too much - the sensor seems to be fixed in place, so I’m stopping there.

    Bill Magley - Reply

    It’s probably a better idea to tighten the screw into the printed part “before” fitting the sensor to cut the thread first, then remove screw, fit sensor and then tighten as described..

    Martin Wright - Reply

    My sensor looks dirty as well. Almost like some grease on it or something. Do I need to be concerned?

    Steven Maguire - Reply

    Hi Steven, finish the assembly and you will see during the prints, if the sensors triggers false alarms, then please contact our support. You will change it in the matter of minutes.

    Jakub Dolezal -

    I’d add a “wiggle” test to the instructions, and note there will be no other opportunity to tighten the screw again.

    You don’t want to discover the filament sensor is still loose in step 28.

    Don’t ask me how I know.


    Andrew E. Mileski - Reply

    Hi Andrew, I hope the instructions are pretty clear on this matter, you have to completely prevent the movement of the sensor.

    Jakub Dolezal -

    I also had an issue with this, and I was brought back to this from Step 28. It seems like the screw is going in a very long way, but the head of the screw is what holds the sensor so it won’t move. I had to pull the idler off (and those small plastic washers), which was easy to do, but it gives some concern when putting it back together since the washers are small, thin, and seemingly difficult to put back in place. Turned out easier than it appeared. I’d recommend some added info describing how the head of the bolt is what holds the sensor in place. After the head of the bolt contacts the sensor, it only takes a little extra turning to lock it solidly in place. A picture showing the head contacting the sensor would be self-explanatory.

    George Miller - Reply

    Ditto on the wiggle test problem for me at step 28. There is so much emphasis on “don’t touch the pcb or chips” that I only tried to wiggle it from the side, which it did not. Unfortunately on step 28, when you attach the cable, it becomes obvious that a front wiggle test is needed as well. So, take off your shoes or rubber flipflops, touch something metal, then grab the four prong connectors and see if it wiggles up and down against the screw head. Tighten as needed. Use the connector cable if you don’t want to touch the prongs, it’s also in the box.

    Murphy Chesney - Reply

    Can anyone elaborate on the “be careful?” What is it I need to avoid doing so as not to “damage the sensor?” Does it mean overtightening or what?

    C. Smith - Reply

    My sensor wiggles as well. Looking closely perpendicular to the screw, it seems the screw has gone through the sensor board and into the plastic below. It sounds like it shouldn’t be able to do that without a lot of pressure, but I didn’t use what seemed undue force to turn the screw. Two questions: 1) Did I just wreck my sensor and need to get a replacement? 2) Should this step have instructions to stop turning as soon as the board no longer wiggles? I haven’t exerted undue force at any other step (where others have broken plastic, stripped plastic etc.) so I don’t think I used more force than others would.

    Brad Needham - Reply

    Do you mean the screw *head* has gone through the PCB, or just that the thin end of the screw has gone through? I was initially confused by the instructions, but I think what it means is that the screw is supposed to be tightened until the head is touching the front of the PCB (with the screw thread deep into the plastic), but no further.

    Arthur Tombs -

    A picture from the side of the part would have helped me to understand this step. I didn’t immediately realise that the head of the bolt should be tightened all the way to the PCB.

    Arthur Tombs - Reply

    Inserting screw worked, but I agree with Magley & Wright about putting the screw in first, before inserting the sensor, to “cut” the threads into the plastic.

    However my question was the alignment of the sensor once it is tightened down. Mine tilted off-vertical slightly. Is this correct? I agree a picture showing the inserted and screwed-down sensor from that side would be helpful.

    Jay Rabe - Reply

    Hi there Jay, try to have the sensor centered in the hole for the sensor to sense the filament sensor. Luckily, if you would find any problems with the filament sensor after the assembly, it is quite easy to re-do this later on.


    Tomáš -

    Thank you, Tomas. The shiny sensor pad does seem to be centered in the view hole, but it couldn’t hardly be otherwise given the geometry of the holding screw. And while I got the connector plug installed OK, the pcb does sit at a slight angle, meaning the sensor is at a slight angle. It just seemed odd since the pictures, as well as you can see, show the pcb pretty straight in its slot. When I get it powered up, if it works no problem, just seems odd.

    Thanks, Jay

    Jay Rabe - Reply

    I see what you mean, I still believe you should be perfectly fine. In case you encounter any problems with the filament sensor, then please contact our Live chat at shop.prusa3d.com :) our techs will help you out 24/7.


    Tomáš -

    I suggest adding a note in the instructions that the head should touch the PCB… that gives some idea on when to be concerned about tension.

    Robert Eden - Reply

    Hi Robert, thanks for the suggestion ;)

    Jakub Dolezal -

    • For the following steps, please prepare:

    • M3x30 screw (1x)

    • M3 translucent washer (2x)

    • Translucent washers are placed on the idler for better visibility. No need to place them as in the picture ;)

    Picture in Step 9 is a bit confusing, this part is not in the MK3 kit in this form ;-)

    Jeroen - Reply

    Hi Jeroen, since the washers are transparent, some darker background was needed ;) and you will use the idler in the next step anyway.

    Jakub Dolezal -

    Being a bit pedantic here, but the washers are translucent, not transparent :P

    H4irBear - Reply

    Hi, yes you are right, thanks for the correction ;)

    Jakub Dolezal -

    I find the text for this illustration is written in poor English and therefor hard to understand at first.

    I suspect “due to better visibility for the viewer “ should be something like “so the viewer can see them better“

    Bernard Higonnet - Reply

    Thank you, description updated.

    Jakub Dolezal -

    • Turn the Extruder-body like in the picture. The pins must be facing to the right.

    • Insert M3x30 screw in the hole.

    • Place a washer from the other side.

    • Assemble the idler on the M3x30 screw.

    • Finish the assembly with the second washer.

    • If the second washer keeps falling off. Use a M3n nut from the spare bag to fix it temporarily.

    only one washer in picture use. please arrow secound washer on picture

    theerachai - Reply

    Look at both photos for Step 10. It’s between the extruder body and the idler.

    J Allen -

    Hi, first washer is marked with green arrow, second washer with orange circle. Please, check the pictures again ;)

    Jakub Dolezal -

    How tight should I tighten the screw?

    Raynald Bijosono - Reply

    Hi Raynald, you can’t tighten the screw right now as there is no counterpart. However, this screw is holding the idler, so later on, don’t tighten it too much. The idler must be able to move/rotate.

    Jakub Dolezal -

    I used a nut temporally do hold the second washer on place. This after I dropped it two or three times to the floor and had to search it ;-)

    Wolfgang Peters - Reply

    second this ^. I did the same thing as a safeguard.

    tim -

    Clever, thanks :D

    Tor Andre -

    Thanks Wolfgang! I think this should be worth to include it as a step, or perhaps not adding this washer until step 14.

    David de León -

    We will think about it David :) thank you for your feedback.

    Tomáš -

    +1 - I did the same thing.

    moacg - Reply

    Thanks guys - So did I

    Michael Tillbrook - Reply

    +1 works like a charm.

    pariah_l@yahoo.com - Reply

    M3x30 doesn’t seem long enough to go through the extruder body the other piece. Additionally the hole that the M3x30 goes into is angled down and doesn’t look like the picture. Yes this just on my piece or are others seeing that was well?

    Arezendes - Reply

    As in the photo they use a m3x40 but saying it is a m3x30. The 30 is too short and the 40 is exact the same as the picture

    Robin -

    EDIT: keep in mind there is a m3x25 and m3x30. I took the 25 instead of the 30.30 fits right

    Robin -

    Yep, I made the mistake of using the M3X25 thinking it was the X30 and was about to switch to a X40. The measurement chart on the bag is very helpful if used. This is the first time I used it as I was not able to eyeball the correct length with a 5mm difference. These comments are helpful! Thanks everyone.

    Jarem Frye - Reply

    Should say in write up something to the effect of “slide 30mm screw threw hole, then place washer and slide onto the screw, then slide idler and finally second washer”. It would just be clearer.

    IPT - Reply

    • For the following step, please prepare:

    • Extruder motor (1x)

    • Bondtech pulley WITH the lock screw (1x)

    • Ensure you are using the correct motor, there is a label on the bottom of the casing. The reason is, each motor has different cable length.

    • There is a second spare lock screw in the package.

    Ah, this is the other Bondtech not noted on the 5.E-Axis bag. This should be noted and also a special note above for the first Bondtech to use the one with no set screw.

    Graham McIntyre - Reply

    If you are upgrading from MK2S, remove the old extruder-gear/pulley

    Michael Hermes - Reply

    I got two equal Bondtech pulleys. There is no lock screw hole in any of them

    Tor Andre - Reply

    Hi Tor, please contact our support at info@prusa3d.com to get the correct one.

    Jakub Dolezal -

    I did. Got the correct pulley now. Awesome service :)

    Tor Andre -

    My BondTech pulley with the grub-screw (set-screw) had the grub-screw locked in place and the internal hex was stripped so I couldn’t remove the screw. Turns out the threaded hole wasn’t tapped quite far enough and it left a burr that locked the screw in place. It looked like they tried to remove it, only to damage the screw, but it was shipped to Prusa, anyway. I got Support in on this and they were great, telling me they would send me a replacement. I found an SAE (non-metric) Allen wrench (key) that was just barely large enough to get the screw out, so I contacted Support again to cancel that shipment of a new gear. I ran a tap through the threads, cleaning them up, and installed a new screw so everything is good now.

    George Miller - Reply

    I had the exact same problem as George Miller, but I manage to press a torx 6 in it and was able to tight it that way.

    Martin Olesen - Reply

    The grub-screw is extremely delicate and can be easily stripped if you are overeager with tightening it.

    If you need to remove a stripped grub screw, try using a small torx bit and tapping it into the grub screw with a hammer, you may be able to get enough grip on the socket to extract the stripped grub screw from the bondtech pulley.

    Jeff Getrum - Reply

    • There is a flat part on the motor shaft, rotate it towards you.

    • Slide the pulley on, note the CORRECT orientation.

    • The screw must be facing directly against the pad (flat part) on the shaft. Slightly tighten the screw, the final adjustment will be done later.

    • Don't press the pulley against the motor. Leave it on the very top of the shaft, see the picture.

    • Before moving to the next step, rotate the pulley 90-degrees clockwise ( the lockscrew will be facing to the left).

    This step and picture should be revised to show the motor shaft flat and pulley rotated 90-degrees clockwise (set screw facing to the left, in the current picture). In the orientation this step requires currently, you end up being unable to access the set screw in the next step without turning it.

    Guido Kimble - Reply

    Hi Guido, good point. For now, you can rotate the shaft even if assembled in the extruder body. Use pliers.

    Jakub Dolezal -

    Came here to say the same thing. This step seems to imply the orientation of the shaft matters, but then has us align it 90 degrees off from being useful.

    Zachary Loafman -

    +1 yes, an additional image of the rotated gear would be helpful.

    Gene Dahilig - Reply

    I find that you can rotate the shaft-with-pulley when mounted inside the extruder body with just a finger, no need for pliers. (I’m twitchy about scratching things, I guess.)

    steve hix - Reply

    PSA: DO NOT TIGHTEN THE LOCK SCREW. Better yet, skip this step. not needed at all

    You will need to be able to move the motor pulley in step 15. When you get to that step, put the motor pulley in there

    Karim - Reply

    Well, at least in my version of the extruder body (B7), the tightening was important. I actually didn't thightened enough and let it free to move up and down. When I got to step 14 it ended up stuck in the bottom of the motor shaft and I had to unscrew the motor and this time make sure the pulley was in the top.

    David de León - Reply

    • Find in the package two M3x30 screws and insert them into the holes.

    • Before mounting the motor, ensure the second translucent washer is still on the top M3x30 screw.

    • Mount the motor on the extruder body as shown in the picture, double check the proper orientation of the motor cables.

    • Tighten both screws firmly.

    • Tighten the screw, but only slightly, keep in mind the idler must rotate freely.

    It's easy for the outer nylon washer to fall off when doing the steps between step 10 and here. Maybe put in the other 2 30mm screws first? Or at least note here to make sure the nylon washer is still in place.

    Graham McIntyre - Reply

    Thanks Graham, note added.

    Jakub Dolezal -

    I agree with Graham, the outer washer from stem 10 might be moved to step 13 to avoid losing the washer each time the piece is rotated.

    Carlos Rusconi - Reply

    It doesn’t seem like there are too many threads that catch the stepper motor for the screw for the idler.

    Ira Schonfeld - Reply

    It says the idler must rotate freely. I assume that means you can “close it" so it fits inside the extruder body?

    Even with the screw loose it feels like it's bumping over two or three ridges as the idler moves over the extruder body.

    Jeff Spencer - Reply

    Hi Jeff, the idler must be able to move a bit, because when you insert the filament it will push the idler a bit out. However, without the filament inside, the idler should be “closed fully” (aligned with the extruder body).

    Jakub Dolezal -

    Doesn’t make a huge difference, but this section deviates from the “Please prepare” step that the rest of the guide does.

    Evan - Reply

    Hi Evan, thanks. I'm aware of these steps and will “correct” them soon.

    Jakub Dolezal -

    I think the pictures would be clearer if they included the idler assembly we mounted in step 10 (unless I’m missing something). It made me scratch my head at the orientation or wonder if I was missing a part.

    Zachary Loafman - Reply

    Oh, I just caught that the idler was “tucked in” in the pictures. My parts didn’t initially rotate freely that way so I didn’t try it.

    Zachary Loafman -

    Don’t forget to take out your nut holding the plastic piece before you screw in if you read the comments earlier!

    Erin Baker - Reply

    The idler door doesn’t fully close. The two Bondtech pulleys, even with the teeth engaging, don’t allow it. With some force, I can push the door to align with the extruder body, but that doesn’t sound right and is probably hurting the bearings in the idler pulley, which spring the door back open again. There are about four layers of idler door print material “sticking out” of the body. The M3x30s and their springs will probably exert a similar force as I can apply myself, but with Jakub’s above reply that “without the filament inside, the idler should be closed fully”, I’m not sure if I should just proceed.

    Christophe Lermytte - Reply

    In the forums, people stated that the door does not need to be fully closed and pointed me to the picture in step 24, which indeed seems to agree with that.

    Christophe Lermytte -

    Hi Christophe, you should be able to fully close the “door” (idler), however, slightly opened is still ok. Most important is to have the filament correctly flowing through the Bondtech gears.

    Jakub Dolezal -

    I found it important to get each of the 3 screws engaged in the threads of the motor housing a small amount and then tighten each of them a bit at a time to ensure the motor and the fixture remained parallel. Initially, I had tightened the screw hold the idler on and it sort of tilted the assembly relative to the motor making it tough to engage one of the other screws. When I backed them all off and started again it went fine.

    Darin White - Reply

    The 4th screw mounting the motor just appears installed. first we have the screw that dose for the swinging cover then the 2 other motor mounting holes and then in this step the 4th screw shows up in the picture in step 16… aside from the hinge screw what is the proper torque for the other three? are they also the 4 NM mentioned early in the build instructions?

