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  1. 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

    I suggest 3 - 3.5 hours. It took me just under three but I had completed some of the belt work with the mk3 guide earlier.

    Cory - Reply

    Hi Cory, thanks for the suggestion. I will wait for some more and then make an average :)

    Jakub Dolezal -

    3 hours 15 minutes for me.

    John Merling - Reply

    It took me 5.5 hours, but this is my first 3D Printer. I went slow, to make sure I got everything right.

    I’m glad extra hardware was included. The screw bag had a hole in it. I lost one of the nuts (I was able to find the screws).

    Ken biles - Reply

    I needed a bit over 2h, but i have to mention i have a background in Mechatronics so may be 3-4h are a good suggestion.

    Patrick Kirsch - Reply

    1hr 45min according to my video timestamps. I have assembled a MK3 with the old extruder design before, however this was a MK3S.

    Tyler - Reply

    Took me 4h05m but I was only able to work in 40 minutes/day, so some of that time was spent getting the project out and putting it away each session. I went slowly and took my time, checking my work carefully.

    Dan Green - Reply

    It took me 02:28:31.92. Of course, the proof’s in the pudding, as they say. I haven’t done any calibrating or anything yet. :)

    Greg Sanders - Reply

    Took about 3 hours

    Darin - Reply

    Pretty sure about 4 hours.. pay attention to the square nut insertion, as much as I thought I was paying attention, I missed putting one in completely, which ended up in a disorganized dissasembly/assembly process. Hopefully did not miss anything.

    Recommend taking another look with each nut insertion and doublecheck yourself.


    Kelly L Gann - Reply

    Yikes…. 3-4 hours and a five out of five difficulty…. I think I will go to the gym and walk my dog first… mental preparation.

    Alan Crooks - Reply

    I was a bit stumped at Step1 but after some patience I managed to get through it.

    Just under 3 and a half hours for me. I couldn’t get the rubber band back onto the pliers for ages and the specific order of the allen keys threw me initially too.

    Len Williams - Reply

    Some of us need a new section that show the steps for assembling MK3S and MMU2S at the same time. Or at least notes that tell us when to look at the MMU2S instructions (and which step there). Following these instructions leads to needing to disassemble the X-Axis when doing the MMU2S assembly

    Al Nettleton - Reply

    Hello Mr Nettleton, yes and that is correct. You need to make sure the extruder is assembled correctly and you need to know precisely how it is made. Besides, we advise to use the MK3S without the MMU first so that you could make sure it is working correctly and gain some experience with it.

    Martin L. - Official Prusa CS -

    I have already built MK2 and upgraded twice so it is now MK2S.

    It turns out that 3A. MK3/MK2.5 - extruder upgrade (MMU2) at 3A. MK3/MK2.5 - extruder upgrade provide the instructions, although I learned that after installing the magnets.

    I’ll report back after completing my assembly.

    Al Nettleton - Reply

    When assembling MK3/s and MMU2 at the same time, I was able to follow 3A. MK3/MK2.5 - extruder upgrade instructions for building the extruder and filament chimney. After that section, I returned to step 6 in these instructions and completed the basic printer build and performed the “Live adjust Z: steps.

    The next step was to test a print. Here I found that it was necessary to turn off both auto feed and auto filament detection to successfully print.

    All is OK and now the rest of the MMU can be assembled.

    So, I do not think it is necessary to build the extruder without the MMU chimney first and then partially disassemble it to install the MMU.

    Yes, it is absolutely recommended that calibration be completed before assembling the MMU add-on.

    Al Nettleton - Reply

    3.25hrs for this section. My first full kit 3D printer build. I could see it taking 4+ hrs easy if you don’t tinker a lot… or 1.5hrs if you’ve built few of these. Probably should put 4hrs on it.

    Chris - Reply

    Took me about 4 hours, no breaks. Almost as hard as getting the frame square, back in chapter 2; but much more detailed. So I agree with 5 out of 5 for difficulty.

    Gerard Sharp - Reply

    2.5 hrs, this was the easiest part of the build.

    Charles Fico - Reply

    Hello, thank you for the feedback!

    Martin L. - Official Prusa CS -

  2. This is the most important and hardest chapter, take your time, don't rush. Properly assembled extruder is essential. The bag with fasteners includes M3x20 and M3x18 screws make sure you won't mix them! Pay attention to the instructions, when to use the M3x20.
    • This is the most important and hardest chapter, take your time, don't rush. Properly assembled extruder is essential.

    • The bag with fasteners includes M3x20 and M3x18 screws make sure you won't mix them! Pay attention to the instructions, when to use the M3x20.

    • Keep the magnets apart in a sufficient distance. They can break each other!

    • The hotend for MK3S needs a shorter PTFE tube compared to the MK3 (more info here). The PTFE tube is already installed, MK3S compatible hotend is marked with an yellow/orange dot on the heater block.

    • Pay great attention to the cable management, if you miss some important step you would need to disassemble the extruder.

    • This bag includes extra fasteners. Don't worry if you finish with few unused screws and nuts.

    To share my concern. I’m used to standard when it comes to fastener sizes. M3x20 is the machine screw that is slightly longer then m3x18 by about 1/16 of an inch. So the emphasis of separation is important. I hope that helps someone else that this gave pause to. - Reply

    What size is the steel ball? - Reply

    It is 7 mm, same as the one used in the MMU2

    Martin L. - Official Prusa CS -

    Keep a close eye on parts before hand. My kit had 4 M3x18 screws and only 1 M3x20 screw (luckily there’s spares!).

    Logan Kelly - Reply

    The “PRUSA CHEATSHEET” v1.00 paper reference does not have the M3x20 size screws shown (included with the MK3s kit). It would be handy to add it.

    adrian.w7 - Reply

    Yes, sorry for that. Thanks for reporting this!

    Martin L. - Official Prusa CS -

    If the magnets can damage each other, why are they shipped in the same bag?

    Dominik - Reply

    Hey Dominik,

    They are usually already attached to each other when packed inside the bag.

    Filip M - Official Prusa CS -

    The damage might occur when the magnets attract and slam into each other. No risk of that when they are already touching, but once apart it’s best to keep them out of each other’s “range”.

    Wout -

    I finished this section with 2x M3x10s, 2x M3x18 and one 1x M3n left over.

    Keith Illg - Reply

    Thanks Keith!

    I was just about to go through each step and count what was used..

    I have exactly the same:

    2x M3x10s, 2x M3x18 and one 1x M3n left over.

    ### Prusa.. please take note:

    I inventory the hardware bag so I know if I have the correct amount for every step.

    What I don’t know (without a lot of extra work) is what parts are supposed to be left over at the end.

    Having a spare parts bag was a hugely excellent idea but this instance of having a “few unused screws and nuts” .. No..

    Ken.Lotts - Reply

    Hello Ken,

    note taken. It’s simply that we prefer to include some more screws rather than risking that some screw will be missing.

    Martin L. - Official Prusa CS -

    The hotend for MK3S needs shorter PTFE tube.” Could this be worded more clearly? I see a PTFE tube sticking out of the hot end. Is it too long and will need shortening later?? Or is this informational only, saying the MK3S tube is shorter than the MK3 tube?

    john brenner - Reply


    The shorter PTFE tube is included in the Hotend in the MK3S kit. The PTFE tube must be changed if you have old MK3 Hotend and doing the upgrade on the MK3S. It is info for customers.

    Martini H. - Official Prusa CS -

    funny how important it is to not mix 18 and 20’s.. and then prusa mixes them up themselves.. good thing there is a bag with spares :)

    Johnnie - Reply

    100% positive I received three M3x20’s instead of the two needed for this step, and only two M3x18’s of the three needed for this step. Inventoried all of the parts needed for this step, measured them carefully.

    Picture of the screws with one M3x40 on the far left, and one m3x10 on the far right for reference:

    Picture of the inventory sticker on the bag:

    What should I do?

    Kinsky - Reply

    Just noticed the bag of spares, thanks for including that!

    Kinsky -

    My kit also came with two M3x18 and three M3x20, which is opposite of the quantities shown on the bag of parts.

    Jeff Keyzer - Reply

    Hi Jeff, in this case you can always take some screws from the Spare bag.

    Filip M - Official Prusa CS -

    Same here. Two M3x18 and three M3x20, opposite of the bag's inventory. Will use spare.

    David Stosik -

    It looked like I had a few extra parts left over at the end of this chapter. Around five or six. I returned to the end of the chapter to step 66 to find I hadn’t eaten all my allotted bears. Be more careful with this chapter than you have been on the others and learn from others mistakes. It will save you a lot of checking, rechecking and rereading in the long run! Read all the comments. ;)

    Michael Nolte - Reply

    What do I do if my heater block doesn’t have the yellow/orange dot on it?

    Nathan S - Reply

    Hi Nathan,

    If yours didn’t come marked with a dot, you can determine if it’s an MK3S hotend by taking the PTFE tube out and measuring it. If its ¬44.2mm, you have the correct one.

    Marcin M. - Official Prusa CS -


    based on experience in taking my 2 week old 3S apart to add the MMU and having 2 problems coming from the stainless bolts seizing on the nuts. I should have known better having built loads of things with S/S screws before for the ham radio gear and always been told to use it. I now cannot get the hot end out as the seized bolt means that the captive nut is no so captive any more. So I have to print another part and basically destroy the existing one to try to get to the nut.

    A drop of anti seize grease would have solved that.

    trevor clapp - Reply

  3. For the following steps, please prepare:
    • For the following steps, please prepare:

    • Extruder-body (1x)

    • Adapter-printer (1x)

    • FS-lever (1x)

    • The list continues in the next step.

    Adapter printer and fs lever are not in a bag for 5. E-axis. In case your kids are fetching parts bags for you.

    Aaron P - Reply

    Glad I am not the only one who does this :-). For others like us, its in the “MK3S/ MK2.5S Extruder” bag.