    Mustrum Ridcully - Reply

    Hi Mustrum, the last screw (top-right) is used after the extruder is installed on the X-carriage. There is no recommended torque for this part of the assembly. I recommend using the Allen key for the last turn or two to ensure you won't overtight the screw.

    Jakub Dolezal -

    • Open idler fully to have direct access to the pulley.

    • Use a piece of 1.75 mm filament (from the spool) to align the pulley with the openings for the filament (see the picture). Arrows only indicate the direction. Don't use the 3mm nylon filament!

    • Adjust the pulley and tighten it with 1.5mm Allen key. Use reasonable force as you might damage the thread.

    • When ready with the alignment, please remove the filament.

    Adjusting the pulley and tighten it with 1.5mm Allen key failed as both Allen keys stripped at very low force. I used my own.

    Valentin Bulbuc - Reply

    Same here, the included allen wrench is not working. Had to use my own.

    Alexander Buschek -

    Have to agree with ++Valentin Bulbuc++ 1.5mm Allen key supplied is not adequate quality. Allen key stripped at very low force ++Jakub Dolezal++ , A change of supplier is easy an fix for future customers considering Prusa kit QUALITY is key to sales.

    PaulHarris -

    Do we remove the filament peice after? I’m just finding out I need to remove it to insert the e3D hot end later on

    Alex Wilkie - Reply

    Hi Alex, yes please. Remove the filament when done with the alignment.

    Jakub Dolezal -

    It would be nice if there were more pictures showing the feed path of the filament from other angles.

    Gabe Yoder - Reply

    Hi Gabe, from these pictures you can see most of the “filament path”, other angles always cover something. What issues do you have with the aligning?

    Jakub Dolezal -

    Agreed, like a back angle where the filament sensor is.

    Erin Baker -

    The black pieces of extruded plastic included in the box are NOT filament. I fiddled with how to get these to route through for a little while before opening my box of actual filament. Adding a note here wouldn’t hurt to have someone else avoid my mistake.


    Joe Pighetti - Reply

    This threw me off as well. I was about to get a drill before I read your comment.

    Travis Howard -

    Hi Joe, that is why it is said to use a filament from the spool.

    Jakub Dolezal -

    For clarity, you might mention that a piece of orange filament is used in the picture to show the routing, and the green arrows are parallel to the correct path.

    Bill Magley - Reply

    Please just include a 100mm piece of filament in the bag for the step. Save so much time.

    phr0ze - Reply

    This. I was surprised I had to pop the filament in this step.

    Zachary Loafman -

    Exactly. Reluctant to break the humidity seal of a new filament that will not be used right away and I haven’t got my dry storage boxes fixed yet… 100 mm of orange filament would be nice in bag 5. E-AXIS. I have the black version and black filament is harder to see inside the black parts.

    Anders Permats -

    Also second this idea. It would be much easier to have a small piece of filament included for this step.

    David de León -

    I agree with adding a small piece of filament. I live in Costa Rica with ridiculous humidity and am not yet set up to drybox my plastic. So I feel I should do that before cutting open one of these rolls of plastic, then having it absorb humidity for the next few weeks.

    Rich Rector -

    You can use a piece of the silver filament you got with the printer. We strongly recommend to use this filament for calibration once the build is complete so you will open it anyway couple steps later.

    If you don’t want to use the filament, that’s fine, you can use one of the hex keys. The idea here is just to have something straight as a reference.

    Martin Lexa -

    Agree it’s hard to see where to insert the filament from the back … a top view with an arrow of where to insert would help. I see now it inserts right next to the sensor.

    Rich - Reply

    In my case the filament (yes, from the spool) didn't fit through the small hole, it got stuck. So I widened it with a 2mm drill bit. I hope that's no problem?

    Tako Schotanus - Reply

    Hi Tako, small diameter increase should be fine.

    Jakub Dolezal -

    At the end, remove the filament, close the idler and look through the “exit” hole to check alignment.

    Etienne van Ballegooijen - Reply

    The grub screw socket is EXTREMELY shallow!

    Allen key failed to turn the grub screw, and I noticed the Allen key was slightly worn / rounded / deformed.

    I used the second Allen key, which was in better condition, and it worked slightly better.

    I didn’t think to blame the Allen keys, but rather thought I had stripped the grub screw.

    I’ll go back with an Allen key from my toolbox and try it again.

    Andrew E. Mileski - Reply

    Hi Andrew, there is a spare lock screw in the package (bundled with the Bondtech pulleys) ;)

    Jakub Dolezal -

    My grub is stripped and I can’t even get it out

    Craig Bennett - Reply

    I got it out after a few hours of trying. I about damaged my hands doing it

    Craig Bennett -

    Mine also got stuck, but didn’t manage to get it out… Not sure if the tools were bad, the thread wasn’t good, or I just didn’t put it correctly. Just in case I got a pack of good allen keys while waiting for the replacement. Could be god to add a note to be careful when using this one.

    Jesus Berjano Pena -

    I had the same problem as Craig. I didn’t think that I tightened it too much, but clearly I was wrong. Given how shallow the inset is on the screw, it doesn’t take much to round out the inside. It took 30 min of effort with multiple different allen keys (both mm and SAE) to finally get a grip on the inside to get it out. I thought that I was going to need a new extruder motor and hobbed gear.

    TrinityEllis - Reply

    Barely tightened the lock screw at ALL. And it’s in the wrong place. And now it will not unscrew at all… so… no functional printer as no filament will load. SERIOUS design flaw for a kit this pricey.

    Neil F - Reply

    agreed these photos are not helpful - which hole should the filament go through? need a clear shot from the rear circling the correct hole - is it the one right next to the circuit board or one above in the idler thing??

    Nick - Reply

    also clear note NOT to use the filament looking piece in the box it is too thick

    Nick - Reply

    That is not filament. You are suppose to use a bit of the filament from the filament spool as mentioned above :)

    Tomáš -

    Picture is confusing. Just show the filament path and add arrow head at each end to show direction. You tell us to “align the pulley" but what does that mean? There is a knurled band on the pulley. I assume this just needs to be in line with notch for the filament. Other than that i cannot tell what the path actually is.

    William - Reply

    Yeah same here on stripping the lock screw. Thanks to you guys I didn’t give up and finally got it out. May have stripped the new one too but least it’s in the right spot, I think. I agree a less shallow one would definitely help.

    Mark - Reply

    How did you manage to remove the lock screw ? I’m in a similar situation as you were? Thanks!

    Karim -

    The gears are meshed, but I can’t push the filament through. Is it too tight or will the motor do it?

    Cathy O'Malley - Reply

    Hi Cathy, make sure you are not using the black nylon filament, which is too thick.

    Jakub Dolezal -

    Previous comments talk about using the filament from the spool. Can you add that as an item needed for this step. ‘Open spool and insert filament.” The word ‘open’ would help as I was not familiar with filament thickness by touch alone therefore ‘1.75 mm’ filament was not helpful.

    Once I realized my error, the instructions and image seem clearer. But that is not the case when you reading the instruction for the first time.

    Name - Reply

    Hi, thanks for the feedback. I assumed “filament (from the spool)” was enough, but I will adjust the text.

    Jakub Dolezal -

    Hi, building my second printer with the new B7 part. With the gear loose, when I attach and screw down the motor (leaving the extruder door side loose enough to move), the gear on the motor is touching the part—I cannot turn it with pliers. Anyone seen this or know how to resolve?

    Gavin Adams - Reply

    • For the following step, please prepare:

    • M3x40 screw (2x)

    • Extruder spring (2x)

    • Assemble springs on both screws, see the second picture.

    • Close the idler, so the screws can reach it.

    • Place both screws into the Extruder body and tighten them. The screw's head should be almost aligned with the printed surface.

    Are we supposed to close the idler first here?

    Neal Tibrewala - Reply

    Hi Neal, yes you must close the idler first as the screws must reach nuts inside the idler. Step instructions updated.

    Jakub Dolezal -

    So what distance should the screws be tightened for optimal grip/release? Almost flush? Is there a more scientific method to this, or does it not matter that much.

    Douglas Shelfoon - Reply

    Hi Douglas, on MK3 align the screws with the surface of the printed part. We've improved the design this way, so no special measurement is needed.

    Jakub Dolezal -

    Impossible to tighten even slightly on the part i received, the idler had no thread and the screw/bolts just slip though….

    It would be better to use a metal nut here since the printed part was useless

    Tom - Reply

    Hi Tom, You might have missed step 3 first picture? There should be nuts in the idler.

    Orne Brocaar -

    Thanks Orne,

    Just now saw the 2 holes for the square nuts as you said, so it was my fault being stupid :(

    Great news though, I can continue building now :))

    Tom -

    Is this supposed to mean align the top or bottom of the screws head? I'm now assuming you mean bottom. If so, can you revise it to say “The bottom of the screw's head should be almost aligned with the printed surface.” ? I originally assumed it to mean the top of the screws head and it's been causing me issues with my filament breaking for days because my gears were way too tight. It's actually hard to tell from the pictures how is lined up.

    Andre Fowler - Reply

    So top or bottom of the head? If I set it to the bottom of the head, the spring has almost no tension.

    Ira Schonfeld - Reply

    I was not sure either. So I put the filament back in again. While tightening the screws I push and pulled on the filament until it feels right. OK, not really a scientific method, but it turns out I had a good grip with the screw head TOPS just above the printed part. Hope I’m not wrong, let’s see later :0

    Quentin -

    Thanks Quentin, that helped me as well putting the filament back in to see what this area was doing. Had some good tension on the filament, but could move it back and forth - so now I understand a little better what’s going on in there, and so far it seems good. Sounds like it’s adjustable for various filament widths, so good to go on for now.

    Gregory Thomson -

    The screws “extrude” from the back as two small 2-3mm bumps post the idler - is that what expected?

    Peter Larsen - Reply

    Any closure on whether it’s the top or bottom of the screws’ heads?

    I did closer to the bottom and seem to be getting good tension on the filament.

    Evan - Reply

    Hi Evan, the screw heads should be almost aligned with the surface, but this is something you will adjust during the prints and also in case you buy filament with larger diameter (larger tolerance), you might need to adjust it again.

    Jakub Dolezal -

    It might be good to remind people that the back side of this part looks similar to the correct side (a hex opening over two holes) so if you aren’t paying attention and accidentally put the screws in the wrong side you can get the screws compeletely past the threads such that they can’t be removed without destroying the 3D printed part.

    Don’t ask me why I know this……

    (I had a friend re-print me the part, after a 30 minute freak-out)

    Brian Bishop - Reply

    Same happened to me. Is it possible to remove the screws without damaging the print?

    Lakshman -

    My build has ground to a halt. The shoulder of the smooth shaft is stuck on the square inside the idler part. Not sure of function of this square but. Is the m3/40 supposed to be secured against extruder body?

    Steven Comisso - Reply

    I made the same mistake. The instructions are misleading because they tell us to close the idler, then put the screws in, but there is no mention of turning the part over between the two steps.

    Arthur Tombs -

    D'oh. Just saw my mistake.

    Steven Comisso - Reply

    As I have it assembled now, the idler turns in opposite direction of the motor. I cannot figure out how this should work?

    Georg Zueblin - Reply

    Hi Georg, can you record a video? Post it in section “Answers” (see top of this page). At this step, there is no movement needed, so I need more information.

    Jakub Dolezal -

    Instead of telling us at the start to put the nuts in then. Why not here? If the nuts are screwed in, then they fall out super easily

    Craig Bennett - Reply

    This is the first step I did backwards - definitely needs a callout to ensure users are entering the screws from the correct side of the idler. Also the slots that hold the square nuts are too loose/sloppy - one of my square nuts is rotating in place. I have to stick a small allen key in from the side to hold the nut from rotating as I tighten the screw.

    Connor H - Reply

    I did this too, and unfortunately, tightened one screw so the nut was past the threaded part of the screw. I thought I was never going to get it backed out because the square nut was slipping, but your post gave me the idea to trap the nut with something thin. The smallest allen key proved too big, but I ended up trapping a paper clip in one side as it turned, and then I was able to get enough purchase to re-thread the screw and back it out.

    Definitely think this step should call attention to the fact that the part needs to be turned over.

    JDR -

    It was at this step that I re-inserted the square nuts from Step 2. Without the screws, they nuts will not stay in place through the constant rotation of the part. I would suggest moving the nut insertion to this step.

    Ed Barton - Reply

    Make sure the gears of the idler pulley and the motor pulley are meshed before you tighten the screws. I somehow managed to tighten the screws with the idler teeth touching, rather than meshing with, the motor pulley teeth. Strangely, the gears didn’t just fall into place as I turned the pulleys. Fixed it by unscrewing the screws, turning the idler pulley so the gears meshed, then closed the idler and tightened the screws.

    Brad Needham - Reply

    Definitely need to let people know to turn it over, this is way too easy of a mistake to make. I had to brace something in there to get the nut to stop spinning inside the printed part.

    Jason - Reply

    +1 for warning us to flip the piece over - just wasted 45 minutes getting the screw back out (used a paperclip to jam the nut in place, like many others have)

    Joe Bourrie - Reply

    The gears are meshed, but I can’t push the filament through. Is it too tight or will the motor do it?

    Cathy O'Malley - Reply

    • Take two M3nS nuts from the package and insert them in the slots, all the way in.

    • Check proper alignment with the 1.5mm Allen key.

    Minor nit, I missed this step and inserted the nuts after the hot end was attached. Maybe put a picture of two M3nS nuts with green arrows, like is done in the rest of the manual? There is enough information to figure this out, but having the extra cues would help.

    timcdoc - Reply

    Hi Tim, this step will be updated. Thanks for the suggestion.

    Jakub Dolezal -

    Yeah, when 95% of the rest of the manual has a “prepare X, Y, Z” step it throws you off for the 5% of the time it doesn’t.

    Evan - Reply

    Hi Evan, I will update these steps soon.

    Jakub Dolezal -

    I just found a trick for inserting the square nuts. You can push them down deeper with another square nut. you can take them out with the smallest Alan wrench (1.5mm)

    Zerg620 - Reply

    There are plenty of options how to push the nuts inside the plastic part :) yet we suggest using our procedure .

    Tomáš - Reply

    • ATTENTION: 3D printed extruder parts were improved in order to achieve better cooling. More information including direct comparison can be found on our Prusa Research forum.

    • The assembly of the printed parts is mostly the same. You will be informed in case extra attention is needed. Before you proceed to the next step, let's learn how to recognize your parts:

    • Previous design has a grill on one side of the extruder cover and extra fan arm. This iteration has a label B6 (printed by us) or R2 (available on GitHub).

    • The latest design has NO grill to improve airflow and also the arm is not present. This iteration has a label B7 (printed by us) or R3 (available on GitHub).

    • In case you are using this guide to upgrade your MK2/S to MK3 and have already printed the B6/R2 design you can proceed or if possible reprint the latest parts B7/R3.

    Add Comment

    • For the following steps, please prepare:

    • Extruder-cover (1x)

    • M3nS nut (1x)

    • M3n nut (1x) - skip for R3/B7

    • M3x25 screw (2x)

    • B6/R2 - insert both nuts in the printed part. See the second picture.

    • B7/R3 - insert only M3nS nut. See the last picture.

    • In case you can't press the M3n nut in, don't use excessive force. Take M3 screw thread it from the opposite side of the printed part, as you tighten the screw, it will pull the nut in. Be careful not to break the printed part during tightening.

    In case you can't press the M3n nut in, don't USE excessive force.

    Keith Manley - Reply

    Thank you Keith, corrected.

    Jakub Dolezal -

    I cannot manage to fit this in whatsoever. The screw trick does not seem to be working whatsoever in getting the m3n nut in. Is there any better advice for this?

    Kyle Campbell - Reply

    EDIT: It eventually worked after brute force involving a rubber mallet.

    Probably not the best idea though…

    Kyle Campbell -

    Hi Kyle, the “screw trick” should work always, but you need to ensure three things:

    1) the thread is on the entire screw

    2) there is nothing blocking the nut from going down

    3) nut is aligned with the shape of the slot

    Jakub Dolezal -

    I had a bit of a problem getting the M3nS (square) nut in the hole. It was really tight and I had to poke in the hole with a small Allen key and also a bit with the Phillips screw driver to clear it just enough to be able to push the nut all the way in using some force pushing it in with the screw driver.

    Kari Söderholm - Reply

    Hi Kari, we are checking the printed parts before shipping them to you. However, sometimes it might happen there are some residues, which are blocking the nut. Use the smallest Allen key, to check it out and if needed clean them.

    Jakub Dolezal -

    The instructions say to pull the M3 nut in with an M3 screw, but the shank of the M3x25, used in this step, is too long to get force on the nut against the printed part. I recommend using any of the full-thread M3s from the package, such as the M3x10, in order to get the right pull-in.

    Trey Hilyard - Reply

    I think there is something wrong with this design of this step. I’ve dealt with “the thread trick” already, and I’ve had to use “excessive force” because the product does not meet the specifications otherwise. I just spent an hour on this step and I am not done..

    The nut is supposed to bottom out on the arm, and that surface is rough extruded surface. Of course that interface is going to cause problems! Then you have the corners to deal with and other little bits of extrusion will get in the way.

    Prusa should avoid this fastening method at all costs.

    Even when I take the nut out, I can see only a few threads of my specified M3x18 screw making it through to the nut area. I’m seriously tempted to take a drill to the whole thing. If they’d just given it a little tolerance by making the screw a 20 instead of an 18 it would have probably eliminated all problems.

    Really the arm should just be solid with a slot in it like so many other pieces are.

    Ben Ruppel - Reply

    One M3x25 screw missing from the 5. E-Axis bag, but the spares bag came to the rescue!

    HugoB - Reply

    I believe the screw trick only works if you use a fully threaded screw, not the ones shown, also I had to hold the nut until it started sinking in.

    Daniel Brynych - Reply

    Please re-write the last paragraph for clarity (and grammar). I suggest the following: “Take an M3x10 screw and thread it from the opposite side of the printed part; as you tighten the screw it will pull the nut in. “ It is important to use an M3x10 rather than one of the M3x25 because the shorter screws are threaded all the way to the head.

    Arthur Tombs - Reply

    • Take the E3D hotend and place it inclined into the Extruder body. Make sure the white PTFE tube fits in properly.

    • Insert the hotend into the Extruder body, see the picture.

    • Make sure the hotend is fully seated and the upper part aligned (almost in contact) with the surface of the printed part.

    • Note the CORRECT orientation of the hotend.

    • Be VERY CAREFUL with the hotend wires from now on, you can damage them.

    The PTFE tube insertion should be its own step prior to this one, please.

    RWReese - Reply

    Agreed, I didn’t see any mention of the PTFE tube anywhere else, and completely missed this passing mention when I was building. Only after some frustrating filament feed issues weeks later did I realize I was missing this part.

    Ian Tyssen -

    Nevermind, please.

    RWReese - Reply

    What should be another VERY CAREFUL point here is to pay attention to the hot end being inserted ALL the way. Pay close attention to the final picture and how far in the hot end is. Mine was not in all the way. It went together and all was good until the XYZ Calibration. Yikes! Took 2 weeks to finally figure out what the heck was wrong. Finally got it thanks to the build forums.

    Mike Kwiatkowski - Reply

    You should perhaps mention that the white tube will need to bend a bit for you to be able to get the hot end all the way in.

    Andreas Sjolund - Reply

    The manual says, “Push the hotend into the Extruder body, see the picture”. What it should say is “Insert the hotend into the Extruder body so it fits around the collar in the Extruder body. This will make the bottom of the hotend even with the Extruder body.”

    This little misstep on my part caused 10 days of total frustration because when I ran the XYZ calibration it always failed with the message “XYZ Calibration failed, check the axes”. So that what I did but it had no relationship to the axes. It was a hotend location problem.

    E Duane Rose - Reply

    Hi Duane, I'm aware of this and we are already working on a new design, but you are right. The instructions will be updated ;)

    Jakub Dolezal -

    I had to take mine appart after it failed at calibration. So making it easier to fit the extruder body and cover to make it easier to fit in the correct position would be a massive improvement.

    Martin Wolfe - Reply

    Ditto. Thankfully, it only took hours to fix and not days. I realized there was a problem after I attached the the wire routing when the nozzle was loose and rotating freely. Still, it should be more difficult to snap on the extruder cover if the extruder is not in the right position. It’s never fun disassembling a part immediately after spending an hour assembling it.

    Mahir Abrahim - Reply

    It was ten days for me as well. After failing XYZ calibration, I noticed I was repositioning of the P.I.N.D.A. probe a lot. Then when I found that 6-8” of filament disappearing inside the extruder. I decided to run the extruder motor until the nozzle started to extrude filament or something physically come out. The nozzle was what came out. I then knew, I needed to open up the extruder block. Went back to the online page for the extruder nozzle assembly and found the same error codes I had by your comments. I then remember being unsure of putting this part together. I ended up taking out the PTFE tube from the nozzle assembly and reinserted that first into the plastic body case and pushed it into the hole and up to the Bondex gears. I then inserted the extruder nozzle over PTFE tube and then was able to place the aluminum nozzle easily into place. I put the unit back together and the printer calibrated then properly.

    Thanks for your comments it make the difference!!!

    Frank De Abreu - Reply

    Is the Hotend from the MK2S (which has my Olsson ruby) the same as the one in the MK3 upgrade? I’m trying to avoid changing the nozzle

    Thanks, Stuart

    Stuart Kurtz - Reply

    Contacted Support. It is different a Voltage. ?24V vs 12V. Installed ruby prior to assembly> I’ve realized what is ultimately needed is a quick change nozzle system to accommodate different sizes for different print types. The current nozzle change system is too difficult to make swapping a print nozzle a regular experience .

    Stuart Kurtz - Reply

    Hi Kurtz, there are some significant differences, you have to use the MK3 hotend.

    Jakub Dolezal -

    I made the same mistake of not having the hotend flush with the extruder body. I didn’t realize I made that mistake until I went to calibrate the PINDA probe and realized the hotend was sticking out way too far. Took me about 30 minutes to take apart the extruder and install the hotend correctly. Getting the hotend to fit right wasn’t easy, I had to bend the ptfe tube just the right way to get it to fit. I may be oversimplifying the problem but it would be nice if the hotend could only fit in the extruder body only one way. Other than this one problem the assembly of my printer went smoothly and the printing is perfect.

    M Goddard - Reply

    Is our E3D V6 supposed to arrive without the blue collet clip? If you look at Step 18 on their assembly instructions, this part is included. I am only seeing the black collet and PTFE tube on my V6 here. https://e3d-online.dozuki.com/Guide/V6+A...

    Todd Scott Anderson - Reply

    I found a decent clip on thingiverse. Also I removed the tube and made sure everything inside was clean. the loose collet’s teeth had made a couple little ptfe strips and the bottom of the tube wasn’t exactly square. Removed the fluff and fixed the bottom.

    The initial collet ring’s dimensions were a bit flimsy so I scaled it X, Y, Z : 0.95, 0.96, 1.2


    Todd Scott Anderson -

    Hi Todd as far as I remember the collet on E3D V6 was always black. Was your missing completely?

    Jakub Dolezal -

    Jakub, Todd is talking about the blue clip, not the black collet itself.


    My E3D didn’t come with one either, so I printed one just now. https://www.thingiverse.com/thing:247854...

    But I’m not sure if It will fit into the extruder body, but here goes!

    Daniel Lee -

    This is very important like many who did not make sure the lock was in the up position and have a collet clip ready to keep it there the PTFE tube rode up just a bit either during the build or during the initial testing prints and when I did my first filliment change while printing the the Prusa bottle opener a blob had formed in the space between the bottom of the slightly raised ptfe tube and the top of the heat break… when this happens many times the only fix is the removal of the hot end from the extruder body to fix it. There should be an instruction to insert a (Provided clip) after making sure the ptfe tube is in contact with the top of the stainless steel heat break. I have helped many folks both on this facebook page and on the official forums with this problem https://www.facebook.com/groups/Prusai3u...

    Mustrum Ridcully -

    It will be extremely helpful if you guys add the specific measurement for the PTFE tubes both for the hotend and the one for the sensor cover they can get easily damaged and you will need to replace them but is hard without the proper measurements.

    Cesar Lopez - Reply

    Had to come back to this step after seeing the same “Calibration failed, check axes" error message. Double check the hotend/PTFE tube is way up there, y’all.

    Taylor Mills - Reply

    The problem is that the hotend can be fitted perfectly in two positions, but only the upper one (as mounted on the axis) is the right one. The instructions fail to show the good and the bad position. Like all the guys above I got stuck at the calibration procedure and had to go back to this step.

    Georg Zueblin - Reply

    Inserting the PTFE tube took more force than I was comfortable with applying.

    After only the first attempt, I noticed the chamfered top of the PTFE tube had been deformed.

    It took several attempts before I finally went YOLO and pushed it in.

    There has got to be a better way to do this!

    Andrew E. Mileski - Reply

    Well the tube has to sit in tight, that is why the insertion takes slightly more effort than the other parts of this printer.

    Jakub Dolezal -

    To make sure my tube was fully in, I opened the idler door and could see the tube inside within just a few millimeters from the idler itself, which seemed proper to me. Is there any reason having this picture/step shouldn't be included in this assembly

    343 Guilty Spark - Reply

    The hot-end PTFE tubing appears to be 45mm long. My hot-end overheated, causing the filament to become stuck in the tube, and I had to disassemble the extruder enough to remove it, then clean-up quite a mess. The PTFE tube was replaced but the hot-end needed to be disassembled, the heater block cleaned and the nozzle replaced. Not what I expected from a 5-day old printer. The original PTFE tube I removed had been stretched due to the heat, and it measured 50mm in length. I started with a replacement tube of the same length, then cut it down after trial and error assembling, ending up with 45mm. I seem to remember during my printer assembly that the length of that tube was mentioned but I couldn’t locate that information again. Hope this helps future assemblers.

    George Miller - Reply

    Hi George,

    I recently placed all the PTFE tubes in one place https://help.prusa3d.com/l/en/article/nt... and correct length for the MK3 PTFE tube is 50 mm.

    Jakub Dolezal -

    Would it be possible to add pictures from the top?

    Brian Dobson-Lewis - Reply

    Hi Brian, by the “top” do you mean to see the nozzle, or the other side?

    Jakub Dolezal -

    Jakub, sorry to reply so late. I meant the other side, where the filament will be loading from, where the ptfe tube is. The instructions mention aligning it to the printed part, so it might be nice to see that.

    Brian Dobson-Lewis -

    it’s ok to the cables of the thermistor to be touching the heat block? mine are touching but i’m afraid to break them if i try to move the cables away

    Gustavo de León - Reply

    I came back to comment this after some issues appeared after 4 weeks of printing. Make sure, the heatblock is firmly attached to the hotend (but be careful don’t use too much force or the heatbreak will get damaged). Mine came loose after a few days…

    John Wilhelm - Reply

    Should say VERY VERY CAREFUL. I was only being VERY CAREFUL and I just now realized that one of the more-insulated wires has snapped… Not sure when that happened. It broke so close to the hotend there’s no way I can solder it back.

    Of course I broke the most expensive piece :(

    Alex Mauger - Reply

    It would have been helpful for me if there was a third photo that shows the proper fit of the hot end in the extruder body. Assuming I have mine assembled properly it would be nice to know that the cooling fin closest to the nozzle aligns with the collar on the body and the way the body fits around the channel in the top of the hot end. I hope that make sense.

    Ben Vizzier - Reply

    Hi there Ben, thank you for your comment. We will take a look at it once we will be updating our assembly manuals.


    Tomáš -

    If your worried about breaking the wires on the heater cartridge and thermistor then just remove them by loosing the two screws on the bottom of the heatblock and set them aside in a safe place. They are easy to reinstall after you have the extruder mounted on the X-carriage. This makes building the extruder much easier since you don’t have to worry about sensitive bouncing around. Just make sure not to over tighten the screws when installing the heater cartridge and thermistor so you don’t crush them.

    Paul Betz - Reply

    • Rotate the extruder as shown in the picture.

    • Take the extruder-cover and place it on the extruder body. Both printed parts must be in direct contact.

    • Using M3x25 screws tighten both parts together.

    • Ensure again the hotend is properly assembled. The surface of the heatsink (part of the hotend with cooling ribs) must be aligned with the surface of the printed parts. See the last picture.

    • Note the B7/R3 extruder cover is missing the "arm". Otherwise the assembly is the same.

    On the last step of this part, using the 2 screws to tighten both parts together, I accidentally got the screw and nut cross thread. Now, the nut just turns in the 3D printed part and they’re stuck. I think you should use a longer nut or better quality nut so they don’t get cross thread. Like a coupling nut.

    Bob - Reply

    With the hot end in and the two parts together, the outside part rocks, as of there is some interference. I had to clean up a bit of the inside of the extruded cover and had to use a 30mm on the right side. A 25 mm wouldn’t even contact it

    Steve Worcester - Reply

    I agree. M3x25 was too short on that right side. Had to go with a 30

    Christopher Tilley - Reply

    M3x30 here, too.

    RWReese - Reply

    M3x25 worked for me.

    David Beach - Reply

    I also had to use 30mm, the 25mm would not grab the nut

    Dennis Sladek - Reply

    25mm were ok for me.

    Andreas Sjolund - Reply

    I got the same problem as Bob. The 25mm screw started to go cross thread and the nut started rotating in the printed part (as the nut wasn’t completely straight) without tightening. I was luckily able to get it out by pushing on the nut with the Phillips screw driver to keep it from rotating while unscrewing the screw. Then I used a 30mm screw to get it more easily on the correct thread.