    Sean -

    Thx. I searched like crazy! For that price.. this help me a lot ;)

    mullcom -

    To expand: They are in the bag called “MK3S/ MK2.5S Extruder”

    Sam Hudson - Reply

    Thank you :)

    Morris -

    Legend! - thank you :)

    Ismael Abufon -

    Thanks! i was looking around a bit

    david -

    OMG! Thank you! :)

    Dániel Szászi -

    Many thanks bud

    Julian -

    Just to point out that the CheatSheet have M3 x 20 Bolt drawing 20mm from the top of the head to the end of the screw. That should not be. Shoulder screw are measure from under the head to the end of the screw to my knowledge. Might be the print i have V1.02 and its scale . I did separated the 20mm with seeing the longest one and confirm on the sheet but doesn’t look right so grab the galiper ;o) a few spot check on the sheet don’t match might not be 1:1.. Maybe use a ruler in doute… Hope it help Happy Building! - Reply


    We’ve noticed an error with the scale and will fix the cheat sheet shortly.

    Filip M - Official Prusa CS -

    Extruder-body (1x) in in a different bag than Adapter-printer (1x) and FS-lever (1x). All 3 parts should be put in the same bag. Luckily I reread the comments before I stopped and asked CS for replacement parts….

    Dave Bear - Reply

    Dear Dave, these small parts are sorted separately because they are small, and also because they are not included in the MMU2S upgrade (they are not needed there).

    Martin L. - Official Prusa CS -

    Agreed with previous comments, please mention which parts are in which bags. I would have assumed that because we are working in the E-axis section, the necessary parts would all be in the E-axis bag.

    Lessian - Reply

    You guys took time to label the bags, and produce a nice list of whats in the bag, also visual cheat sheets for bolts and nuts and other parts with them. yet Throughout the build so far you have yet to list the proper bags needed for each section. I would recommend that at the beginning of each chapter with the tools needed list the proper bags of parts that are required for that section. This makes sure the builder has the proper parts bags out to complete the section. and alleviates the scampering and frantic looking around on the floor and under the printer and any where else close to where people are building their printer looking for lost parts that you all packed in a different bag. Luckily for me I read through each section before I started building and read all comments along the way, making my build less frustrating because you guys stuffed a few parts in a separate bag and failed to mention it.

    Michael Thompson - Reply

    Hello Michael,

    The problem with this is that the bags and their contents tend to change and we don’t have the capacity to change these small details every time in all manuals. But you’ll always have the preparational steps such as this one that will tell you which parts you’ll need to have at hand.

    Martin L. - Official Prusa CS -

    It may be helpful to double check the shape of the FS-lever. I had to trim some excess plastic from the light-sensing edge to make this work correctly. I didn’t find this until after fully assembling my printer, but I’m hoping this will help others avoid this issue (because disassembling back to this point is painful.)

    The IR blocking lever was ever so slightly too large when it was printed. Now that I’m more familiar with 3d printed parts, the elephant’s foot is easy to see, but as this was my first printer, I had no idea at the time. The extra material kept filament detection from working correctly because it blocked the light, therefore the printer thought that filament was always present (confirmed via built-in diagnostics menu) because the light was blocked. This also means that the auto filament loading didn’t work. (To load filament, I had to disable the sensor via: LCD Menu - Settings - Fil. sensor [on/ off].)

    Example picture:

    Lowell Carl Alleman - Reply

    My FS-Leveler has an ovalish cutout all the way through in the main body, unlike the picture.

    Ronald Smith - Reply

    Hello there,

    Do you mean that the hole for the screw in the FS-lever is not round but oval?

    Martin L. - Official Prusa CS -

    Oh sorry, I get it now. You were talking about the socket for the magnet. It is shown in Step 5.

    Martin L. - Official Prusa CS -

    Hmmm…Seems I have 3 @ M3x20’s and no M3x18’s in the bag for this step.

    Steven Smith - Reply

  4. For the following steps, please prepare:
    • For the following steps, please prepare:

    • M3x18 screw (1x)

    • M3x10 screw (1x)

    • M3nS nut (2x)

    • M3n nut (2x)

    • Steel ball (1x)

    • Magnet 10x6x2 (1x)

    • Magnet 20x6x2 (1x)

    Keep magnets together util you are gonna use them. They’re attracted to anything ferrous.

    Miguel Barroso - Reply

    One important add on to Miguel: LEAVE THE MAGNETS ALONE until you’re going to use them. I separated mine then set them down a few inches apart and they slammed together snapping the long one in half. Thank the almighty Josef there are spare parts!

    Anthony M. - Reply

    What is the diameter of the steel ball? (I lost it, but can probably find another)

    Peter Longfield - Reply

    Hi Peter, it’s 7mm.

    Filip M - Official Prusa CS -

    I found the magnets stored in the SD card case. Great way to protect them from the rest of the stuff in the bag.

    Daniel Friesen - Reply

  5. Take two M3nS nuts and insert them in the Extruder-body. Make sure both are all the way in. Push them in, the upper nut must "disappear" inside the printed part. Ensure correct alignment of the nuts using the Allen key.
    • Take two M3nS nuts and insert them in the Extruder-body. Make sure both are all the way in. Push them in, the upper nut must "disappear" inside the printed part.

    • Ensure correct alignment of the nuts using the Allen key.

    • Secure the nut using an M3x10 screw. Tighten the screw just slightly, later on we need to add the P.I.N.D.A. sensor.

    • Take two M3n nuts and insert them in.

    • Use the screw pulling technique.

    • Take the smaller magnet (10x6x2) and insert it carefully in the FS-lever. Majority of the magnet will be hidden inside the printed part.

    It may help to have a sharp knife to hand for this step. I had to cut some excess material away from the PINDA clamp.

    Jonny Badley - Reply

    +1 to this. I was unsure if that was supposed to be fused or not. The M3x10 also seemed a little short to reach the threads on the nut

    agregorie -

    Use the screw pulling technique.

    What’s that?

    David Grant - Reply

    Hi David, it’s exactly as Virgil says below. here’s the link: 1. Introduction

    Martin L. - Official Prusa CS -

    @David Grant

    Step 8 of the Introduction.

    Virgil McLaren - Reply

    Be careful with the hexagon nuts on this part… they fit with a lot of interference within the plastic part.

    If you use the Josef Prusa, method of assembly nut you may end with the bolt and nut screwed with the hole of the nut deformed to a point that the nut will turn when turning the bolt.

    If that happens try to force the nut not to move with the plier, if you can’t grab it (like me) insert a thin pointed metallic object between the plastic parte and the nut, just to try to fixit after securing the nut try to unscrew the bolt. if you are successful, glue the bolt on the hole. By the time you reach the point you have to use that bolt with a screw the nut may be already glued to the plastic part.

    Jorge - Reply

    Had the same problem here. A friend is reprinting the piece for me because it was beyond repair… Should have read the online manual instead of the handbook to read the comments.

    My advice: avoid the screw pulling method for this part!

    Ruben Rybnik -

    Heads up, several people needed to undo part of their work once they got to Step 42, because the picture here in Step 5 makes it look like the square nut on the left should be flush (not pushed all the way in). “Ensure correct alignment using the Allen key” is important here. Read the comments in Step 42 for details!

    Dan Green - Reply

    Thank you for saving people a load of time Dan!

    Charlie Miller -

    Thank you Dan.

    Elliott Bridge -

    How I wish I had seen this comment in time! I was going purely by the printed manual, not this online one. Sure enough, when I got to step 42, I found the nut wasn’t aligned with the hole, and I had to work my way backward to this step.

    The manual should emphasize this more than “Ensure correct alignment using the Allen key.” It should say in boldface text “The nut at the top of the picture should be deeply recessed!”

    A. Matulich -

    Be sure to put in the top middle M3n nut fully seated (it will sits in much further than picture show) to avoid having to address it later as many have noted in step 42. It will save time and the hassle of being back here to push that M3n nut fully (which is a bit misleading based only on the picture).

    adrian.w7 - Reply

    be careful with the part where the pinda sensor goes in, when i was trying to align the square nut with the allan key, it snapped the plastic part. i have hopefully fixed this with plastic glue, how long this will hold up remains to be seen.

    Shane Hobson - Reply

    Is there anything that will put strain on that part during printing etc? It’s not something I can clamp to make sure the glue sticks awesomely, if the worst strain it sees is screwing in that screw and putting the sensor in I think it will be ok/

    Shane Hobson - Reply

    Hi Shane,

    This part is not strained during printing. You can fix it with glue. However, be careful when installing the PINDA probe. The probe should be left freely - do not fix it firmly, as the probe height may need to be changed, just tight the screw as much as necessary. to fix the probe possition

    Martini H. - Official Prusa CS -

    Yeh made sure to only get glue in one place and not on any moving part, as it turns out the left fan doesn’t spin yet so may have to redo the extruder and then the nut for the fs cover will go in and i wont need to glue it lol.

    Shane Hobson -

    See the two M3n nuts which are not square nuts? When I tried to service the E axis assembly later, one of them started rotating together with the screw and I couldn’t take the screw out anymore. I had to tear down the entire assembly and had to break the piece in the end. This can be avoided with square nuts.

    Mihai-Andrei Stanimir - Reply

    Really make sure to get all nuts in at this stage, triple check if you must, because later there is a step when screwing the fs cover on, if that nut isn’t there you’ll have to go back 40 steps not joking lol.

    Shane Hobson - Reply

    my Nut, Bolt', and Extruder Body are fused together for all of eternity, and since this is my first 3d printer I can’t just print a new one.. Can somebody please print me a new one?

    Andrew Hessin - Reply

    We managed to grind off the head of the bolt with a power sander and push out the screw and nut with only minimal cracking of the part, so with fingers crossed we’ll be trying to print a new extruder body with a cracked and slightly melted one.

    dear God please let this work.

    Andrew Hessin -

    Make sure you used the M3-10 for the purple step! I accidentally used an M14 and was looking all over for it in a later step!