    25mm screw might work ok if you get it perfect, but I think it’s just easier to use a 30mm screw here and even putting in the screw first and then the nut it was easy at this point. Still the screw is totally inside the printed part. I would suggest switching to 30mm screw here for more foolproof assembly.

    Kari Söderholm - Reply

    Guys, thank you for the feedback. 25mm screw should be enough, but we will adjust the design for easier assembly.

    Jakub Dolezal - Reply

    The last statement in RED is a important point. Be sure to make this check as you assemble the extruder housing. if your installation is incorrect it will show up as an axes error when you do the XYZ Calibration.

    E Duane Rose - Reply

    Thanks Duane for your comment. I read it and quickly realized I needed to back up and reassemble the Extruder Cover. Removing the E3D Hot End took quite a while because the white PTFE tube was stuck. My solution was to enlarge the hole for the PTFE tube and then use a Dremel tool to widen the interior of the Extruder Cover so the two halves would fit around the E3D Hot End without rocking or stripping screws.

    Bill Waring - Reply

    The M3x25 screws worked for me. I actually had hard time positioning the Extruder Cover around the Heat Sink, it all looked misaligned. Eventually I inserted the two M3x25 screws all the way in into the Extruder Cover, and using them as guides towards their nuts managed to align everything properly and to tighten the two screws (hope the later Calibration step will pass OK!). It is a rather delicate step…

    I read in the messages above that two of you had problems with “misaligned” nut. Maybe it helps to someone reading this, but I discovered one “M3” nut that would not fit., no way I had problems with it, tried several M3 bolts, it would not work. Then I took it out and tried it with several M3 bolts just to test it, nut and bolt only. None worked. I believe that nut is not Metric! It got somehow mixed in, and I was fortunate not to force it and create a bigger problem with some part…

    Alan Rusyak - Reply

    M3x25 worked for me, eventually. Biggest issues were:

    1) Realizing just far down the hole those nuts need to be placed. It helps to pre-thread the two screws into the cover as far as possible, and checking the depth the head CAN reach. Hold the cover to the side, and you’ll see the screws DO stick out approx 1/4”, which is easily enough to catch the captured nuts IF they are also sunk as far in as they are supposed to go. Not easy, but it works.

    2) Also, for me, I needed to realize that we’re not trying to catch BOTH of the square nuts on the left side (looking at the photo above), only the closer one. I assume the farther one has yet to be used, but will be soon.

    Kalani - Reply

    I’m worried about the hot end wires bouncing around while they are still bundled. Is this a good time to cut the zip tie holding all the wires in a bundle, or is it safer to leave it tied up? for now? (I realize the zip tie holding the two hot wires should probably stay)

    timcdoc - Reply

    Just an idea… once I had finished this assembly, I noticed that I had bent the delicate pins of the the filament sensor. Would it not be a good idea to include a little plastic cover that can be slid on to protect the pins at this fiddly stage?

    David - Reply

    Here is how I captured the nut: use a longer screw on the nut side, but only grab 2 threads. Use that screw to push the nut against the M3x25 coming from opposite dir3ction until the 25mm grabs the nut, then back out the screw that is barely attached, then finish tightening the 25mm screw.

    Rich - Reply

    Great tip, worked perfectly thanks.

    Shaun Collins -

    To get the M3x25 to work on the right side, I first used an M3x30 and by tightening that it pulled the receiving nut in all the way inward … I then removed the M3x30 and was able to screw in the M3x25.

    Gerry P - Reply

    The left side fits perfectly with the M3x25 but the right side does not go all the way. Seems like its block after 3/4 of it… what do I do??

    Pablo Traversat - Reply

    M3x25 fits fine. When you put the nut in (Step 6) it says to put a longer nut in from the other side and tighten it up to pull the nut further in.

    I used an M3x18 and that was enough to get the nut all the way in. If your M3x25 is not reaching, give that a try.

    SteveP - Reply

    Nut traps for M3 nuts are the bane of my existence. Had the same issue on the Mk2. Some nuts just don’t stay put…

    Cristian Sandu - Reply

    I also needed to use M3x30s to get the nuts to catch.

    Gene Dahilig - Reply

    2 x 25mm worked fine…

    Martin Wright - Reply

    This is another step where I had a nut that was not perfectly straight in the hole, the screw got cross-threaded, and the nut started spinning in the hole. I will use more care in the future, but some of the suggestions in the comments such as using a M3 standoff rather than a nut and/or using a longer screw do seem to be useful.

    Harold Toler - Reply

    The cool-end fan side, top edge, doesn’t meet perfectly. There is a gap.

    The pictures here seem to show the exact same defect.

    It is hidden behind the fan, so it doesn’t really matter, but it shows that a part redesign may be required to increase clearances.

    Andrew E. Mileski - Reply

    I have this as well. Not sure if it's gonna be an issue yet or not, but at there's a sliver of comfort knowing someone else made similar note

    343 Guilty Spark -

    I also had a challenge inserting the M30x25mm screw on the right side because it wouldn’t grab the nut. I found that the nut wasn’t seated correctly so what I did is I took a M30x40mm screw instead, screwed it in all the way and this allowed me to fasten the nut to the end of the longer screw which when tightened, pulled the nut in place. Then I removed the 40mm screw and put the M30x25mm back in and it worked perfectly.

    Ben - Reply

    I have a similar small gap at the top edge near the cool-end fan. It is about 0.75 mm. Did it work out OK for you all?

    TrinityEllis - Reply

    Regarding the last comment in red, for the R3 version, there seems to be no axial ambiguity - the heat sink surface is about 1.3mm below the surface of the print part - intentionally not flush?

    Ricky A - Reply

    There’s a lip on the two halves of the part that holds the hotend fan (fan in next step). See image two. That lip makes it impossible to pull out the thermistor cartridge to replace it (which you will need to do eventually, as it’s tiny and delicate). The cartridge hangs up on the lip and you have to bend it to get it in/out or do crazy tricks with the heat block. I took a small file and made a little more room so I could easily slide out the thermistor and heat block heater when the time came to replace them (already have replaced the thermistor with a dual-cable assembly). It’s worth doing NOW before you get everything together.

    Neil F - Reply

    The extruder cover that I got, has a different design. It is squared has a vertical vent slot and it is missing the protruding arm with the nut at the end :(

    The fan I got is black … that doesn’t seem to be an issue, but the extruder cover concerns me.

    Sergio - Reply

    would recommend to use the M3x25 screws or you might have difficulties some steps further ahead when you want to mount the extruder. the M3x30 blocked the nut for the screw you would have to use then.

    Michaela Noemayr - Reply

    • For the following steps, please prepare:

    • M3x18 screw (6x)

    • Left hotend fan (1x)

    • Front print fan (1x)

    • The left hotend fan has two sides, but the side with sticker must be always facing the hotend (not visible when the fan is mounted). Otherwise, the cooling won't work properly.

    • The left hotend fan (Noctua) can be also in a black colour without the rubber corners, other hardware parameters are the same.

    • Note for B7/R3 design only five M3x18 screws are needed, at the end of this chapter you might have one M3x18 left, which is OK ;)

    Bolting on the Front print fan took me a lot of time because I could not get the nut inserted far enough (so the 18mm screw was too short to ‘reach’. 10 minutes of careful scraping/cutting and pulling later the connection was established ;-)

    Jeroen - Reply

    I suggest using a m3 washer on each m3 screw holding in the front print fan to avoid cracking the casing of the fan at mount points.

    Natalie Crandall - Reply

    Hi Natalie, thanks for the suggestion. For each fan assembly there is a warning regarding cracking the plastic cover while tightening. If you follow the instructions, you will be fine :)

    Jakub Dolezal -

    I’m guessing this is a good time to unroll the fan wire, but maybe a note saying it is, or it isn’t.

    timcdoc - Reply

    I would recommend taking a second to check the condition of the terminals on both of these fans. My print fan had a burnt and broken connection to the yellow terminal and I didn't find out until I put everything together and did cable management.

    Ryan Latimer - Reply

    Hi Ryan, fans are tested in our factory before shipping, but I forwarded your findings to my colleagues. Please contact our support for a replacement (info@prusa3d.com).

    Jakub Dolezal -

    My fan is missing one of the brown rubber grommets. It’s on the outside, so I don’t think it should make a difference with noise or vibration.

    Mark Tellier - Reply

    Don’t overtighten the screws securing the front fan, it is easy to, I think I did and it failed the first self test when complete… or there was another problem I didn’t know about, anyways, that was the only issue I had after completion and printed my first part shortly afterwards :-) , well happy…

    Martin Wright - Reply

    Same here with the missing outside rubber grommet on the hotend fan.

    TrinityEllis - Reply

    Yeah I’m completely missing the rubber grommets on the noctua fans. Mk3 kit ordered 7/22/18.

    Ricky A - Reply

    This note advising I may have an extra M3x18 screw just brought me a LOT of relief as I was beginning to panic when I had one left over, though I did appear to be missing a x10 screw, which I pulled from the spare bag…not sure what happened to it…

    Jason Wells - Reply

    Hi there Jason, for this sake we have the spare back :) hope you were able to finish the assembly without any major problems.

    Tomáš -

    my noctua fan is black. Not complaining just making sure its the right one?

    Russell McCampbell - Reply

    Hi there Russell, these are exactly the same ones as the brown ones. Black ones are made by Noctua just for us, unfortunately they were not available from the day one.


    Tomáš -

    Has the lack of rubber dampeners on the black version of the fan had any effect on the noise/vibration? Why are these rubber bits not included on that fan?

    Mathias - Reply

    Hi there Mathias, not sure what you mean with that. Both of the fans are the same, just the color differs.


    Tomáš -

    I also have the black fan and the rubber dampeners are missing

    An Einstein - Reply

    Hello! No worries about that, from our experience, the “dampeners” function is mostly aesthetic here, so you don’t really need them. We have tested that the functionality of the fan is not influenced by the presence/absence of these tiny rubber parts.

    Martin Lexa -

    • Place the Extruder on the side and guide the wires from the motor in the slot.

    • Place the Left hotend fan on the Extruder. The cable must be placed in the top left corner, see the picture.

    • Note the correct orientation of the fan. The sticker has to face towards the hotend!

    • Insert M3x18 screws and tighten them slightly.

    • Now, tighten ALL screws, alternating diagonals. After tightening, check that the fan can still rotate freely.

    • DON'T tighten the screws too hard, all parts are made of plastic and you can break them.

    turn the fan so the cable go out to the rear of the carriage

    Heiko Schultz - Reply

    Hi Heiko, thanks instructions were updated.

    Jakub Dolezal -

    Not sure where to route the extruder motor wires after they’ve been routed through the slot, as indicated in picture 1.  I currently have them running down the back of the noctua fan and in the slot so it can be routed behind the extruder and wrapped.  In other photos the motor wires are not visible from the back of fan so I’m not sure if there was a step that I missed?

    The wires are as snug as I can get them routed around the noctua fan but I'm afraid that they may come loose in time and make contact with the belt.  I think better pictures of the cable routing for the motor could help.  I’ll remove the noctua fan and see if there’s anything I missed.

    Corey Dryja - Reply

    I think I found the answer to my question and I may have asked my original question out of order.  It looks like there is a slot on the x-carriage that the extruder motor wires are supposed to run through that sits on the side of the fan.  This is not the slot that’s detailed in the steps below.

    Corey Dryja -

    In this step it calls for M3x18 screws for the Nactua fan. I didn’t like how in the second picture the screw that is inserted near the edge by the heater cartridge protruded. It was really close the thermostat wires. I replaced it with a M3x12mm. Later I will switch it to a M3x14mm for better anchoring. Prusa Team may want to look at that.

    Donnell - Reply

    Hi Donnell, thank you for the feedback. The design is safe and working if the printer is built according to the instructions. Longer/shorter screws might be working as well, but I can't guarantee it and also it will be extra screw size in the package. We tried to lower the amount of sizes across the entire build.

    Jakub Dolezal -

    Agreed. I understand the need to keep the part list minimal, and I agree with this. Perhaps this could be another place to have a square nut or hex nut there, as this seems to tap into the PETG part. However, I fear I may have overtightened here because I wasn’t sure how much to torque this screw. It seems it would be better with a metal to metal connection here instead of the longer screw nearing the hot end, as Donnell was saying. With a nut, perhaps a 10mm screw would be adequate and not come near the hot end.

    Ben V -

    Ditto. I didn’t have M3x14 handy, but swapped for M3x16 that I had on hand. Still ~2mm protruding out into air, near the heater & thermistor wiring, but less. Will definitely be swapped later.

    michael_farady -

    The protruding screws were also unexpected for me, as every screw so far in the build has been of perfect length. Maybe a clarification in the instructions would be welcome.

    Andreas Sjolund - Reply

    You might mention these are plastic self taps….

    Rich - Reply

    Why is the cable location for the fan described as top left? As you face the fan it is more like bottom right. It would be nice to show a picture if it is intended to be close to the extruder motor wire bundle coming out of the groove.

    Charles L Miksch - Reply

    Hi Charles, if the extruder is mounted, then the cable is in the top left as described in the instructions.

    Jakub Dolezal -

    “Top Left” corner doesn’t really make sense when the extruder is laying on its side. Perhaps “The fan wire should be next to where the motor wire was just placed in a channel.” Especially since no image was shown from the side where the wire could actually be seen on the fan.

    Dave S - Reply

    Yes. This is much more clear than “Top Left.”

    Brien C -

    the raceway for the extruder wires over the fan is very small can you enlarge the raceway so we have more room for the wires. please.

    vincent bartlett - Reply

    1. The Left Hotend Fan I was provided with was black, not brown. Jakub, you might want to add to the instructions of this step that this is the same fan to avoid confusion if the Left Hotend Fans you’re providing with the MK3 kit are all now black and no longer brown.


    2. Be extremely careful when tightening the M3x18 screws into the Left Hotend Fan. Unlike many of the other parts that you/we/I have tightened so far, you won’t get any sort of torque feedback when tightening these screws, other than the visual cue of them sinking right below the top surface of the fan. The screws will just keep spinning without any indication that they’re fully screwed in, and you’ll strip the plastic threads on the printed pieces below. Again, just screw them in until they’re down right below the top surface of the fan and call it good.

    blueyes13@gmail.com - Reply

    My noctua fan didn’t come with the rubber feet…Mk3 kit ordered 7/22/18. Is that normal? I thought they had to be there for noise reduction. Also they are black.

    Ricky A - Reply

    Since we have to judge necessary screw depth by looks alone, it would be great if we could see a picture of what “far enough” looks like for those of us with the fan type without rubber corners. Are the screw heads supposed to be even with the inner (taller) part of the corner? Because my “lower right corner” screw started squeaking alarmingly when I tried to turn it that far…

    C. Smith - Reply

    Seconded. This was the first time I had to switch to the long handle of the allen key because they were so stiff.