    Keith Illg - Reply

    I had a problem inserting one of the M3nS. I did not want to break the body by pushing it too hard on the table.

    Took another M3nS and it worked like a charm. As Josef mentioned: tolerances!

    Hans G. Schnieder - Reply

    When inserting the M3nS nut (between the two holes specifically) push it all the WAY IN! As others have said, this is crucial, I got to step 42 and had to read back to here to realize my mistake. Fortunately Laurel W - March 11 comment on Step 42 shows good steps to correct for this.

    Posting this to try to highlight this to Prusa to update this step with a bold warning.

    Reinhardt - Reply

    Well, it is not in bold letters, but the step does say: “Make sure both are all the way in.” and “Ensure correct alignment using the Allen key.” there’s even Josefs’ avatar, is that not enough?

    Martin L. - Official Prusa CS -

    A groove plier made getting the M3n nuts into the extruder body EXTREMELY easy, with almost no stress put on the plastic body. Found one of the nuts impossible to get into place without it.

    Kinsky - Reply

    Hello Kinsky,

    Sometimes there might be some extra printed material in the holes, so we recommend cleaning those with a hobby knife for example. However, pushing it in with groove pliers or flat end screwdriver is also a good solution :)

    Marcin M. - Official Prusa CS -

    NOTE: The upper square nut needs to be FULLY pushed into the slot (not flush as in the picture). Otherwise you will find in step 42(!!) that you need to come back to this step. (PLEASE emphasize this in this step’s instructions!)

    Preston L. Bannister - Reply

    Hi Preston, it’s emphasized in the first part of the description.

    Filip M - Official Prusa CS -

    Hi Preston,

    Thank you for the feedback, we will look into adding that information and possibly changing the picture.

    Marcin M. - Official Prusa CS -

    Exactly! I’ve made this mistake :)

    Alex I. -


    Take two M3nS nuts and insert them in the Extruder-body. Make sure both are all the way in.


    High or you will be an unhappy camper


    THE M3nS nuts require some gentle force to insert, but you won’t be able to push them all the way in just using your fingers. Once it’s seated, I use one of the smaller allen wrenches like a nail and gently start to tap the top with the head of the needle nose plyers. The nut will easily wedge all the way in.


    The nut on the left that doesn’t have the M3x10 screw (as photographed) needs to be seated at least a 1/4” so I feel the above tip works better than using the allen key for alignment. I use the allen key as a nail and it won’t damage either the tool or the plastic part.

    Ron - Reply

    Had a very hard time putting in the hex nut m3n, the screw pulling technique didn’t seem to help it was way to difficult, what I fount was get the nut in with screw pull technique once its seated in and does move so much use the pliers to apply pressure from one side of the nut to the other side to push it in flip the printed part down so the nut is facing you. My pliers had a blunt end and not a sharp end so it did not go in the nut hole. And don’t apply to much pressure so the part doesn’t brake but just enough to get one corner in a little bit down then another corner until the whole thing is down, also one side of the printed part has more plastic then the other side so use that to your advantage when you flip it over apply pressure so it goes towers the side with more plastic on it and not the thinner side, then start off by gently applying pressure and gradually add more so you know how much is needed .

    mat - Reply

    pushing the 4 nuts in took me over 2 hours but i didn’t crack anything, stressed and sweating!

    Mathew Boon - Reply

    The M3n nuts (green arrow) i have are not going completely in. Will that cause a problem later on?

    Douglas - Reply

    Hi Douglas, both nuts should be at least flush with the flat part. Try using the screw pulling technique.

    Filip M - Official Prusa CS -

    Pushing the magnet into the FS-lever, use some parallel jaw pliers! I used a set of Knipex 86 03 180 and they works PERFECTLY!

    ben_r_ - Reply

    PUSH THE NORTH EASTERN M3nS NUT ALL THE WAY IN! It needs to disappear from view, or else it causes problems at step 42 and you have precariously disassemble the whole unit to fix it… like had to. :(

    Sean McInnes - Reply

    I was following the printed book manual, not this online one. This online manual corrects a deficiency by stating “the upper nut must ‘disappear’ inside the printed part.” The printed manual doesn’t say this. Needless to say, when I got to step 42, I had to disassemble it all back to step 5 to push the nut in all the way.

    Please correct the printed manual ASAP for future customers! And if you have a backlog of manuals without this correction, insert an errata sheet before shipping it! Not everyone has a computer handy in the location where we assemble the printer. We rely on the printed manual.

    A. Matulich - Reply

    Hi Matulich,

    we reprinting the manuals very often to update its content based on the online version. I'm sorry yours was missing this update.

    Jakub Dolezal -

    wow a lot of people had to back track because they didn’t push the nut all the way down and come here to complain that the instructions are not clear on this.

    Come on guys take some responsibility for yourselves here. I know your frustrated maybe even angry that you had to back up, but did you not look in the screw hole leading to that nut to verify that it was far enough in the hole to align? I’m not sure how much clearer they can make the guide if you fail to simply check that the nut aligns with the bolt hole.

    I don’t say this to !#^& people off, but just point out that slowing down and thinking it through. the guide stated to verify you pressed the nut in far enough. Use a flashlight to look in the hole to see if you see the silver threads of the nut. Furthermore you could take a long bolt from the extras bag and insert it to verify that the nut is lined up and threads into the nut. it’s that simple. Not verifying you have the nut lined up with the bolt hole is not the fault of the Prusa guys.

    Michael Thompson - Reply

    Thank you, Micheal! Slowing down and thinking stuff through is exactly what we recommend, though we understand that people usually can’t wait to have the printer built and operational.

    Martin L. - Official Prusa CS -

    “Take the smaller magnet (10x6x2) and insert it carefully in the FS-lever. Majority of the magnet will be hidden inside the printed part. “

    Dose polar alignment matter? Is the part supposed to push or pull? sort of important information that is not stated here. I would hate to have to push the magnet back out it’s quite a tight fit to get in.

    Michael Thompson - Reply

    Found the information in the next step. This info might be better suited or at least referred to in this step instead of the next step.

    Michael Thompson -

    Hello Michael, that’s true. unfortunately, this step is quite loaded with information already so we had to split it this way.

    Martin L. - Official Prusa CS -

    I love magnets. The spares go into my magnet drawer. Everyone has a magnet drawer, right? Otherwise, where do you keep all your magnets?

    Ronald Smith - Reply

    I did the stupidest thing ever. I put the small magnet where the square but should go. I stupidly thought: these magnets are going to fly at something and break so I better get them in right away. I doubt there is any way to get it out.

    David Grant - Reply

    Argh, that will be hard. Maybe drill a 2mm hole to the bottom of the magnet and push it out with a small rod?

    Beardape -

    If it’s the top one, it is not that bad, you can use the printer without the filament sensor and print that part again. How did you manage to cram the magnet in? :D

    Martin L. - Official Prusa CS -

    I had a bit of trouble getting the M3nS into the “upper left” (on the picture) slot of the Extruder-body (the one that needs to be pushed in all the way for step 42). What worked for me was: (1) Use the pliers to hold the nut and push it into the slot (2) Flip the extruder-body over and push it onto a table to push the nut further in (3) Put another nut just onto the end of a M3 screw so that it can be held against the nut in the slot and push the nut in a little more (4) Hold the extra nut with the pliers now and push the nut all the way in. Depending on how tight the tolerances on the nut slot are, you may not need all these steps.

    Ian Dunn - Reply

    Small improvement: The color coding in the previous step (step 4) is pink : M3x18 screw, dark blue : M3x10 screw. In this step the M3x10 screw is pink, which got me confused for some time. It would be easyer if the M3x10 screw has the same color in both steps.

    Beardape - Reply

    Hi Beardape.

    Thank you. If possible, we will unify the marking.

    Vojta Z. -

  6. Insert the FS-lever in the body. Secure the part with a M3x18. Tighten it, but ensure the lever can move freely.
    • Insert the FS-lever in the body.

    • Secure the part with a M3x18. Tighten it, but ensure the lever can move freely.

    • WARNING: make sure the following procedure is done right, otherwise the filament sensor won't work!!!

    • Insert the bigger magnet (20x6x2) in the Extruder-body, it will stick out:

    • Correct setup: magnets are repelling each other, thus the lever is pushed to the right.

    • Incorrect setup: magnets are attracting each other, thus the lever is pulled to the left.

    Careful! Since the magnets are arranged to repel each other and the smaller magnet’s groove on my unit was loose, the smaller magnet literally flew across the room when I let go of it. I had to add the smaller magnet only just before step 14 and use a piece of filament to hold it down until the cover was attached to keep everything in place.

    Ante Pettersson - Reply

    How did you use a piece of filament to hold it down?

    David Grant -

    What I did to fix it was cut a little piece of plastic, from the big packing material that was in the box, and squeeze it into the hole with the magnet, which made it fit snugly, and I then trimmed edges carefully with a hobby knife.

    David Östman -

    Small magnet flew out hit large magnet and snapped it in two

    Peter Green - Reply

    Hi Peter, sorry to hear that. It’s a neodymium magnet, the dimensions are 2x6x20 mm. You should be able to find suitable replacement in your local hardware store.

    Martin L. - Official Prusa CS -

    Yep, happened to me, too. (But there are spare magnets in the upgrade set).

    Prusa should improve the design of the FS-lever: If the magnet is inserted from the other side it would be fixed together with the FS-lever and there is no risk that it jumped out.

    (Note to myself: first read the comments!)

    Ulrich Homann -

    I had the issue of the magnet flying out but was fortunate that it didn’t break. I stuck a piece of masking tape on one side of the smaller magnet to add a bit of friction which held it in the socket.

    Adam Reid - Reply

    Used super glue to fix the small magnet.

    Marc Pflugmann - Reply

    Tip: Rotate the larger magnet on the outside of the assemble near the slot where it goes until it repels the smaller magnet in order to find the correct orientation before inserting it into it’s slot.

    larry - Reply

    Would you recommend actually gluing the small magnet inside the part so it doesn’t fly out?

    Fabien Duchene - Reply

    Hey Fabien, please follow the manual closely - I suggest you don’t do any additional steps that aren’t mentioned in it unless you’re absolutely sure about it.

    Filip M - Official Prusa CS -

    I also was unable to complete this step as described because the small magnet insisted on flying out of its slot and attaching itself to the larger magnet.

    Eric Jaeger - Reply

    I kept the big magnet in the sd card case it came in, and set the smaller one to the side. When it came time to insert he big magnet, I made sure I knew which side was the repelling side (did it through the plastic case). That way when I inserted it, there was no attraction, and no risk of the small magnet flying out and sticking.

    Nathan Charlton - Reply

    I wrapped the magnets in one layer scotch tape to prevent them flying out of there slots….

    Paul Saba - Reply

    I had the Flying Magnet problem and cut the bottom corner of one of the polythene parts bags out to make a little cover for the smaller magnet, which kept everything in place. Otherwise I could not get it to stay in there. Big Magnet, although loose, doesn’t want to somersault.

    Richard Simpson - Reply

    Put your thumb over the smaller magnet while inserting the bigger one.

    Corinne McConkle - Reply

    The FS lever in the photo appears to sit nicely and lie flush with the surrounding surface. Mine sits 1.5mm proud. The surfaces seem clean so it looks to me as if it is intended to be like this but it doesn’t match the manual photo. Is this important ?

    Bernard McIntosh - Reply

    Hello Bernard, it’s just the angle of the photo. The lever will not be flush with the surrounding surface and that’s ok.

    Martin L. - Official Prusa CS -

    Got to say….. the idea of using the magnets is pretty clever.

    As long as your small magnet is in tight, you shouldn’t have an issue with the magnet flying out.

    Alan Crooks - Reply

    Came here to say that. I’m learning so much about making my own stuff by seeing the way you guys went about designing the printer :D

    Miquel Martin -

    Just wanted to point out on this step when screwing the lever into place, it may be a good practice to gently BOTTOM OUT THE SCREW TO THE POINT THAT THE LEVER WILL NOT MOVE FREELY, THEN BACK OFF the screw until it does.

    It wasn’t until after everything was assembled and I was testing the filament run out capability that I realized there was a bit too much play in the lever and it was hanging up on the IR sensor. Autoload of filament was working and sensor info was showing the sensor was good (pushing filament in and out changed the value), however, during a test print I cut the filament and it kept moving right along laying down nothing.

    I was initially trying to be careful to not over tighten it as instructed, and the screw itself was tight, but the lever moved side to side about .2 mm. I just assumed there was more clearance between the grooves in the sensor than there were.

    Anyway, hope this saves some headache!

    John Maranto - Reply

    Thanks for this. I adjusted the screw as you stated before adding the large magnet. then once I added the large magnet there seemed to be too much wobble in the small arm. So I held the arm close to the left (next to the large magnet) tightened the bolt down to where it didn’t move then slowly loosened the bolt until the arm slapped back into it’s proper position to the right. verified that the arm moved freely and no binding occurred. and now I have zero extra play in the arm. I would recommend adding this check in the guide to verify the correct tightness on the screw and freedom of the lever to after the part and magnets are installed.

    Michael Thompson -

    I would like to add to your comment to help people out:

    Put the M3x18 screw in. Then put the big magnet in the slot. CLOSE the switch and screw the screw until the switch does NOT open . Now slowly, loosen the screw until the magnets repel. That should have good amount of play.

    Luis -

    I can get both magnets in their slots just fine but holding my thumb over the little magnet. But, as soon as I let go, the smaller one jumps out of its socket. Not sure what to do here.

    Bruce Kaplan - Reply

    Hi Bruce,

    You could crumble a small piece of paper and wedge it inside together with the magnet to keep it in place better, or use a dab of glue (be careful not to glue the other parts).

    Marcin M. - Official Prusa CS -

    I had the same issue here, magnets wouldn't stay in the parts, I used some tape around the magnets and now they stay in. Thanks to those who suggested the idea.

    Matt Barendrecht - Reply

    The two magnet slots were a bit loose, and the magnets would undo themselves and slam into each other as soon as I'd stop holding them. I wrapped one or two very small pieces of masking tape on the bottom end of the magnets before inserting them, in order to add a bit of thickness, and they now sit tightly in! Here is a picture.

    David Stosik - Reply

    I used 2 wee bits of blue tack to hold magnets in place.

    Miles Odonnol - Reply

    I agree with John Maranto’s comment; the M3x18 screw securing the FS lever should be tightened down just to the point where the lever doesn’t move freely and then backed off a quarter turn to allow it to move freely.

    Also, maybe note that there is no nut for this specific screw (unlike many prior parts, it’s threaded into the 3D printed piece of the extruder body.

    Steven Woodbridge - Reply

    it seems like a thin washer (or even a pair, for metal on-metal slide) under the FS-lever would help it move more freely. maybe something for the next revision.

    Robstar - Reply

    (not that i saw any problems; just seemed like a nice idea)

    Robstar -

    Wait, is the FS-lever new in a particular revision of the kit? I disassembled my extruder recently, and it doesn't seem to be present in mine. My Mk3’s been working well, but I’ve been having trouble with it since installing the MMU2 and upgrade to Mk3s. Perhaps this is why? I’ve only had one truly successful mmu print so far. Also in the upgrade kit, there are still two magnets in their package, should one of those have been used in this step?

    Patrick Leiser - Reply

    Hello, I’m not sure I follow. The MK3, the MK3S and the MK3S+MMU2S have each a completely different filament sensor mechanism. The guide here is for MK3S or MK2.5S. If you have the MMU2S, do not follow this guide, there is a separate one for you.

    Martin L. - Official Prusa CS -

    Hi Prusa, how tight should it be in “Secure the part with a M3x18. Tighten it, but ensure the lever can move freely.”?

    pcprusa - Reply

    Hi Pcprusa, for example, you can tighten the screw all the way in, and then start loosening until the lever is able to move smoothly.

    Martin L. - Official Prusa CS -

    these comments are great ! I used johns suggestion and bottomed out the screw and back the screw out 90 degrees and it was still not freely moving then backed it off another 90 to achieve the free movement, a total 0f 180 degrees. Also I left both magnets in the shipping container until i needed them, no use creating a condition of loose magnets that i have to manage.

    jim Jacobson - Reply

    I thought I had completed this correctly but when I got to Step `8, the M18 screw was sticking out of the assemaby and prevented the tow parts fomr fitting together. What next?

    Beginner - Reply

    De-assembled to this step and found I had used an M20. BEWARE!!

    Beginner - Reply

    Brilliant approach to creating a spring loaded assembly without a spring!!

    Marvin Fuller - Reply

    Why is my screw not fitting in the hole?

    Rain Li - Reply

    How tall should the stick out be?

    Rain Li - Reply

    Hi Rian, the screw should be almost fully tightened. Make sure the lever can move freely, though. If it’s unable to do so, then loosen the screw.

    Filip M - Official Prusa CS -

    If you manage to put the magnets in backwards like I did (by bein a big dummy) I recommend extracting the magnet in the FS-Lever rather than the one in the body. I was able to do this by gripping the part with a very small Irwin quick-grip and pulling the magnet out using a pliers

    Ian Lovrich - Reply

    This lever is cruical for your IR-Sensor and tends to get stuck in later build steps, even if you had a lose fit in this step. Test during the following steps every now and then, if the lever is still moving. Especially when you reach step 18. I added some PTFE-Oil on the screw to reduce chances of jaming.

    Andreas Renn - Reply

    Dear all, I have searched everywhere on line to check if I am the only one that has this problem. The FS for me was always in the way of the IR sensor slot with result to always give a logic “1” at all times. When the filament runs out, the lever is repelled by the magnets in completely (the screw is tight enough to allow the lever to move freely) but still the edge of the FS lever that cuts through the IR sensor was still significantly in between. I have checked the ball with the spare one in case it was bigger than normal (which that would made sense) but I measured both ones and they have a diameter of 6.9mm (it would be nice to know if this should be the size). If the ball was two mm smaller the edge of the FS lever would not interfere with the sensor when the filament runs out and hence give a logic zero.

    As a workaround i modified the lever. I have cut the edge a little bit so that it does not interfere with the sensor but only if the filament pushes the ball and consequently interferes with the sensor.

    Spyros Efthymiou - Reply


    I confirm that the steel ball diameter should be 7 mm +/- 0.1 so your ball is in specs. The problem was probably that the lever printed accidentally a little bigger, maybe the first layer was slightly more squished? Anyway, you dealt with it correctly.

    Martin L. - Official Prusa CS -


    I had the same issue here. The IR sensor was always on (logic 1) and didn’t know why. When the motor part wasn’t mounted on the main body of the extruder, the FS-lever didn’t trigger the sensor and work properly. Each time I mounted the motor part the lever went in the way of the IR sensor triggering the sensor. I noticed that the FS-lever had elephant foot ( first printed layer was bigger than the others). Therefore I had to take it out to cut out the excessive material all around the lever and installed it again. Now everything works as expected.

    To Prusa development team: I would suggest you to try to expand the hole around the FS-lever on the part that is mounted on the motor. The hole is a bit small and once the motor is mounted, the lever may move a bit and interfere with the sensor triggering it permanently. So, even if all the pieces are correctly assembled the lever would be slightly in the way of the sensor and trigger it. The whole sensor system may not work properly because of this little detail.

    Raoul Cenan -

    I also had this problem, what I found is that the lever is in the right place until you put the two sides of the extruder block together.The underside of the lever gets between the two pieces and as you bring them together, it forces the lever up, tripping the sensor.

    I carved away a small bit of the underside of the lever, and a small bit from the front half of the extruder housing where the sensor was being wedged out, and everything worked great. Support sent me another set of parts, thinking that mine had been misprinted, I haven’t tried them because shaving things down worked so well, and I don’t want to take the extruder completely apart!