    Arthur Tombs -

    Suggested re-write for clarity and grammar: “Now, tighten ALL screws, alternating diagonals. After tightening, check that the fan can still rotate freely. “

    Arthur Tombs - Reply

    Thank you Arthur for your feedback :)


    Tomáš -

    • This step is valid only for the design B6/R2 if you have new parts B7/R3 without the grill please skip to the Step 26

    • Rotate the Extruder as in the first picture.

    • Mount the Front print fan using two M3x18 screws.

    • DON'T tighten the screws too hard, all parts are made of plastic and you can break them.

    Myself and at least a few other users have run into the self test X axis failure due to the fan or extruder wires slightly sticking out behind the fan where the extruder carriage will pinch these wires when it runs into the left Z axis bracket. It would be helpful to new users to show a closeup of how the fan & extruder motor wires need to be tucked into the slot that runs over & behind the fan.

    Jason Livingston - Reply

    not into the slot, the slot is fine for the extruder cable, but the fan wire is too thick.

    the MK2(s) fan cable is without '''cable sheath''' and would fit into. To push both wires into the slot on the MK3 would result in high pressure to the wires.

    I’ve guided them directly out at the back of the carriage.

    At step 21 you can see the wire and the cutout at the fan


    Heiko Schultz -

    Hi Jason, cable management for extruder cables starts at step 24. It is shown you need to place the cable inside cable clip and then inside the slot on the X-carriage, but I will recheck this part again.

    Jakub Dolezal - Reply

    Yeah, but the problem is with how the extruder cable has to be positioned for step 28. It was not clear enough nor obvious enough how to put the extruder cable, and I ended up having trouble on the X Axis calibration too, I almost had to unmount the whole head to fix this.

    Thelvaen Mandel -

    the top M3x18 screw is too short. And no, there is nothing blocking the nut ;((

    Guntor H - Reply

    Hi Guntor, are both nuts properly installed? You can use longer screw to pull them closer and then replace with M3x18.

    Jakub Dolezal -

    I’m running into the same problem. Nothing blocking the nut. I did notice that it kept falling out between the step where you insert it and this step. I’ve used a longer screw to try pulling it in. I’ve used this screw without the fan to try pulling the nut in. Still won’t fit. The screw keeps turning, because it doesn’t quite reach the nut to lock into place. :-(

    Jennifer Krull -

    The 18mm will do nicely for the top nut, but you have to pull the nut in before the 18mm screw can reach it. Take a short screw, add a washer and screw it in from the the back side. Tighten it till the nut is fully seated (but do not use excessive force). Remove the short screw and washer and use the 18mm screw to mount the fan. You’ll find that the end of the screw now levels nicely with the top of the nut.

    HugoB -

    I Had the same problem. The M3x18 is just a millimeter or two too short. I went for a longer screw from the spares. Doesn’t look perfect, but should work perfectly.

    Pål Driveklepp - Reply

    I had to do the same

    Craig Bennett -

    Ditto. Spent half hour on the problem, resorted to a longer screw. If it impedes something down the road, out comes the Dremel with a cutting wheel. First unsolvable problem not following the instructions. Find these comments invaluable. Onward.

    Gary Shumway -

    Same problem with the top m3x18 bolt (or screw if you prefer), it is too short - I used a 20mm one I had for another project

    Chris Tipney - Reply

    I had today the same problem - i fixed it with a lighter - just screw the nut (in the sparepart) on a screw and heat them a little bit (not to hot!!!) and pull them into the hole (just a little bit) - thats works perfect ;)



    Max007 - Reply

    thanks for that (frightening) tip! works! better would be a 20mm screw!!!

    Christoph Stahl -

    Max, I was going to do the same thing but resorted to just using a longer screw. Figured less likely to screw something up…or more likely in this case.

    Gary Shumway -

    I had the same problem with the one screw seeming too short. After I tightened a 20 mm screw in, I noticed there was about 2 mm sticking out and I was able to take it out and put the 18 mm one in. So, I t is just barely long enough and the nut has to be completely seated which I wasn’t able to do without the slightly longer screw to tighten it in.

    Ross Stenersen - Reply

    Use long screw, tighten it, remove and replace with correct one works for me.

    Jan Tomek - Reply

    I did the same as Jan above, worked great

    Alistair - Reply

    The seam on the fan popped open on me when I tightened the screw down. No big deal but now the fan has a little crack on the mounting hole. A bigger screw that overhangs the fan mounting hole a little more would help with that, or a washer. I think the M3 head is almost small enough to fit in the mounting hole so it exerts a side force on the fan housing when tightened. I was not over-tightening, it popped pretty early. Fortunately the fan requires very little force to hold it on so even with the crack I was able to loosen the screw a bit and still keep it in place just fine.

    A warning to tighten very gently would be good.

    Bryan Jackson - Reply

    Be careful out there. I over-tightened the screw on the “crows nest” which cracked the very delicate fan housing. Just a small crack, though and it’s still quiet as a mouse FWIW.

    Andreas Sjolund - Reply

    The M3x18 screw is long enough. Use the top of a longer screw to press the nut in while screwing in the M3x18.

    Edwin Martin - Reply

    it was exasperating getting this 10mm screw in the top part fan shroud. I tried the heating method, that didn’t do anything, but then I used a longer one and wrenched it down really hard, then was able to use the 10mm screw.

    Brian M - Reply

    Also cracked one of the fan mounting points, but a bit of super glue (and an even lighter touch) worked fine. I agree with earlier comments that a washer might be a wise choice on both of those screws.

    Bill Magley - Reply

    I had to use a M3x25 screw to help set the nuts in the printed part (with the fan in place), then the M3x18 screws were able to reach the nuts properly.

    Paul Bowers - Reply

    I had to use a separate small screw to get the nut on the arm in tight so the screw would reach it to the fan mount hole (top one). It’s a very tight fit and the screw will not reach unless you ensure the bolt is all the way in and very tight,

    Peter Larsen - Reply

    18mm is too short. A 20mm from old MK2 fan worked for one and used a spare 25mm for the other deeper one.

    Ray Ackley - Reply

    Thanks for the tip Ray! I checked my spare bag from my MK2 and found a 20mm screw. It’s much better :)

    Gael Lafond -

    I 2nd Ray’s comment. Was unable to get an 18mm into the “arm” nut. Using a 25mm for the meantime…

    Woger Dog - Reply

    Agree with all who commented that 18mm screw is too short on upper left corner of fan, especially with warning about not tightening too much or risking cracking the fan housing. I’m using extra 25mm screw for now. May use cutoff wheel on my Dremel later if it’s an issue sticking out.

    Dave Templeton - Reply

    I over tightened the screw and it split the fan mounting tab a little. It seems the fan mounting hole is over-sided for the screws provide leading to this issue.

    Luke Davis - Reply

    Same problem as others : but different solution : with the (black) front print fan, the M18 bolt furthest from the extruder motor, wouldn’t catch on the nut. The nut we’d noticed was quite loose previously, (it dropped out when turning the assembly back and forth), although was not proud of the surface, so we’d presumed it was in far enough. We’d tried pushing it in with a flat head screw driver : We tried pulling with a longer screw, [having first removed the fan] : but that didn’t work.

    What worked in a nicely controlled fashion was :

    We put another hexagonal nut on top of the one that was apparently flush with the component surface, then used Maun flat parallel jawed pliers to drift it into place. by squishing the (proud) extra nut against the opposing face of the plastic arm the nut we wanted sunk into beautifully. Throughout the build we have used our Maun (200mm) parallel pliers time and time again, whilst reading comments from people about it being difficult to drift things into place.

    Jonathan Histed - Reply

    It took me nearly 1 hour just to get the 18mm screw to catch the nut. Nut kept popping out, even after I would tighten it using a longer screw. I finally just pushed my thumb against it, after tightening it with a longer screw, and it caught.

    Jeff Gannon - Reply

    I also had a problem with the front print fan 18mm screw. I used a small set of files (or even sandpaper) and ground off approx 1mm from the plastic screw hole ledge, effectively making it the depth less shallow for the screw to extend. I kept the original screw and it was able to catch the nut on the opposite side and easily pull it into place.

    ben686@hotmail.com - Reply

    I also had the issue with the upper screw being too short. I was able to get it to start, but only would engage a couple threads before starting to get too tight. I like to see a couple threads extending beyond a nut. So I grabbed a spare 25mm screw, ground a little off so it didn’t extend much past the nut and this worked perfectly. So I’d say this should be a 22 or 23mm screw.

    Russ LaValle - Reply

    I think you meant “..please skip to Step 26”.

    Timothy - Reply

    Hi Timothy, yes it is step 26. I'm just updating the manual and added one step to this chapter, so all steps moved +1. Fixing this now.

    Jakub Dolezal -

    • Following steps are valid only for the latest design B7/R3, if your Front print fan is already mounted, then skip to the Step 29

    • For the following steps, please prepare:

    • nozzle-fan-45deg-support (1x)

    • nozzle-fan-45deg (1x)

    • M3x18 screw (1x) - already prepared in step 22

    • M3x10 screw (2x)

    • M3n nut (1x)

    • In case you are missing one M3x10 screw please use the spare bag.

    My kit came with the orange piece but not the black one. I suppose I can print the part later and upgrade to this fan set up? Where would I find the file for that?

    Joshua Reed - Reply

    Hi Joshua, I'm sorry about this. All printable parts are on our website, look for the MK3 bundle: https://www.prusa3d.com/prusa-i3-printab...

    Jakub Dolezal -

    My kit seems to have come without the “nozzle-fan-45deg (1x )”.

    Cody Jensen - Reply

    Hi Cody, please contact our support to get one, or in case you have a spool of ABS, you can print it on your own: https://www.prusa3d.com/prusa-i3-printab...

    Jakub Dolezal -

    • Press the M3n nut in the support, all the way in. It will be barely visible. If needed use a screw from the other side to pull the nut in, then remove the screw.

    • WARNING: there are pins of the filament sensor on the other side of the extruder. Be careful as you might bend them during the following steps!!!

    • Connect the support to the extruder using the M3x10 screw. Note the correct orientation in the picture.

    • The inclined part of the support must be facing to the extruder cover.

    • Place the nozzle-fan on the extruder and secure it using M3x10 screw.

    Connecting the fan support was super difficult (yellow step) - I had to cut down my Allen key so that I could fit it in the gap to engage the screw head, and even then it was really fiddly. Am I missing something? Did anyone else have problems with this?

    Phillip Wright - Reply

    Thank you for your feedback Phillip.

    Tomáš -

    You can slide the Allen key (long part) between the motor and the outside edge of the part and angle it in there to tighten it up.

    Kacey Shupp - Reply

    Yep, that’s what I done as well.

    Vilius Kraujutis -

    I managed to slide the Alan key between the wires to get to the screw.

    Zerg620 - Reply

    Broke my nozzle-fan-45deg-support on this one. Only a crack that could be glued, but be careful on this one.

    Troels Larsen - Reply

    That happened to me too. It’s great, that all parts were available online, so an hour later I was able to continue assembly after I 3D printed the latest part. Thanks, Prusa team for the open source mindset. It’s awesome!

    Vilius Kraujutis -

    so the yellow step doesn’t work for me….the 10mm screw just turns and turns…it has nothing to grip. is it meant to make its own thread?

    Owen Coorssen - Reply

    I have the same issue - it looks like there is space for a nut. I’m going to put one of the hexagonal nuts in there . I don’t know if I missed this step.

    Jonno Schmidt -

    Hi there, are you sure about that? There is a nut which was supposed to be put in at this 5. E-axis assembly (spiral wrap).

    Tomáš -

    Yellow point: this should be done at step 19.

    Mihai - Reply

    it is the purple step for me that seems to be missing a nut. I put one in anyway but it would have been easier before the hot end was installed. “Place the nozzle-fan on the extruder and secure it using M3x10 screw.” is fine but the screw holding the orange fan nozzle spun without a backing hex nut. Its nut trap was missing a bit of outside edge in the print of the cover but there was enough for it to work

    Mustrum Ridcully - Reply

    Hi Mustrum, for this part no nut is needed. You need only the M3x10 screw.

    Jakub Dolezal -

    • Slide the Front print fan in the nozzle-fan.

    • Secure the fan using a M3x18 screw.

    • In case you are upgrading already assembled printer, it might happen the fan cable is too short and you can't incline the fan. First, try gently pull the cable from the spiral wrap. If this doesn't help unhook the cable (see the picture), but be extremely careful as you might disconnect the wires from the fan!

    I had to use a 3x25, the 3x18 could not quite make it

    Ben - Reply

    Hi Ben, M3x18 is enough, but the nut in the support part must be pulled in. Otherwise the screw won't reach it. See step 26 once more, the nut will be barely visible while properly seated in the printed part.

    Jakub Dolezal -

    I also had to use a 3x25 because the 3x18 couldn’t make it.

    Jay - Reply

    Hi Jay, M3x18 is enough, but the nut in the support part must be pulled in. Otherwise the screw won't reach it. See step 26 once more, the nut will be barely visible while properly seated in the printed part.

    Jakub Dolezal -

    Same here. I am doing the R3 update, and the M3x18 will not reach.

    Scott - Reply

    Hi Scott, M3x18 is enough, but the nut in the support part must be pulled in. Otherwise the screw won't reach it. See step 26 once more, the nut will be barely visible while properly seated in the printed part.

    Jakub Dolezal -

    The M3x18 was too short for me too, so I took off the nozzle-fan-45degree and took a closer look at the nut and noticed there was more room to go in more. originally the nut was about flush with the side it goes in, but what I did was use the M3x10 and screwed it in snug and you’ll noticed it will pull the nut in almost to the mating edge. then I backed out the M3x10 and after that attempted assembly of the fan again and it did fit. I ended up attaching fan first to the 45degree part, then securing them as a whole after that.

    David - Reply

    18 mm is okay. I found it too short at first, but after adjusting the nut making it deeper, I tighten it with 18mm.

    Hsiung - Reply

    M18 worked for me. Use an M10 to pull the nut all the way in and the 18 catches great.

    Brian Lloyd-Newberry - Reply

    I’m starting to learn that if the screw is just barely too short, it’s because the nut is not fully tightened into the printed part. Gotta put a screw into it, tighten it, and leave it for a few minutes. As usual, just be sure not to be too aggressive with how tight it is since the plastic can crack and break if you tighten without stopping. Really big thanks to Jakub to writing this very good guide up. It’s got words, pictures, and colored steps and parts!

    Zerg620 - Reply

    18mm was fine for me, however I had issues getting the fan to actually stay seated inside of the fan nozzle. In trying to get the fan to seat properly, I ended up cracking both the nozzle and the support at the screw. Any way I can snag the STL to print new ones?

    Corey Hastings - Reply

    This step took me the longest so far because it is tough to reach the nut on the other side of the fan with the 3x18. At least 30 mins…

    Mark - Reply

    Broke the black printed part around the nut hole the when pulling in the nut in with an M10, and still the M18 couldn't reach. Maybe a fix with an M25!