    Kelly Hickel -

    It should really be pointed out that the magnets do NOT have North/South poles like you would expect, i.e with one END being North and the other END being South. Rather, it’s the FLAT FRONT and FLAT BACK of the magnets that have opposite poles. In order to figure out the correct way to orient the magnets,:

    1). Put the short one into its position in the FS Lever. It doesn’t matter how you orient this first one. Screw the FS Lever into place in the body so that it can easily move from one position to the other, in a lever action.

    2). TAPE the longer magnet into the inside of its plastic case and securely CLOSE the case. Now hold that case-enclosed/taped magnet next to the Body, near the FS Lever, with the enclosed/taped magnet oriented in the same way as the one in the FS Lever. If the FS Lever moves AWAY from the long magnet then the long magnet is CORRECTLY ORIENTED.

    Roger Garrett - Reply


    Try a couple of times to move the FS lever TOWARDS the other magnet, and if it snaps back to the position AWAY from the large magnet, then you’ve got the long magnet in the correct orientation.

    2A). If the FS Lever does NOT move/snap away from the large magnet but instead is attracted to it, then the long magnet is in the WRONG orientation. Turn the case-enclosed/taped magnet around so that the OTHER FLAT SIDE is facing the flat side of the small magnet in the FS Lever.

    2). ONCE you have the case-enclosed/taped magnet oriented properly MOVE it FAR AWAY from the small magnet and open the plastic case. MARK the SIDE that should be facing the small magnet and MARK the TOP that should be slightly exposed once it’s inserted into the main Body.

    3). OPEN the plastic case and UN-tape the magnet from inside the case.

    Roger Garrett -


    4). FINALLY, carefully, position the long magnet properly in its slot , making sure the flat side that you labelled for facing the small magnet is indeed facing that small magnet and the end that you marked as being the top is extending slightly out of the top of the hole you’re inserting it into.

    Roger Garrett -

  7. Take the printed part Adapter-printer and insert the steel ball in. Roll with the ball to all sides to ensure smooth movement.
    • Take the printed part Adapter-printer and insert the steel ball in.

    • Roll with the ball to all sides to ensure smooth movement.

    • In case of any rough surface, remove the ball and clean the inside of the printed part.

    • Place the printer part together with the steel ball in the Extruder-body. The surfaces of both parts should be almost aligned.

    • DON'T use any screw to secure the Adapter-printer. It should hold inside the Extruder-body by itself.

    What size is the steel ball?

    Jason Fraschilla - Reply

    7mm. 12 characters

    Jason Fraschilla -

    Are we supposed to secure the printer-part adapter with a M3-10?

    Cory - Reply

    Hi Cory, no need for a screw. Instructions updated.

    Jakub Dolezal -

    So the M3x10 in step 4 is not used yet. Correct?

    Bruce Fleischer - Reply

    Hello Bruce, that is correct.

    Martin L. - Official Prusa CS -

    I thought it was used in step 5.

    Alan Ostroff -

    It’s used in step 5, picture 1, PINDA probe mount. (purple arrow)

    Andy -

    My upgrade kit (mk3 to mk3s) didn’t come with this “adapter-printer“ part.. instead it had 2 “FS-Lever“.. oh well. I’m printing it now. I don’t have black though, so it’s grey, hope that doesn’t affect the sensor performance.

    Harukanioku - Reply

    Hi there, we’re sorry for the inconvenience! Please write me an email to

    Martin L. - Official Prusa CS -

    I dropped the steel ball and the cat had fun it took me a while to get it a way from her.

    Ronald Lake - Reply

    Try to distract the cat with gummy bears. They have multiple uses.

    Freddy Jay -

    for those doing MK3S kit with MMU2S go to step 1 3B. MK3S/MK2.5S - extruder upgrade

    Brendan Hall - Reply

    Hi there,

    This kind of shortcut is possible, but we still strongly recommend to go through the entire MMU2S assembly manual to make sure you didn’t miss something.

    Martin L. - Official Prusa CS -

  8. For the following steps, please prepare: Extruder motor (1x)
    • For the following steps, please prepare:

    • Extruder motor (1x)

    • Extruder-motor-plate (1x)

    • M3x10 screw (2x)

    • ATTENTION: there is a set of the Bondtech gears. Make sure you have all the parts and you are using correct ones.

    • Use now: Bondtech gear with a grub screw. If the screw is out, gently screw it in (leave space inside for the shaft).

    • Use later: Bondtech without a grub screw, bearings and a shaft. Keep the bearings somewhere safe, they tend to roll away ;)

    • In the upcoming steps, always check the pictures to ensure the correct orientation of the Bondtech gears. Always double-check your work.

    the ziplock bag doesn’t list the additional bondtech stuff, I was a little confused til I got to this step

    ryman1 - Reply

    I only have one of those black bearings shown in the red square!

    Bill Koontz - Reply

    LOL found it! it was inside one of the Bondtech gears! Whew!

    Bill Koontz - Reply

    I don’t think it is too clever to put the magnets into the same bag as the Bondtech-stuff. It was hard to seperate as all parts always got attracted. I almost lost the tiny spare allen screw. I would suggest putting the Bondtech parts into its own bag.

    rdu - Reply

    Hello Rdu,

    Thank you for the feedback, we will look into making adjustments to the packing.

    Marcin M. - Official Prusa CS -

    Bondtech ‘stuff’ is now in it’s own bag. I can confirm.

    Chadel Pryce -

    Re: the Feb 16 entry, my kit came with the 2’nd Bondtech fully pre-assembled with both bearings and the shaft. Took me a second to figure out, but hopefully a time-saver later on.

    Scott Mutton - Reply

    Just letting you know, I ended up with two screw-locks, one mounted on the gear and one loose

    Miquel Martin - Reply

    even i had 2 screw locks,

    Adidev Jhunjhunwala -

    it would also be good on this bond tech to specify that the lock screw is a “grub” screw, to minimize confusion. same as the pulleys in previous steps.

    Alan Ostroff - Reply

    Hello Alan

    Thank you for the suggestion, we will look into adding a note about the distinction between regular and grub screws.

    Marcin M. - Official Prusa CS -

    The last servo motor in the box is labeled “Y axis”. Will that still work as the “Extruder” motor? I double checked all of the other motors and they were all correctly labeled as: Y axis, X axis, Z axis left, Z axis right. There are 2 “Y axis” motors in the box but they have different cable lengths. I can see that all the servo motors are the same model. Just want to make sure the Extruder motor model is the same as the rest.

    Darrell Goff - Reply

    Hi Darrell, the cable will be too short. Please contact our support if you received a wrong motor, we’ll send you the correct one.

    Filip M - Official Prusa CS -

    May I suggest bolding and capitalizing this part pf the instructions? “Bondtech gear WITH a grub screw.” “Bondtech WITHOUT a grub screw, bearings and a shaft.” I hope this helps.

    Michael Nolte - Reply

    Hi Michael, we feel there’s enough emphasis on that in the description. Make sure you check all the pictures and read the description carefully.

    Filip M - Official Prusa CS -

    Filip, judging by all the coments, there is not enough emphasis

    Jack -

    this was VERY confusing to someone who doesnt know what a grub screw is. This should be made more clear in the instructions

    Jack - Reply

    I was debugging the filament sensor being stuck at “1” all the time, and the solution was to remove some stringing in the extruder-motor-plate, specifically in the top cavity where the filament sensor pivot arm is seated - just around the quarter-circle in the middle top of the plate. This stringing occasionally did prevent the arm from flipping back to “0”.

    Øyvind Aardal - Reply

  9. Take the Extruder-motor-plate and secure it using two M3x10 screws. Use the cable as a guide to properly orient the part. Resist the temptation to place a screw in the third hole! Leave it for later ;)
    • Take the Extruder-motor-plate and secure it using two M3x10 screws. Use the cable as a guide to properly orient the part.

    • Resist the temptation to place a screw in the third hole! Leave it for later ;)

    • Rotate the shaft as in the picture. Flat part must face in the direction of the arrow.

    • Slide the gear on the shaft, the grub screw must be facing against the flat part of the shaft. Tighten the screw slightly.

    • There is a "channel" for the filament inside the printed part. Align approximately the teeth on the gear with it.

    • Don't tighten the screw just yet. We will align it properly first. See the next step.

    Don’t do what I did. That set screw can easily be lost in a carpet xD. - Reply

    try searching for it with the magnets?

    ryman1 -

    If you attempt ryman’s advice, keep the magnet in the SD case to prevent possibly picking up other unwanted debris (or use a different magnet).

    Heimdall -

    It doesn’t look like both sides of the extruder motor plate filament “channel” are at the same depth. It is hard to align the teeth on the extruder gear when your reference on either side are at different levels.

    Miguel Rodriguez - Reply

    It might be helpful to add a note on the correct orientation of the motor wires. I had to come back to this step after I noticed in step 14 my wires were misaligned.

    Jeremy Wood - Reply

    Hi Jeremy,

    I’m not sure what are you referring to - motor wires are clearly visible on both pictures in this step.

    Filip M - Official Prusa CS -

    just because there is a picture, doesn’t mean you cant say which way the wires should face. You’ve done it for all the motors thus far

    Jack -

    Hi guys, it was there the whole time. The last sentence of the first blue bullet point: “Use the cable as a guide to properly orient the part.”

    Jakub Dolezal -

    This is just one example where it would REALLY HELPFUL to have text ON THE PICTURES and not just color-coded reference lines and arrows on the pics. If there were some text on the picture, pointing to the cable, saying “Make sure the motor is oriented so that the cable is coming out of the right side”.

    Roger Garrett -

    Hi ,

    The screw on the shaft is stucked because the Allen key is turning in the screw!I didn’t use any glue and I didn’t tighten the screw.

    Impossible for me to continue to the step ten! I’m screwed!

    Adrien - Reply

    @Adrien If you are talking about the set screw, try using a torx bit, they can often be used on stripped hex screws. If that doesn’t work, you can try using a drill bit to drive the screw all the way through (with the gear off the shaft) taking care not to damage the threads. A new set screw should be easy to source locally.

    John Arild -

    Same problem, screw was already in way to far when unpacked. trying to remove it but was so fixed it ruined the grib. and @ John Torx isn’t in such small size.

    Michiel van Duijn -

    Hi Guys, try to heat the gear up and remove the set screw, there should be a spare one in the spares bag.

    Martin L. - Official Prusa CS -

    After getting replacement part i notticed right away it was again fixed so tight that only the same result would happen, a ruined head for the alan wrench.

    So decided not to force it and submerge the entire part in a thin oil designed to get fixed screws apart and then i could loosen the grub screw without damage.

    My gues is the screw gets fixed in the gear when either the screw or the gear is still not completly set after fabrication, therefore gets locked after the parts have set.

    Advice is to not screw the grub screw so freaking far therefore leaving some tolerance for metals to set, or deliver it seperate. Also not a fan of alan screws on such small parts as the head ruines easely after even a litlle force, Torx would be a far superior choise here.

    Michiel van Duijn -

    The bag with the gears had a setscrew loose in the bag, but the gear had another setscrew already in it (intended as a spare perhaps?) At first I thought the setscrew had shattered, but no, it’s just exceedingly tiny. I put the spare in the plastic case that held the magnets.

    Steve Lionel - Reply

    Did the same. also had an extra hex nut. threw it in the case as well. Once I complete the build, i’ll toss them in with the extras parts bag but for now the plastic magnet case makes a great place to hold tiny spare parts. Or tiny nuts and parts that you haven’t used as of yet if you have to leave your build till another time.

    Michael Thompson -

    In this step: “There is a "channel" for the filament inside the printed part. Align approximately the teeth on the gear with it.”

    which part is the “teeth”. Is it the longer grills at the top, or the smaller ones at the bottom.

    Ravish Chawla - Reply

    Hello Ravish, see the second photo. Align the small embedded teeth.

    Martin L. - Official Prusa CS -

    For the “channel alignment”, it’s helpful to bring the assembly up to eye level and look down it like a sight. Then you can clearly see how the high/low the gear on the shaft should be.

    Stephen Cooper - Reply

    Small typo: on step 3 (orange) 'flat part must in the direction' should be 'must face in the direction'. Picture is clear, but thought I'd point it out.

    Falk Thys - Reply

    Thank you for pointing this out, we will change it :)

    Marcin M. - Official Prusa CS -

    Once on the shaft, my gear does not center in the circle carve-out of the extruder-motor-plate. In fact the motor shaft does not center in that circle. It is off center in the hole toward the cables and the filament channel. This makes it extremely difficult for the filament to fit through the channel. Once assembled later in the process, it presses the filament out of alignment with the PTFE tube such that it wont feed through but gets hung up on the edge of the PTFE tube.

    Jeremiah Barber - Reply

    Hi Jeremiah, please contact our support - perhaps the motor was damaged and the shaft is off-center.

    Filip M - Official Prusa CS -

    This instruction completely fails to mention that there are in fact two sets of teeth on the gear. As a beginner who is just learning about this stuff, I look at the gear and see a very prominent set of teeth at the top of the gear, and I assume that these are the teeth that are being talked about. Instead I find out that whats being referred to is in fact a smaller much less obvious set of grooves. Grooves are very different to teeth.

    Please correct this rather serious error in the instructions.

    Lessian - Reply

    Hello, thanks for your feedback. If you feel uncertain what to do, you can always look at the pictures, you can even see them in fill size by clicking on the magnifying glass icon.

    Martin L. - Official Prusa CS -

    There was no uncertainty, I saw teeth and I knew that they were teeth. I knew that the filament had to line up with the teeth. I didnt zoom into the pictures because there was no uncertainty, didnt think there was any need to clarify what was meant by teeth. It was only when I saw that lining the filament up with the teeth had it being bent fairly badly, then I looked closer at the picture and noticed that the gear was sitting higher than mine. Hence my frustration with incorrect terms.

    Lessian -

    I would venture a guess that my eyes are older than most of yours….rather than trying to line up the teeth by eye, I used the smallest allen wrench and laid it in the printed part grooves to use as a frame of reference when lining up the gear teeth. I know it is not critical as we align it in the next step, but it was helpful for me to judge so I’m sharing.

    Laurie S - Reply

    To add to previous comments a photo and arrow of the “teeth” (more of a recess IMO) would be helpful to ensure 5he correct reference

    Marc LaPlante - Reply

    Hi Marc.

    Thank you for your feedback. When displaying a full-size image it is clearer which teeth they are. However, if necessary, we will create a different step description.

    Vojta Z. -

    I have a lot of trouble getting the gear on the shaft. It definitely does not “slide” on. I tried to push, hoping it would losen up after the first millimeter, but it didn’t, and I almost could not get it off again!

    What should I do?

    FarbrorMartin - Reply

    Hi Martin,

    Please try to loosen up the grub screw first, before sliding the gear on the shaft.

    Marcin M. - Official Prusa CS -

    My extruder gear with the grub screw simply does not fit to the extruder motor shaft . I removed grub screw completely from the gear. The other gear without grub screw fits nicely and tightly on the motor shaft. I am afraid of breaking something if I apply any more force to the problematic gear and shaft. I watched YouTube video “Prusa MK3S Extruder Assembly” ( and on 4:06 gear just slides on the shaft just like my other gear without screw.

    Tvrtko Malkoc - Reply

  10. Take a piece of the 1.75 filament. You can use the bundled one on a spool, don't use the black nylon, which is too thick. Straighten the filament as much as possible. Place the filament along the path and align the gear properly. The filament will be always slightly bent. Use it anyway for the initial alignment.
    • Take a piece of the 1.75 filament. You can use the bundled one on a spool, don't use the black nylon, which is too thick. Straighten the filament as much as possible.

    • Place the filament along the path and align the gear properly.

    • The filament will be always slightly bent. Use it anyway for the initial alignment.

    • For a final check replace the filament with an Allen key. Bear in mind, the key has slightly different size than the filament.

    • Tighten the screw slightly to temporarily fix it, we will make the final check and tightening later on. Be careful, you can strip the thread.

    • Don't use any glue to fix the screw in place, you won't be able to release it, in such case you might have to replace the entire motor.

    No tightening of the gear screw in this step?

    Martin Lundgren - Reply

    Hi Martin, it is assumed the screw is tightened from the previous step, retighten it if needed. Final check will be done later on.

    Jakub Dolezal -

    Be very careful using the Allen key for the final alignment check: it is very easy to deform the printed part.

    Make sure you’re using an Allen key that is slightly thinner than the filament.

    Matt - Reply

    The left side is slightly higher than the right from this angle, looks that way in photos as well so I’m assuming that’s the design.

    Alan Crooks - Reply

    In my case the Bondtech hobbed bolt pushes the filament away from the motor axis so far so the filament dont hit in the middle of the PTFE-tube. My Bondtech is 7.4 mm in diameter (the hobbed part) are there different sizes? Cant see any way to adjust in that direction.

    Mats Paulsson - Reply

    Hi Mats, the second gear (idler) will ensure the filament will go properly into the tube.

    If it’s still not entering properly, then you can always contact our tech support via email or livechat.

    Filip M - Official Prusa CS -

    Please note that this instruction manual does not once mention one key detail in lining up the feed gear. The most obvious protruding teeth are not where the filament gets lined up with. You need to line the filament up with the smaller indented grooves.

    Whoever wrote this document needs to realise that there is a big difference between teeth and grooves. And that if someone doesnt know that its the grooves being used here, its reasonable to use the actual teeth, because these instructions specifically say to use the teeth.

    Lessian - Reply

    Where should I find the filament 1.75?

    Hussain - Reply

    Hi Hussain, it can be from any filament you’ll use for the printer. For an example from the 1kg PLA spool that arrives with the MK3S.

    Filip M - Official Prusa CS -

    > Don't use any glue to fix the screw in place, you won't be able to release it, in such case you might have to replace the entire motor.

    I’m guessing that happened if it’s in the docs.

    Dan Corrigan - Reply

    Hi Dan,

    Indeed. It’s mentioned because it happened.

    Martin L. - Official Prusa CS -

  11. For this step, please prepare: Extruder-cover (1x)
    • For this step, please prepare:

    • Extruder-cover (1x)

    • M3nS nut (1x)

    • Slide the nut all the way in.

    • Ensure correct alignment using the Allen key.

    • Leave the hole on the "arm" empty for now. We will use it later, while assembling a print fan.

    Question - why is the first picture showing a nut resting on all top of the black printed part? It was confusing as it seemed to suggest the nut was to be installed there. Maybe update the picture (it’s not like the other steps)? Thanks.

    adrian.w7 - Reply

    Hello Adrian, it is to show which nut to prepare for this step. As the second picture shows you, you hve to insert it into the hole.

    Martin L. - Official Prusa CS -

    Sometimes the responses to perfectly reasonable questions just don’t properly address the question. In every other image where it’s showing the parts that you’ll need each part is laid out separately. In this case one part, the bolt, is laid ON TOP of another part, and it does indeed look as if it’s implying that it somehow GOES there. Sure, the next photo shows and the related text explains were it will actually be positioned, but it makes no sense to have it where it is in the first picture. The proper response from Prusa should have been, “You’re right, we’ll take another picture and show the parts separated, just like every other similar picture”.

    Roger Garrett -

    My extruder-cover arrived broken. Will the missing chunk effect my prints? (See where the missing bit would be highlighted in red)

    Peter VanDusartz IV - Reply

    Hey Peter, it shouldn’t affect anything. You can re-print the part later.

    Filip M - Official Prusa CS -

    Can I print it in PLA? Also, where do I find the models?