    Peder Bacher - Reply

    Why is there a note here about the fan cable being to short when the fan cable is not hooked up to anything? How can it be to short at this stage? Perhaps I will find out in a later step that I need to do this but the note on this step causes confusion.

    Rick - Reply

    Now I realize that this note is probably for those who are upgrading to the latest version of the mount for the front print fan. Would be nice to make that clear so those of us who have never built this (or any other 3d printer) before are not left wondering.

    Rick -

    Now I realize that this note is probably for those who are upgrading to the latest version of the mount for the front print fan. Would be nice to make that clear so those of us who have never built this (or any other 3d printer) before are not left wondering.

    Rick -

    Thank you for your feedback Rick, we will update this step, so that it is easier to understand :)


    Tomáš -

    same issue as Rick , cable is not attached to anything yet, dont understand the note

    David - Reply

    I fully agree with Rick comment.

    Pierre Hilson - Reply

    • For the following steps, please prepare:

    • P.I.N.D.A. probe (1x)

    • M3x10 screw (1x)

    • M3nS nut (1x)

    • The probe is a sensitive device, please handle it with care during the assembly!

    • The P.I.N.D.A. cable can be either black or grey, both probes are the same.

    • Make sure there are FOUR wires in the connector, if not, please stop the assembly and contact our support asap.

    My sensor looks very different… it says “JIMOU Sensor, P/N: IB2T-B0802N-C4B.7/35/PUR”. Hope it works…

    Mr Cookie - Reply

    Hi Mr Cookie, it will be fine. This is our PINDA probe.

    Jakub Dolezal -

    My sensor doesn’t look anything like this one, it doesn’t have a label at all. I’m not entirely sure how far down to place the sensor.

    Brian M - Reply

    same here (required text)

    Joe Cabezas Campos -

    My pinda has a black tip. July 18 kit.

    Derrick Smith - Reply

    Hi there Derrick, that is perfectly fine :)

    Tomáš -

    How do we contact support? I have a 3-pin Pinda

    Mark O'Brien - Reply

    Hi there Mark, through Live chat at shop.prusa3d.com, which is located on the bottom right corner.


    Tomáš -

    I had to request a replacement part. I was really looking forward to printing this weekend.

    Name - Reply

    • Start with the front print fan wire and place it under the cable clip.

    • Gently push the P.I.N.D.A. probe through the mount.

    • Create a loop on the probe wire, then place the wire under the cable clip.

    • Slide the M3nS nut in the slot and tighten the probe SLIGHTLY with the M3x10 screw.

    • The exact position of the P.I.N.D.A. probe will be adjusted later (in Chapter 9, Preflight check), so there is no need to adjust the probe or tighten the screw fully at this point.

    Ok here I almost screwed up and inserted the nut in the upper slot. Fortunately I realized in time and managed to take the nut out again. Maybe it’s worth mentioning this in the text? (Be careful to use the correct (lower) slot!). I know that there’s the picture but maybe a small reminder wouldn’t hurt.

    Francesco Santini - Reply

    I’m back at this point after completing the build and realized I had installed the nut in the wrong slot because I could not get the P.I.N.D.A. tightened up properly. Thanks for commenting on this as it was exactly my problem.

    Rich Rector -

    I’m finding the PINDA probe exceedingly difficult to mount into the described hole… I’ve attempted several time and it appears the hole is either exactly the same size, or slightly smaller than the probe.

    Thomas Hansen - Reply

    I found wiggling it side to side helped, but also pushing from other side to help clean out hole, then moving back to correct side.

    Rich -

    Same. I had to scrape around the inside of the hole with a drill bit a few times to make it large enough for the probe to fit.

    Arthur Tombs -

    Figured it out. The very thin bridging directly above the washer hole was preventing the “arm” which clamps down on the PINDA probe from moving upward. For me, removing that slight bridging solved my problem.

    Thomas Hansen - Reply

    THIS! I read this earlier and didn’t understand … now that I inserted the nut and retention screw, I understand - the probe is held in with “jaws” and the two thin bridges at the end of the jaw keeps the jaw from expanding. Without the bridges you could even pry the jaws open a little to help the probe go in.

    Why are those bridges there….?

    Rich -

    I’m assuming we’re not supposed to remove that bridge, right? They clearly designed it to be there, as it would have printed just as well without it.

    Evan -

    I couldn’t insert my PINDA without removing these bridges!

    Taylor Mills -

    The ‘bridge’ is only a layer or two thick . I too didn’t understand it’s purpose (which is apparently none). I ended up screwing the sensor in and then breaking the ‘bridge’ so the M3x10 screw and nut could eventually do their job of securing the sensor (but it’s being screwed in seems to make the screw and nut somewhat redundant. I hope by screwing in the sensor I didn’t mess up the wire connections to same. The more one screws around trying to get things to work the greater the chance of bending the sensor prongs or creating other problems.

    Gary Shumway -

    I used the flat screwdriver as a wedge to open it up just enough to be able to slide the probe in quite easily.

    Kevin Chua - Reply

    This technique worked well for me, too.

    Zachary Loafman -

    This worked well for me also. I just pryed with the flat screwdriver ever so slightly in the top slot and wiggled the pinda and was able to get it.

    Joe Banach -

    Me too. The bottom “bridge” got stretched a bit by doing this, but I didn’t want nor have to remove them completely as suggested in another comment. Still, pretty sure removing the bridges is OK too, the probe feels tightly secured anyway.

    Christophe Lermytte -

    This worked for me too but both bridges broke. But there is no way this was going to fit with the bridges in place.

    Jason Cumiskey -

    so i use pliers to keep screwing it in. rotatiing in 45 degeeres at a time. is this how its supposed to be done? i mean there is a thread.

    Brian - Reply

    Hi Brian, inside the P.I.N.D.A. probe holder should be no thread, you can slightly widen (open) the holder and push the probe through, but be careful as you can break the printed part.

    Jakub Dolezal -

    I found it very difficult to more the probe to its position. It is hard to grab the probe because your fingers hit the cable clip. I ended up cleaning up the hole for the probe with a 8mm spiral drill (by hand).

    Etienne van Ballegooijen - Reply

    I found that I could rotate the probe and the threads on the probe would pull the probe through the hole. Be careful not to damage the cable when doing this. I found it easier to do this before routing the fan wire under the clip.

    Jeff Heck - Reply

    I to used my pliars to gently screw the PINDA into place.

    Ray Benjamin - Reply

    Also had problems setting the Linda probe. I pushed it I. From the other side in order to loosen it up and that seemed to do it. I like the idea of using the screw driver to widen it a tad when inserting. That would do it too I think.

    Gene Dahilig - Reply

    Well, certainly this is a pain in the….neck. My probe required some work to get it in. This method doesn’t seem too good for making fine adjustments once you get to the point of setting sensor height. I used a small screwdriver to slightly open the clamp. It maybe better to test and smooth as necessary, the inside of the clamp prior to getting to this point so it will be a known factor prior to PINDA installation. It seems to me that using nuts above and below the clamp would be better in the long run. Since the sensor is already threaded, this should be an easy modification for future updates.

    George Miller - Reply

    Stuck half way through, removed probe from holder. Found a small tag of plastic sticking out from joint between plastic cap and metal body. Cut it off and slide probe back in. Went in better, and along with gently prying with the screwdriver probe went fully in.

    Brian hughes - Reply

    Could use a recommendation from Prusa about whether to remove the plastic bridges. The instructions photos above show them attached and present, but after trying a few times it seems there's no way I can just gently push the probe through like the instructions say to. It seems to me like the bridges must be removed so I can pay the clamp open a bit.

    343 Guilty Spark - Reply

    First printer and first build so sorry if this is a simple question but would appreciate an answer to the bridge question also. I was able, with much trepidation, to get the probe through with the bridges intact but am concerned that adjusting the pinda on any fine scale later will be impractical. Should/can the bridges be removed? I saw on the Sanladerer build video (3:14:42) that the bridge wasn’t present at all but its an older build and many of the parts/steps are different. Thanks.

    George Carson - Reply

    Hi George, the “bridge” is there to prevent you from opening the PINDA holder over certain limit, which will result in breaking the holder. Proceed carefully while inserting or moving with PINDA probe and you can remove the “bridge”. However, you should be able to move the probe even with the “bridge” in place.

    Jakub Dolezal -

    My Pinda sensor was very difficult to insert and move, but I managed to do it at this step. However, when it came to Pinda calibration, I couldn’t adjust the probe height on the assembled extruder without risking damage to the probe. So I cut the two paper-thin bridges on the Pinda holder clamp and inserted a jeweler’s flathead screwdriver to gently force apart the jaws of the Pinda probe holder enough for me to slide the probe up and down to calibrate the probe height. I have no idea how I would have done the Pinda height calibration without cutting the bridges and (gently) forcing the clamp jaws apart.

    Brad Needham -

    Wiggling it with my pliers worked pretty good!

    Marijn - Reply

    Couldn’t push it in at first, then I pulled out the little bit that I was able to push through and then took my round file and just ran in along the edges a little. I suppose sandpaper could work too. It was still snug, but definitely easier to push through.

    Darrell Dudics - Reply

    At first, I also had problems with getting the probe though the hole. What worked for me was wiggling the probe in circles as if I was stirring a bowl of batter. I didn’t even have to modify the hole, just wiggle the probe in circles with a stirring motion.

    Zerg620 - Reply


    Take a look ahead at Step 3 of the “Preflight Check” - you will see that they recommend a spacing of one zip-tie’s thickness from the bed of the printer to the PINDA probe (assuming the nozzle is just barely touching the surface). DO THIS STEP NOW because trying to adjust the PINDA probe once the whole printer is assembles is a minor nightmare. Just hold the whole extruder assembly over a flat surface, with the nozzle lightly resting on the surface and the assembly at a clean perpendicular 90-degree angle (ie: straight up) and get your one-zip-tie-thick distance from the PINDA to the surface happening now.

    I’m assembling multiple MK3’s and when I did this for my second one I felt like I’d given myself a gift.

    Jascha Narveson - Reply

    excellent idea! I mean its in there but adjusting it well thats a pain in the ***…. Not sure why or how but heck if it works like this, its good enough for me :)

    Miguel De Langhe -

    • Place the Extruder on the Y-carriage as in the first picture. Ensure the correct orientation of the printer, shorter extrusions must be facing towards you.

    • If needed, lower the X-axis so you can see the entire X-carriage. It is needed for the next step.

    • Take the cables from P.I.N.D.A. probe and front print fan, slide it between the lower smooth rod and belt.

    • Take the cables from Extruder motor and left hotend fan, slide it between the lower smooth rod and belt.

    • IT IS VERY IMPORTANT to place the cables as shown in the pictures. Please double check your steps.

    • Cables from hotend will be placed under the lowest smooth rod. We will arrange them later.

    First I had the printer the wrong way around (front facing me instead of back) and put the cables in from wrong side and had the extruder the wrong way. Would be helpful to add a note in the beginning to check that you have the printer oriented he same way as in he picture (back facing yourself).

    Kari Söderholm - Reply

    Thanks Kari. Almost did the same thing..

    Øyvind Taknæs -

    Thank you both for the feedback, description updated ;)

    Jakub Dolezal -

    Unless I missed a step, need to add how to lower the Y axis to match the picture, assuming rotate both axis until down ‘far enough’ being careful not to go out of level? (I used a ruler to make sure the sides were more or less level)

    timcdoc - Reply

    Hi, I updated the description of this step ;)

    Jakub Dolezal -

    Yeah, please add that you need to lower the Z axis down to get the the screws in the next step in, I don’t believe I missed a step about this…

    blueyes13@gmail.com - Reply

    Step description updated ;)

    Jakub Dolezal -

    Could do with a larger image on the printed manual. Too small to see cable routing.

    Brian Knight - Reply

    • For the following step, please prepare:

    • M3x40 screw (1x)

    • M3x30 screw (1x)

    • M3x18 screw (1x)

    • Place the Extruder near the X-carriage and check no wire is pinched. There is a slot in the X-carriage for the motor wires, see the picture. Bend the motor cable in it and leave the fan wires straight.

    • Mount the Extruder on the X-carriage using the screws above. Tighten all screws, but not fully. We need to arrange the cables again, this time in the back.

    • Guide the cables through slots (channels) on both sides of the Extruder. On the left side, it is Front Print fan and P.I.N.D.A. probe, on the right side Extruder motor and Left Hotend fan.

    • Ensure again the cables are in the channels and not pinched between printed parts. Now, tighten all screws, equally to prevent issues.

    First guide the cables (especially from the extruder), then tighten the screws.

    Michal - Reply

    Make sure the stepper and hot end fan wires are in the side channel otherwise they will get pinched and damaged when tightening the three screws.

    William Dutcher - Reply

    +1 William, I had to take mine off and reroute the cable because I missed that.

    Austin Hampton - Reply

    Guys, thank you for the feedback. Description of the step adjusted.

    Jakub Dolezal - Reply

    The M3x18 head sunk in far deeper in the part without any added resistance where it should have stopped. A print quality issue?

    Bo Gustafsson - Reply

    Hi Bo, the M3x18 screw will be fixed by M3nS nut inserted earlier. No resistance along the path (hole) is needed.

    Jakub Dolezal -

    Jakob, please emphasize placing the extruder stepper motor wires in the channel above the beige fan. I still missed that.

    RWReese - Reply

    Hi, there is an extra picture for this in step 21. At this step it is too late as the fan is already mounted, if we are talking about the channel above the fan.

    Jakub Dolezal -

    Agreeing with all the above comments!

    My suggestion:

    Mount the Extruder on the X-carriage using the screws above. Take especially care that the cables from Extruder and hotend fan are guided in the slot/channel.

    Tighten all screws, but not fully. We need to arrange the cables first.

    Tom Ludensen Christensen - Reply

    Hi Tom,

    thanks I will stress out, which cables must be arranged.

    Jakub Dolezal -

    Additional information needs to be provided for routing the extruder stepper motor cables.  I tucked the cables into the channel above the fan but didn't realize that there was another channel on the x-carriage assembly that the cables needed to be routed through.  I ran the motor cables through the top channel, then around the side of the fan, and into the slot pictured above with the orange arrows.

    Corey Dryja - Reply

    Corey, had the same cable routing problem as you and I agree there needs to be better instructions.


    Ezra hawkins -

    I definitely agree, the second channel in the x-carriage assembly is on the back side because of the orientation of the printer so many people like you and me have and will miss it. A picture that shows that part of the x-carriage assembly must be addded.

    Levent Alpsal -

    Hi guys, this step will be expanded to cover the process step-by-step. However, there are some limits, I need to reorganize the steps.

    Jakub Dolezal -

    thanks Corey, I completely missed that second channel.

    There definitely needs to be a picture and some instructions clarifying the guiding of the motor cables!

    Adam White - Reply

    Really happy that you learned from my folly and didn’t make the same mistake that I did.  This suggestion really needs to be added to the guide because others have been posting pictures of their build with the same cable routing mistake I made.  People are going to start cutting their cables in half.

    Corey Dryja -

    Yeah, a second picture is absolutely necessary to do it correctly.

    Mr Cookie - Reply

    Yup and I’ve been seeing people post pictures of their builds with their extruder motor stepper cables routed properly.  This really needs to be added to the guide.

    Corey Dryja -

    Guys, thanks for the feedback. It is on my to-do list.

    Jakub Dolezal -

    Don’t forget to place the cables in the channels before you start to screw. This is very important, you can even destroy your cables, if it is not placed properly before placing the screws.

    Miroslav Piencak - Reply

    Might want to emphasize earlier the location of the extruder fan wire out the back. I guessed (wrong) from the pictures. Maybe a picture of the fan wire in the step where the fan is mounted would be nice.

    Joe Pighetti - Reply

    I saw the side channel for theextruder motor and fan wires on the mount, but didn’t realise that channel leads to a larger opening on the back of the mount. The wires need to go in there, then through the side channel. That way they don’t get pinched between fan and mount.

    Bas - Reply

    I did it wrong the first time, too. Perhaps something like, “Note that there is a vertical channel on the X-carriage - ensure that the extruder and fan wires are positioned in this channel and then routed back along the horizontal channel before tightening screws.” (And a front or side view picture would help to clarify that.)

    Bill Magley - Reply

    This is probably the most confusing step if you don’t realize that there is a cutout and path for the stepper motor cable. This is something I spent a while with on my MK2S build and luckily remembered here. Maybe add a picture of the pocket in the X-carriage so builders get the idea. I suggest adding more detail. What worked for me was to hang the assembly from the 40mm screw, attached loosely. Then route the cables into the slots. Maybe add a warning that if the extruder won’t easily seat against the X-carriage, stop and readjust the cables. When you have it right, it comes together perfectly. When you don’t the temptation is to start to force things and that always ends badly. As with everything else PR does, it’s well engineered but not easy to explain sometimes.

    Ira Schonfeld - Reply

    Almost broken my cables, since I did not realized how to route the extruder cable from the first channel to this one…

    Lyubomir - Reply

    If you can‘t reach the top m3x40 hole you need to lower the X gantry.

    Per Hansen - Reply

    Lowering should have been a much earlier step, as if it’s too high you’re already balancing the assembly in one hand.

    Evan -

    Thanks, I was totally confused here. Simple, once you point it out.

    Shelley Dorbin -

    Cable placement should go first on the list. I almost messed things up too. The M3x18 screw can go way too deep, it should be mentioned not to do that.

    Zsolt van den Mar - Reply


    thank you for the feedback, step will be updated.

    (@mirecxp, @pighetti, @bas, @bmags, @irasch, @lpetrov, @vandenmar)

    Jakub Dolezal - Reply

    ARGH - could you use something other than two colors that look like eachother? Symbols or something would be helpful for those of us who are color challenged …

    Peter Larsen - Reply

    Is the one circled in Green really supposed to be M3x30? Mine sticks out too far. The M3x18 seems to fit here. It could be that the nut on the other side got turned a bit and is the reason I can’t fit the M3x30 in there, but I want to verify before I have to pull the whole thing apart…

    Brien King - Reply

    Hi Brien, yes it should be M3x30, make sure there is no obstacle in the hole. Use for example and Allen key.

    Jakub Dolezal -

    I would suggest making sure all the screws thread nicely before mounting on the X-Axis. I had to take mine off and remount it after the M18 wouldn’t catch.

    Shaun Collins - Reply

    Unfortunately I had to glue back one of the tabs on the left side of X carriage, where the motor wires go. It came off earlier in the X carriage/belt assembly. Maybe if you Strengthen the walls of the printed pieces a bit it might help.

    Erin Baker - Reply

    The lower wing on the right hand side (for the nozzle fan) said “crack”. Let’s pray this wing is not crucial …

    Mario Witdoek - Reply

    My M3x30 screw (the green one) isn’t catching on anything. Did I miss a nut somewhere?

    Brenda Bell - Reply

    Hi Brenda, have a look at step 6 5. E-axis assembly (spiral wrap)

    Jakub Dolezal -

    Just thought I would share. I had a struggle getting the m3x30 to thread in properly. It is still not fully threaded in (1/4) of head exposed on surface. Tried removing the bolt and it is stuck in the extruder assembly. After the 1st attempt I tried using an Allen key to make sure holes were properly aligned and there were no obstacles. It is possible I pushed material in the hole on the 1st attempt. I also saved the m3x30 to tighten last to ensure all 3 holes were as close to aligned as possible. Everything seems to be secured tightly as is :). Maybe add a step where you hand thread the bolts through the holes on the extruder side before mounting to the carriage? It’s been a great build so far!This has been my only real issue thus far. May have ruined the filament sensor in my haste trying to remove the bolt. But all pins are bent back straight, sensor is secure again and solder remains intact. Take care :-)

    Cameron Freitas - Reply

    Update: I believe the nut is slipping inside the hex hole where you press the nut in. I'm going to try a small pair of needle nose pliers to pin the nut down as I try to screw the m3×30 bolt in from the carriage side and see if I can get the bolt to thread on the nut the rest of the way

    Cameron Freitas - Reply

    Also had problem with the 30mm nut sticking out from the surface. In step 16 you install two M3nS nuts; in this step the 30mm screw goes into both of them. Turns out that the further back M3nS nut had gotten push too far in so its threads no longer lined up with the screw hole. It is fully wedged - I can’t get it out. As Brien noted the 18mm nut works. It’s only engaged with the first nut. I’m hoping this will be strong enough.

    Brian hughes - Reply

    Like a couple of others, I had one of the thin tabs on the x carriage crack almost completely off as well. It was the one on the right side, where the extruder motor wires route through. I’m not sure exactly when this happened, but I just noticed it, and I am attempting to repair it with super glue. I suspect that this was a result of the tab being fragile and colliding with lower x carriage bearing, which is not secured to the x carriage. I think if the carriage had been attached to the bearing with a zip tie like the top section, this would not have happened.

    Jeff Webb - Reply

    I discovered that I pinched the motor cables a couple of steps later. I read the warning but I didn’t realize there are cable channels on both parts until later. Hopefully, I didn’t break them. Since pinching the cables can destroy them, one of those big red phases would be good here: “IT IS VERY IMPORTANT to route the cables in their channels on BOTH parts to make sure the cables will not pinched and damaged.”

    Lee Clark - Reply

    Definitely the most complicated step. Had my wife hold the cables in the cable holder clip for me while I tightened the screws. Cost me half a bag of gummies.

    Daniel Bowman - Reply

    Feels good to have the extruder on there. Time to keep on going!

    Zerg620 - Reply

    One of the dificulties with routing the cables is being able to see what exactly is happening. The advice to have the short extrusions towards means that the wires are obscured by the frame; in addition, black on black makes the slots and channels difficult to see.

    Setting the assembly at a suitable angle give an easier view of both sides.

    The fundamental problem here is that one needs an extra pair of hands. The wires are safely held only when the extruder is attached to the extruder to the X-carriage. It is too easy to get one side in ok, only to find that the other has fallen out of position.

    It would be helpful if the extruder holder wire channels could be modified to retain both pairs of wires in position before attachment takes place.

    Les Grosvenor - Reply

    It could really do with a full-size picture of just the cable slot in the X-carriage plate. When I assembled this, I read the instructions about the slot, and thought it was referring to the one on the side of the X-carriage: I didn’t realize there was a vertical slot as well. It’s really only visible when you look at the high-resolution version of the image, because it’s all black plastic.

    I just came to look here again because, after I assembled everything, the motor cables were touching against the belt. Now I see I must have pinched them between the two parts and may have broken them. At a minimum I’m going to have to take all the cabling apart, take the back off and reposition the cables.

    Mark Grant - Reply

    • Locate the cable for filament sensor in the package.

    • There are two types of the connectors on the cable:

    • The 4-pin connector for the sensor (used now)

    • The 5-pin connector for the EINSY board (used later)

    Are these backwards? Einsy board can’t accept a 5 pin plug?

    Cameron Lamont - Reply

    Hi Cameron, 4 pin connector (marked green) goes to the sensor and the 5 pin connector (marked orange) goes to EINSY. In case you can't fit it, you must be using different port on the EINSY board.

    Jakub Dolezal -

    I had the same problem. Since the picture doesn’t label the lines (that comes in step 32) you think the PINDA probe is where the filament sensor goes. I just took it all apart - and realize that the picture isn’t a good guide on section 8. Suggest starting with a complete map of all plugs and some help identifying the cables.

    Peter Larsen - Reply

    • Use the cable from the previous step and connect it carefully to the filament sensor.

    • Note the connector has two different sides. The side with safety pin must be on the left (white wire is facing up and red down).

    • Gently rotate the cable (clockwise) to create a small loop, see the second picture.

    • Push the cable through the opening on the X-carriage.

    rotate the cable to the loop because a straight wire directly ouf of the plug will collide with the frame or spool holder on top when reaching 180mm or more.

    maybe, if you’r using a custom spool holder, the spool holder itself won’t work any longer on a MK3

    Heiko Schultz - Reply

    Good point, this should be added to the step to explain why you need to loop the cable as people might not bother.

    Robert Hunt -

    As silly as this sounds, now is the perfect time to notate that the bottom bearing needs to be properly seated, mine was loose to the right of the extruder mount at this point as it was not attached in any way (so still on the rod, not seated in the printed mount) and I didn’t catch it until self=test calibration on the X-Axis failed as the bearing was impeding travel…. it was a major pain to go back and fix it.

    Benjamin Curto - Reply

    Is it normal that my filament sensor wiggles a bit in place?

    Andreas Sjolund - Reply

    Hi Andreas, it is not ok. The sensor must be secured in place. Please see Step 8.

    Jakub Dolezal -

    I think that you should reiterate in the instructions that it shouldn’t be wiggling at this point, or maybe reiterate much further back to recheck so we don’t have to disassemble so much.

    Erin Baker -

    I think I found an easy solution if someone else comes across this problem. Unmount the extruder. You don’t have to disassemble anything else! Take the long side of your allen wrench get it sideways into that screw hole that is mounting the sensor and you should be able to screw it in more securely.

    Erin Baker -

    Be careful that your “small loop” is not too big vertically! Despite the fact that I attempted to check to make sure the wire did not interfere with the horizontal travel on the X axis, the loop I made was a little taller than the connector, and a little wider than the bearing. This made it collide with the spool holder, which of course one doesn’t install and thus find out until the VERY LAST STEP OF THE ENTIRE BUILD! Be sure to look at the third picture of this step and make sure your final loop is “tight enough”. It should not extend beyond the back of the rod bearing, nor should it go higher than top of the connector.

    Joel Kozikowski - Reply

    Hi Joel, thanks for sharing your experience with the build. The loop should be really small as in the picture. Just few mm.

    Jakub Dolezal -

    Does the “safety pin” actually click into place to lock it in? I was expecting it to do that, but even with a good amount of pressure it doesn’t click for me. It seems stable and doesn’t look like it will come loose, but it worries me.

    Bill Magley - Reply

    Nope……………… .

    John Hawkins -

    … and it was at this point when I discovered the filament sensor was loose :-(

    It seemed secure waaaay back in step 8 ¯\_(ツ)_/¯

    Remove the extruder (3 screws). Remove idler door (3 screws and 2 translucent washers), tighten sensor screw, replace idler door (don’t forget the 2x translucent washers), and back to step 25.

    Andrew E. Mileski - Reply

    Erin Baker you are a life saver!!! Mine was just a little lose but I was able to angle the long end of the allen wrench in there and tighten it! Phew!

    Jeffrey Gillespie - Reply

    Also had a problem with the sensor being loose, Erin Baker’s solution worked perfectly!

    Ben - Reply

    OK, my sensor was loose too even though I thought I tightened it down properly in step 9. I think it was adding the cable to it in this step that, if it’s not really secure, makes it apparent. Perhaps back in step 9 you could suggest plugging the cable in temporarily to ensure the sensor is locked down and won’t wiggle. Seems like this is a common issue.

    Rick - Reply

    • Starting mid of February 2018, there will be only one 50cm NYLON filament included.

    • There are two NYLON filaments included in the kit with lengths 50 and 30 cm. Both have Ø 3 mm. For this step please use the longer one and DON'T TRIM any of them!

    • Using the pliers cut one end of the filament to create a tip.

    • Check the tip is similar to the third picture.

    its is better to cut out with a cutter

    printminion - Reply

    We are using the tools, which are included in the kit, but if you have some more appropriate tool at your disposal, use it ;)

    Jakub Dolezal -

    The included pliers/cutter just squished the filament without breaking it. I had to use a different tool.

    Neal Tibrewala - Reply

    Hi Neal, it is possible to do it with included pliers (picture above is the proof), in case you have a better tool, please use it :)

    Jakub Dolezal -

    use nail clippers

    PaulHarris -

    You should write the line with “Starting mid of February 2018, there will be only one 50cm NYLON filament included.” on the top. I searched all my boxes twice because i searched for the second nylon filament mentioned in the first sentence :(

    Sebastian Moder - Reply

    Agreed. Just added 15 minutes to the build time and 100 swear words to my tab.

    Jeremy -

    I spent a good 15 minutes looking around for the “second” nylon filament. Absolutely agree

    Woger Dog -

    Agreed! I only have the one filament and I’ve been digging and searching for the second. Please update.

    Oh…you did update in bright red letters…. Thanks

    Larry Boehm -

    This step was updated :)

    Tomáš -

    Mine did not ship with ANY nylon fillament

    Eric Harten - Reply

    Same with me - no nylon filament in the package. Is it OK to just go ahead without it?

    Mark Chu-Carroll -

    Guys (@hartene, @markcc), please check the package once more, the nylon filament could be stuck at the bottom of the box. If you can’t find it, please contact our support at info@prusa3d.com

    Jakub Dolezal -

    It would be easier to understand this step if the purpose of the filament is explained, that its purpose is to support the cables and not to be used for printing. I used the printed guide, hard to understand this step, why insert filament between the cables? Very hard to se the hole, with the X-carriage back mounted according to the printed guide.

    Hans Palm - Reply

    Very agreed!

    Huebners -

    Agree, I am looking for this answer

    Hsiung -

    DO NOT TRIM! It's critical that you preserve the length of the Nylon strip.

    Make sure you make a nice, clean tip.

    For lack of a better analogy, it’s more of a circumcision, than anything else. Please consult a Mohel.

    Bleak Disclosure - Reply

    Better still than write the line with “Starting mid of February 2018, there will be only one 50cm NYLON filament included.” on the top

    rather amend to say There is a single NYLON filament included in the kit with Ø 3 mm and length of 50 cm. DON'T TRIM it!

    PaulHarris - Reply

    I must be lucky - I have a single piece about 50cm long of black filament - but mine measures 3.25mm diameter !!!

    No amount of pushing or twisting is going to get that through a hole which is 3mm diameter AFTER clearing out all the stringy bits!! Grrrrrrrr!