    Peter VanDusartz IV -

    My bad! I totally forgot to link them.

    All the files are here:

    You’re looking for the MK3S Extruder Cover. It shouldn’t be printed with PLA - it must be able to withstand heat.

    PETG is the best choice, you can also use ABS.

    Filip M - Official Prusa CS -

    In no way is that second nut space a slot. That is a hole. There is a very big difference between a slot and a hole. I spent five minutes looking for a second slot but being unable to find it.

    The frequent use of incorrect terms is incredibly confusing and misleading. If its a language barrier, please have an english speaker go over the document.

    Lessian - Reply

    Hi Lessian, could you please explain what the difference is?

    Martin L. - Official Prusa CS -

    A slot could be considered to be a type of hole, but the opening is much longer in one axis. An example of a slot would be where you put a coin into a vending machine.

    Gawain Reifsnyder - Reply


    1) that the text “Leave the second slot empty, we will use it later. “ be changed to say “Leave the hexagonal nut trap (marked with the red X) empty, we will use it in a later assembly”.

    2) possibly add a picture showing the actual M3 screw hole that the inserted square M3nS nut is to be aligned with; it is currently hidden on the bottom of the printed part in both pictures.

    Jonathan Lin - Reply

    to be consistent the nut should be at the front with a white background

    Alex - Reply

    Is it normal for the thin part of the extruder cover to deform with heat? The bit next to the Pinda probe has sagged down on my machine as you can see in these pictures:

    GTO - Reply

    Hi, it can deform when printing high temperature materials for extended periods of time.

    We have recently updated the part, you can find it here:

    Filip M - Official Prusa CS -

    @Filip M - Do you guys have a changelog as to when and what parts get updated? It would be helpful as if we’re doing some maintenance, we could swap out some parts at the same time.

    Ernest Yee - Reply

    Hi Ernest, you can track any changes to our printed parts here:

    Filip M - Official Prusa CS -

    We had trouble getting the nut to go all the way into the slot — it stopped about 2 mm short of where it’s supposed to be. And of course it was jammed in there tight enough that it wouldn’t come back out either. Solved the problem by (very carefully) using the tip of a hot soldering iron to push the nut the rest of the way in. Took about 20 seconds for the nut to heat up, and then it popped right into place. No visible damage to surrounding plastic.

    Steve T - Reply

    Hello Steve, that seems like a great solution, if you do it carefully. Thank you for sharing it!

    Martin L. - Official Prusa CS -

  12. For the following steps, please prepare: Hotend for MK3S (1x)
    • For the following steps, please prepare:

    • Hotend for MK3S (1x)

    • M3x40 screw (2x)

    • M3x10 screw (2x)

    • "MK3S ready" hotend is marked with an orange/yellow dot on the heater block. If yours differs contact our support immediately!

    • We will use one M3x40 later on, keep it around.

    • Don't cut the main zip tie on the hotend cables, or remove the rubber band, it will make the assembly easier.

    What’s the difference between a “MK3S Ready” hontend and a) a MK3 hotend, or b) a standard E3D V6 hotend?

    Jonathan R Swift - Reply

    Hi Jonathan,

    MK3S hotend = 45 mm tube, 1.85 mm diameter, custom MMU heatbreak

    MK3 hotend = 50 mm tube, 1.85 mm diameter (also 2.0 is ok), custom MMU heatbreak

    E3D V6 hotend = no PTFE, “regular” heatbreak

    Jakub Dolezal -

    Mine came with a rubber band around the cable instead of a zip tie. Good thinking, it prevents cutting the wrong one.

    Brandon Bergren - Reply

    Hi Brandon, we've discussed this with our team and decided to change the zip tie for a rubber band. It makes the assembly easier and prevents you from cutting the wires :)

    Jakub Dolezal -

    The “hotend” is labelled as a “nozzle” in the 2. 3. 4. 5. SUP -box, which caused me some confusion while searching for it without opening the box

    Ante Pettersson - Reply

    Thanks, I was starting to think I hadn’t gotten one because I was looking for “hotend” on all the various parts diagrams and boxes.

    William Rigsby -

    Hello guys, thanks for pointing this out. If it causes confusion, we will check if it can be changed.

    Martin L. - Official Prusa CS -

    My hotend/nozzle has a green dot, what do i do now? (I bought a MK3 and got the MK3S upgrade kit)

    Stefan Profft - Reply

    Please measure the length of your PTFE tube. If it’s 45mm, then you can proceed with the build. If it’s 50mm, then you’ll need to shorten & trim it (How to trim PTFE tube - Multi Material) or use one of the spares that you got with MK3S.

    Filip M - Official Prusa CS -

    Perhaps very subjective, but to me the dot on the hotend in the picture and on the hotend I received looks much more like a yellow dot than orange. (If not for the correlation with the picture, I was very tempted to contact Prusa, reporting that I had received a “yellow-dot hotend” rather than the expected “orange-dot hotend.”)

    If others made the same observation, perhaps worthy of reconsidering. Otherwise, please kindly disregard. If there are only two possible dot colors (green or orange/yellow) then probably not worth addressing.

    Todd Kanning - Reply

    Hey Todd, we’ll add a note about it. Thanks for the heads up.

    Filip M - Official Prusa CS -

    I noticed that there is no collet clip on top of the heatsink as shown on the E3d online hotend assembly guide I assume the collet clip is not necessary ? - Reply

    Hello, as long as the black collet is lifted up and the PTFE tube sits firmly in the hotend and not moving, the collet clip is not necessary. But if you feel like having it, you can print it, the model is available on thingiverse.

    Martin L. - Official Prusa CS -

    “We will use some screws later, but keep them around.”

    I think the assembly instructions are really great but this remark is really strange. Where will the screws be used? How many?

    I finally used the M3x40 in step 22 but I still have an M3x10. It is disturbing to think I might have missed using it somewhere.

    Hans G. Schnieder - Reply

    Totally agree with this. This chapter is not as professional or succinct as some of the others — the extra parts, for instance, should be in the spare parts bag, so that completing this whole chapter leaves you (ideally) with no extra parts. Remarks like this are confusing in a chapter whose preface is so insistent on careful precision and close attention to the directions.

    Wesley Boynton -

    Looking back from a few steps down…I guess this means one of the three M3x40 screws gets used a little farther down the line? I still don’t quite get it, but I got through step 14 and only used two of those three screws, and it seems like I’ve now moved on to another piece.

    Wesley Boynton -

    Okay, confirming the last of these three M3x40 bolts is used in step 22. Wow. That’s really unclear.

    Wesley Boynton -

    Hello Hans and Wesley,

    We will look into changing this note to be more clear about the M3x40 screw for Step 22.

    Marcin M. - Official Prusa CS -

    I think the M10 screw has been forgotten about as well. What is the point of having this screw sitting around doing nothing? If its for a later step, get it out at that point, not a bunch of steps in advance. This is how small parts get lost.

    Lessian -

    Hi! It’s months later and the m3x40 list is still confusing. There should be a separate prep for that. I just noticed the “extra” screw and backtracked from Step 17.

    Scott Spies -

    I’m confused; I’ve gotten to this step (upgrading from MK3 to MK3S) and my extruder doesn’t have any dot. What does that mean?

    Nick Bira - Reply

    Hello Nick, If you are upgrading from the MK3, you are in a wrong manual. Please see this guide: Original Prusa i3 MK3 to MK3S upgrade

    Martin L. - Official Prusa CS -

    I’m in need of a new Hotend for my MK3S, but I don’t see this part available for purchase on the Prusa site. How can I get one shipped to me please?

    Chad Wight - Reply

    Hi Chad, make sure you are logged in - otherwise you won’t see the spare parts section.

    Here’s the hotend you’re looking for:

    If you still don’t have access to it, then please contact us via LiveChat

    Filip M - Official Prusa CS -

    Hi, what is the spare part number for the ‘‘Hotend for MK3S ‘‘. Can i buy it on Amazon?

    Joel Sauvé - Reply

    Hello, here is the link to the inline store: We do not recommend buying the hotend from elsewhere!

    Martin L. - Official Prusa CS -

    Hello, I’m abit stuck on the extruder assembly. I have assembled the hotend assembly using the M3x40 screws but the protrusion out the back side is 12mm and to long for the X-carriage to sit flat against the extruder. The mating holes on the X-carriage are only 9mm deep. Have I misread the the extruder assembly? I have not cheated on the HARIBO’s

    Peter S Scheuter - Reply

    Hi Peter, it’s perfectly fine. Here’s a quote from Step 14:

    Tighten them, but be careful, they are slightly longer (2-3 mm), than the thickness of the entire assembly.

    Filip M - Official Prusa CS -

    We will use one M3x40 later on, keep it around.

    I've got a total of six M3x40 screws. I don't know if that sentence means one of the 3 screws I just prepared, or one of the screws left behind. When reaching step 15, I wondered whether I forgot yo use one…

    David Stosik - Reply

    "MK3S ready" hotend is marked with an orange/yellow dot on the heater block. If yours differ contact our support immediately!

    The word “differ” in the second sentence above should be replaced with either the word “differs” or the words “is different”.

    Jonathan Lin - Reply

    Good idea to check that the PTFE tube is seated snug against the heatbreak. It shouldn’t be wobbling up and down. Had this with mine and caused extruder blockage until fixed.

    Goethe - Reply

    This “Step 12: Hotend Parts Preparation” should also include a pic and entry in the text list of the Extruder Body, which was prepared back in Steps 3, 4, 5, and 6.

    Roger Garrett - Reply

  13. Take two M3x10 screws and insert them in the holes. Doing this now makes the assembly slightly easier, but both holes are shallow and the screws might fall out. If this happens, you can continue without them and put them back later on (you will be notified later). Also note, that in few upcoming pictures the screws might be missing. Hotend assembly was completely reworked from MK3 to avoid bad placement. Also any future maintenance is much easier. Place the hotend next to the Extruder-body, see the grooves in the printed part, which are in the shape of the hotend.
    • Take two M3x10 screws and insert them in the holes. Doing this now makes the assembly slightly easier, but both holes are shallow and the screws might fall out. If this happens, you can continue without them and put them back later on (you will be notified later). Also note, that in few upcoming pictures the screws might be missing.