    Mark Baker - Reply

    So, the reason I consider myself lucky is because I have an old 1/8” tapered reamer Ha Ha, now it is a PERFECT FIT!

    Mark Baker - Reply

    I’m new. What’s the filament for?

    morris.ron1@gmail.com - Reply

    Just follow the manual and you will find out later on :)

    Tomáš -

    The 3mm Nylon filament is a stress relief, to even out the stress on the cable bundle during printing. Without it, the cable bundle would have uneven bending and would easily break after some printing.

    Brad Needham - Reply

    My 3 mm Nylon measures 3.44 mm in diameter. It would be helpful to add instructions to check the Nylon fit way back before you assemble the X-Axis. As things are, I have to do a delicate drilling job on the X-carriage with the extruder assembly almost completely assembled, or tear down the extruder assembly to work on the X-carriage.

    Brad Needham - Reply

    About how far in should the filament be inserted to be confident it will stay in? I can only seem to get it in about 2.5mm, but the hole appears to be about 10mm deep.

    Vince Thyng - Reply

    Grrr! Filiment is 3.12 and the hole is 2.8 and it just won’t go in evern after several tip trims af various angles. Anybody knw what the specx are supposed to me?

    Time to see what size drills I have I guess )-:

    Jerry Vaughn - Reply

    Hi Jerry! Cutting off a piece of the nylon to make one of its ends pointy (see the third picture) should do the job, no need to drill the hole.

    Martin Lexa -

    • Locate the hole for the NYLON filament. Using the smallest Allen key ensure there are no obstacles inside.

    • Using the pliers insert the NYLON filament in the slot. Hold the extruder with your second hand.

    • BE EXTREMELY CAREFUL as the pliers tend to slide and you can easily damage the wires!!!

    • To check if the filament is seated properly, gently pull it with your hand. The X-axis should bend a little, but the filament must remain in the slot.

    • If you have issues, try to adjust the tip on the filament.

    It would be helpful if we knew what kind of connection is happening inside … so we know how to adjust the tip.

    Rich - Reply

    Hi Rich, the connection type is mostly friction. The tip on the filament is used to enter the hole easily.

    Jakub Dolezal -

    Please explain why this nylon filament is only visible on step 30 ?

    I can’t see it anymore on steps 36 &37 pictures.

    Even on video build found on YouTube, this step seems missing.

    What the utility of this piece ?

    Stephan - Reply

    Hi Stephan, the nylon is not visible just because it is hidden under other cables from the Extruder, but it is still there. Nylon is much stiffer (rigid) compared to the other cables, it will hold the entire bundle up during prints and also retakes most of the load introduced by the moving Extruder.

    Jakub Dolezal -

    I’m unable to insert the 50cm NYLON filament into this slot.  I cut the NYLON at an angle as requested in the previous step, though when I try to push it through the slot, the pliers end up scratching the NYLON instead of being inserted.  I tried to cut the NYLON angle another time, with the same results.  Is the NYLON supposed to go all the way through and come out the left side, as the picture shows?

    Richard January - Reply

    I think the nylon just dead-ends into a friction lock inside the hole - the black cable/string on the left is the PINDA

    343 Guilty Spark -

    “insert the NYLON filament in the slot” - about how far? Like Richard above, I find it very difficult to insert, even with a perfect tip cut.

    Gerry P - Reply

    Hi Richard and Gerry (@rjanuary, @gerrypez),

    in case you can't insert the nylon filament in, please try decreasing the diameter of the nylon using a knife (make the tip slimmer). You should insert the filament few millimetres in, no exact value is given, but I will discuss this with the devs.

    Jakub Dolezal - Reply

    Could an approximate length of filament that will be pushed into the extruder assembly be documented? I was only able to get about 1cm to go in (with laborous pushing & twisting) and I don’t know if that is sufficient to support the wiring bundle over time.

    Jeremy - Reply

    For me, this was definitely the most frustrating part of the entire build. As I look at it, it seems to me that the insertion of the nylon is really just to have it be held in place while you get the rest of the cabling under control coming out the back of the extruder assembly. Once past Step 39 the zip ties will assist with keeping the nylon in place. So yes, only a couple millimetres of insertion is enough here.

    But, that is a tough couple of millimetres!

    My suggestion would be to adjust the hole to be a wider diameter with a hook ridge around the circumference 10mm into the hole. Then with a notch on the end of nylon it would simply slide in and lock into place, holding it there while the rest is done.

    SteveP - Reply

    I twist the nylon filament in by hand instead of push it in. Way easy and secure too.

    W Lai - Reply

    Twisting the nylon worked easily for me as well.

    James Shumpert -

    Oh wow! Simply pushing it in there was not working. Twisting in there with the pliers worked wonders. This part should be changed because screwing the nylon in there like a screw with the pliers is the only way to do this step.

    Brandon -

    God bless you the twisting worked so easily.

    Edward Chavez -


    Edward Chavez -

    Didn’t work for me. I tried twisting it in by hand and also with pliers.

    Eric Mathison -

    TWISTING IS GENIUS! And worked for me too.

    Zerg620 -

    Thumbs up!!! Reading these comments BEFORE executing the step really pays off. This tip should definitely be added above.

    Rick -

    This should be the top tip, if not just part of the instructions. Thanks!

    Daniel -

    Thank you for all of your feedback, we are working on updating our assembly manual and we will implement this in it :)


    Tomáš -

    Thanks for the tip. Twist it with the pliers while pushing it got it inserted easily enough to continue.

    Hope this gets better documented for next ones.

    Pierre Hilson -

    Small vice grips are your friend on this step. Tried it with the supplied tools but that was going nowhere.

    Trevor Friesen - Reply

    it was the same for me until I saw this the needle nose viseGrip 40mm long in the teeth an when closed 5mm at the tip at last got this thing in…. by far the hardest part of the build so far.

    Mustrum Ridcully -

    I cleaned out the hole for the filament by manually rotating a 3mm drill bit into it. That worked.

    Etienne van Ballegooijen - Reply

    make a L bracket screw into hole and then have nylon attach to that

    vincent bartlett - Reply

    I whittled the tube for a few mm in order to reduce the diameter, and then used vice grips to hold it. That worked pretty well.

    marhar@gmail.com - Reply

    How deep is the hole we are inserting the filament into?

    tim - Reply

    3 mm Nylon does not go into the hole. Just bends. Rotation didn’t work. Very frustrating. Was liking the build until the last hour. Working on assembling the wires and the 3 mm nylon fell out.

    Kurt M. Sanger - Reply

    Found the problem. 3.4 mm Nylon does not fit in a 2.4 mm hole. Skipped this step.

    Kurt M. Sanger - Reply

    The hole is a full mm smaller than the supplied nylon? Wtf?

    Christian B. Nielsen - Reply

    Hi Christian, this is why the tip is created before, you need the nylon to hold in the hole, so the diameter can't be the same. However, one 1 mm difference is too big. Are you able to place the nylon in?

    Jakub Dolezal -

    No, I can only get the tip in, an only about midway up the diagonal cut I made. I'm gonna have to hope the zipties are enough to keep everything in place.

    Christian B. Nielsen -

    With some random Dremel tools, I sharpened the nylon in a more conical way, to increase the grip/friction. It seems OK now, but I’m not going to pull it to test :)

    Christophe Lermytte - Reply

    I had a 3.25mm thick nylon piece and it did not fit in the hole. I found that using a 3.0mm drill to make the hole bigger worked okay.

    Olivia Williams - Reply

    The hole is too small (as stated above) to properly insert the nylon filament. For those that do not have metric drill bits, the 7/64” (2,78 mm) drill bit can open the hole sufficiently to force fit the filament. Use your fingers to twist the drill bit, just to make sure you don’t over do it.

    David Jerrell - Reply

    Even though I have metric drill bits, I was worried about drilling into the extruder assembly. Instead, I used a hobby knife to carefully thin the first 4-5mm of the nylon filament’s tip. Then I used a set of locking needle-nosed pliers to grip the nylon along the axis of the filament and twisted it into the hole like a screw. Because the pliers locked down, they didn’t slide on the nylon and I could switch my grip to get 2-3 revolutions.

    Federico Hatoum - Reply

    Another comment for this, you may want to state that the filament is for the wire bundle guide, to ensure that it clears/stays out of the way during operations.

    David Jerrell - Reply

    Amen. At first I thought this step was for calibration or gear testing and important. It’s kind of comical to learn that all this futility for inserting the nylon is just to hold some wires lol.

    Mike Krall -

    After trying all the above tricks with no luck, I put the filament in the freezer for about 5 minutes. I was then able to “screw” it in using pliers with not too much force. The cold filament could be a little brittle so be careful. .

    Daniel Smith - Reply

    David Jerrell’s suggestion worked well for me: I used a pin vise and a 7/64” drill bit to open up the hole, then a twisting motion with the nylon. It’s snug now.

    Craig Niederberger - Reply

    This step should be done during STEP 3.12 before attaching the (x-carriage: Assembly) so that the the part can sit flat to give more leverage without trying to hold the whole extruder assembly including to prevent any parts from breaking or bending. The filament would not get in the way of any other assembly from there on and provide maximum leverage to push the filament in.

    Kris Sovers - Reply

    +1 totally agree this should be done without all the valuable/fragile stuff attached. I used a smaller diameter T handle Allen wrench to open up the hole some but even then it was difficult to insert.

    Rich Rector -

    You are asked to insert the nylon here, because it easily falls off so you would need to re-check and re-insert it at this point anyway.

    Martin Lexa -

    I too used a 7/64” bit to clean the hole out. It was still tight enough that I had to screw the filament in, but it was finally possible to get it in there.

    Harold Toler - Reply

    Had to use the 7/64” bit *and* a file on the filament.

    Brenda Bell - Reply

    Doh! We inserted the nylon into the wrong hole. They nylon must go into the hole very close to the lower single linear bearing, beneath both levels of belt. Somehow we put it in the hole with a nut listening inside, above, which later is needed for screwing the back plate on (step 35-38), with the result that the back plate couldn’t possibly close up on the extruder (the nylon would have needed an impossible path).

    Jonathan Histed - Reply

    I also initially used the wrong hole. Perhaps adding a red circle with a slash (no as in do not enter or no left turn) over the wrong hole would help assemblers see that there are two holes and one is wrong..

    Paul Post - Reply

    The filament is curved, not straight.

    Any guidance on what orientation to insert it on?

    Andrew E. Mileski - Reply

    After slightly thinning the pointy bit, and very lightly cleaning-up the hole entrance with a hand-spun 2.9 mm drill bit, the 3.0 mm filament seated firmly using pliers. It went in with two very noticeable “steps".

    No way it is coming out now!

    Filament is bowed downward (there was no orientation guidance given).

    Andrew E. Mileski - Reply

    I received my kit on 6/20/2018. On my assembly, this filament went in just like the pictures show. As they explained, they use the pliers (included with the kit) to cut the end and install the filament into the hole. What I did was to measure the depth of the hole and transfer that to the filament, marking it with the edge of blue painters tape. Using the pliers, I easily slipped the filament into the hole until it was completely inserted up to the edge of the blue tape. A quick pull on the filament assured me that the filament was secure.

    George Miller - Reply

    I cut the notch into the filament then using the pliers blunt section clamped down on the notch, this seems to centre it to more of a point centrally. After this I used the pliers to twist it in and had it tight in place within a minute. Hope that helps anyone

    Nicholas Klein - Reply

    I found a tapered screw in my extra screw bin that had a shank slightly smaller than the hole and threads slightly larger than the hole. I screwed that into the hole, then removed it, and twisted the nylon into the hole. It worked fine and holds on tight.

    TK_bit_twiddler - Reply

    This was one of the most frustrating parts of this entire process. There’s just no way to make this work unless you own extra tools / drill bits / etc..

    Nicholas Nothom - Reply

    I tried freezing, trimming and twisting to no avail. Finally just drilled a little bit to make hole slightly bigger and nylon goes and stays in, didnt even have to cut a point. So once again poor QA as many of us have seen through these steps, just make the hole slightly bigger.

    mjb - Reply

    impossible to push the nylon cable through… my nylon cable makes 3.3mm of diam.

    jorgemarmo - Reply

    This is by far the most frustrating part of the build so far. I've spent over half an hour trying to get the filament in. I ended up having to trim the diameter of the filament with a knife, sand it down with sandpaper, and finally use a vise grip to jam it in. Terribly designed.

    hunnaabraham@gmail.com - Reply

    Not even close to fitting on my kit received a few days ago.

    Chris Combs - Reply

    Hi there Chris,

    Can you please share with me a photo of your current E-axis assembly? I will check it out.


    Tomáš -

    On the MK2S kit, the instructions specify that the filament (which, as others have mentioned, has a pronounced curve to it) is meant to be oriented into the slot so that the filament’s natural bend, moves upward, so that it opposes gravity as it supports all of the other cabling (i.e the c shape of the filament should be oriented so that the opposite end, the end not going in the hole, curves over the top of the printer). I repeated this same scheme while assembling the MK3 but I believe that this needs to be clarified for new builders (if, indeed, it is important here).

    MagicDM - Reply

    I’ll also add, for builders struggling to get the filament to fit, that the key really is to cut a long, sloping point at the insertion end (you really should use clippers for this if you have them, rather than the supplied pliers - I believe that the pliers are not really the ideal tool for the job - understanding, of course, that the Prusa team probably challenged themselves to find a way to do the entire assembly with only the supplied tools). If you have clippers, you want to angle as long of a diagonal cut (about 30 - 35 degrees) across the width of the filament as you can (this technique will also come in handy, later, when you insert filament into the extruder for actual printing).

    MagicDM - Reply

    I am no engineer but a lot of frustration and time spent on such a small part of the job of building this machine. seems to me that a little redesign of the part would be in order. Just made a thread inside the hole. Make the hole slightly bigger to accommodate the shrinkage of the the part with the nylon and then when you screw in the nylon t would bit in the threads and that would be that.

    It’s not worth the i’ll of all the builders when we are having to spend so much effort on something so simple to the build. Agree

    Jerry Albertson - Reply

    Inserting the nylon filament at this step is a mistake. It should have been inserted during X-carriage pre-assembly. I have no leverage, and the slightest mistake can cause significant damage. Please update the instructions so that the filament is inserted before the carriage is attached to the rods.

    Timur Tabi - Reply

    I got it to go in about 3mm or so beyond the tapered part (I think, I put tape on it as a reference). If it’s important that this stay in place through vibration and carriage movement, then this is a terrible design concept.

    Scott - Reply

    No worries, once you install all the cables around and the other plastic parts, the nylon holds pretty well.

    Also, as it is true that this concept has proven itself as problematic, we are now using the the textile wrap for both cable bundles, so you don’t need any nylon anymore.

    Martin Lexa -

    • For the following steps, please prepare:

    • X-carriage-back (1x)

    • Cable-holder (1x)

    • M3n nut (1x)

    • M3x10 screw (1x)

    • The M3x10 screw will be used temporarily, use one from the spare bag.

    • M3x40 screw (1x)

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