    • Hotend assembly was completely reworked from MK3 to avoid bad placement. Also any future maintenance is much easier.

    • Place the hotend next to the Extruder-body, see the grooves in the printed part, which are in the shape of the hotend.

    • Correctly placed hotend. Cables should point in the direction behind the hand, we will align them in the next step.

    Insert the two 3x10mm screws on the lower part of the cooling shroud before you put the extruder in place. You can see on of the holes between the fins of the barrel. I couldn’t get them to go in strait on step 19 without taking it apart again.

    Adam Bavuso - Reply

    I agree. The two 3x10 mm screws should be inserted at this step.

    Virgil McLaren - Reply

    Great hint! Insert the screws now!

    Mihai-Andrei Stanimir -

    Hey guys, thanks for the suggestion, we’ll consider this.

    Martin L. - Official Prusa CS -

    When a suggestion is made, such as inserting the two 3X10 screws at this step, and Prusa agrees with the suggestion and actually MAKES the change in the text to conform with the suggestion, then the comment in the comment list should be DELETED, since it no longer applies. It can get pretty confusing when reading through the comments and it seems to be making suggestions for something and, lo and behold, it turns out that the issue no longer exists.

    Roger Garrett -

    I also installed them at this step. I think the instructions don’t say to do it here because in the next step it doesn’t sit flat on the box. But that’s a minor inconvenience

    Dan Green - Reply

    Hello Dan, yes, that is the reason. But you sure can insert them at this step.

    Martin L. - Official Prusa CS -

    Even that this is not common, but for me the lockring was broken and one piece was blocking the PTFE tube. So, adding a check for the good tube stickout hight might be of help here.

    Marc Pflugmann - Reply

    Hey Marc,

    If something arrived broken, then please contact our customer support - we’ll send you replacements.

    Filip M - Official Prusa CS -

    …Punch 2 hole in line with the screw in the box, put the screw in. You now have a extruder holder and the screw are in , Sorry Nice Box… - Reply

    The next step shows better where the wires should go.

    Nathan Charlton - Reply

    I think that the phrase “you will be noticed” is incorrect. I believe that it should be replaced with “you will be notified later”. Perhaps consider changing it?

    Steve Gowin - Reply

    It will be VERY hard if not impossible to insert the [2] 3x10 screws when it comes to the step to tighten them. They may not stay securely in there (mine weren’t secure) but that doesn’t matter, it does not make it much (if any) harder to handle the part as you work with it and will save you a lot of frustration. This should be in the manual as a requirement instead of an elective. PLEASE UPDATE the manual as this is a real necessity in the assembly process.

    William McCulloch - Reply

    Hi William, we’ve found it doable later - it’s not necessary to insert those screws now.

    Filip M - Official Prusa CS -

    After installing the M3X40 screws to mount the motor, they stuck out about 10 mm, not the 2-3 mm stated in the manual. I hope I haven’t done something wrong, I looked all over for some picture that might show how much these screws should stick out but could not find one.

    Michael Godfrey - Reply

    Hello Michael,

    Maybe it is because at this step, you are not supposed to do anything with the M3X40 screws we are working with the M3x10 screws here.

    Martin L. - Official Prusa CS -

  14. In order to protect the hotend cables and ensure proper orientation, it is highly recommended to use a box. Use the one provided in the kit. Place the Extruder-body with the hotend on the box and make sure the cables are pointing down. Place your finger temporarily on the longer magnet  and place the extruder motor assembly on the Extruder-body. The Bondtech gear might pull the magnet out while assembling parts together.
    • In order to protect the hotend cables and ensure proper orientation, it is highly recommended to use a box. Use the one provided in the kit.

    • Place the Extruder-body with the hotend on the box and make sure the cables are pointing down.

    • Place your finger temporarily on the longer magnet and place the extruder motor assembly on the Extruder-body. The Bondtech gear might pull the magnet out while assembling parts together.

    • Make sure both parts are aligned.

    • Place the Extruder-cover on the Extruder-body. Again, make sure that all three parts are properly aligned.

    • Insert two M3x40 screws, you've prepared earlier. Tighten them, but be careful, they are slightly longer (2-3 mm), than the thickness of the entire assembly.

    • Place the extruder aside for few steps, we need to prepare another part. Leave it on the box to prevent damaging the cables.

    “Insert two M3x40 screws, you've prepared earlier. Tighten them, but be careful, they are slightly longer, than the thickness of the entire assembly. “

    Slightly longer? Try 12mm of bolt will protrude from the body instead - be specific (or even say about 1/2”).

    David Maher - Reply

    This happened to me because of the easy to miss step of attaching the extruder cover.

    Kinsky -

    Whoops, I left part of the extruder off, now they only protrude slightly. If they stick out too far, you have done the same as me. They now protrude 3.5mm after correcting my mistake.

    David Maher - Reply

    Yep, made that mistake too, thanks for writing, missed that whole “Place the Extruder Cover on the body part” . Your post made it easier for me to find and correct it..

    Thank you!!

    Kelly L Gann -

    Me too!!!

    it is shown in the picture but it’s easily overlooked. even when i had the pspare bit in my hand it took me 10 minutes to find out where i’d missed this step.

    Worse for me - I stripped one of the nuts inside the body so the 40mm screw was stuck. I had to dremel the bolt out.

    This got hot and slightly warped the extruder body.

    (all fixed now but cost me over an hour in my build/rebuild/fix steps.)

    PLEASE Prusa add a bit more warning to this step in the next revision. I’m pleased to see I’m not the only mupet who can’t follow instructions but so many places in the manual you highlighted stuff - this needs extra pic/and warning please!!!

    Tony -

    Same thing happened to me too.  Mounting the extruder cover really should be its own step.  Like Tony, I also stripped not one, but both nuts on the long bolts.  Not sure how I did that.  I also had to cut them off with a dremel.  To prevent melting the plastic too badly, I dabbed a Q-tip in water and applied it after a few seconds of cutting to cool the bolt.  Seemed to work alright.  Also, thank god there were plenty of extra M3x40 screws!

    Rob -

    Ah, thank you. I made the same mistake. I can’t fault anybody, there is a clear line item for it, but I would agree making that its own step would help.

    At least the obvious overrun of the screws makes it clear when this happens.

    Chris L8 -

    Me too…

    David Grant -

    I didn’t look at the picture closely enough. When I read “make sure the cables are pointing down”, I oriented the hotend with the wires exiting straight downward. I didn’t notice til step 36 that the hotend was 90º off axis! Fortunately, I was able to loosen the assembly enough to make the correction without taking the whole thing apart!

    Kevin Danenberg - Reply

    I made the same mistake… Where it said “make sure the cables are pointing down.” I incorrectly assumed that meant make sure the cable outlets face down. This appears not to be the case. So in order to not get to this step and have to disassemble, at step 14, make sure the cable outlets face to your left and the cables hang down otherwise you won’t realise your mistake until step 36.

    Matt -

    I made this same mistake as well, and also was able to correct by loosening.

    Could we please get this clarified in the step instructions? “Pointed down” and “hanging down” mean slightly different things.

    Joshua Brannon -

    I just made the same mistake! I noticed in this step that the heater block was touching the petg part and corrected it after I looked closely at the pics.

    A step or revision should be added to make sure the heater and thermistor wires turn 90 degrees towards the rear in the previous step. Mine were straight out of the heater block when I opened the package.

    Scott Spies -

    Hi everyone, please always follow each step closely - that includes checking both pictures and description.

    This step specifies that cables should be pointing down, it also shows the correct orientation of the block on 3 separate pictures.

    Filip M - Official Prusa CS -

    In step 13 we get three m3x40 screws out and then only use two of them?

    Douglas - Reply

    Hi Douglas,

    The third one is used in Step 22 for securing the IR sensor in place.

    Marcin M. - Official Prusa CS -

    Prusa, please fix these directions. That’s a huge number of steps to go by after “preparing” a screw, including context-switching to assembling a different section of the whole assembly. That’s a good way to lose parts for those of us who take out those needed for each section and set them aside.

    Wesley Boynton -

    Hi Wesley,

    You do not need to keep the screw in your hand all the time - you can set it aside for the assembly time, it shouldn’t get lost.

    If you’re worried about losing the screw, then consider purchasing a magnetic tray to safely store it at all times.

    Filip M - Official Prusa CS -

    Filip M - Official Prusa CS : You’re not listening and heeding the feedback that you’re getting. There is NO REASON to “prepare” some part at a given Step and then not actually use it until many steps later. Prepare it JUST BEFORE it’s to be used.

    Roger Garrett -

    When screwing the extruder cover onto the extruder body, make sure the two shorter screws you placed from step 13 aren’t wedged against the heat sink. Because of the way the parts are sitting on the box, these two screws can get pushed up and jammed against the heat sink, making them impossible to screw in later. Make sure these two screws can be turned before proceeding to the next step.

    Jon Humphreys - Reply

    Thanks for pointing this out — you definitely saved me some time.

    My screws were firmly wedged against the heatsink, and could not be pushed in. However, I was able to use the rounded allen key to turn them at an angle and they dropped back past the heatsink and into the holes nicely. Glad I addressed that now.

    Wesley Boynton -

    Prua - Please add an extra picture or warning to rember the Place the Extruder-cover it’s really easily overlooked and causes damage to the other parts if forgotten here.

    Tony - Reply

    Hey Tony, have a look at picture #3 - it shows the extruder cover. It’s also mentioned in the Step’s description.

    Filip M - Official Prusa CS -

    I second that motion. Up until now, every instruction has been very clear. Prusa team, I know it may seem clear to you, but it is not clear at all to us. I also skipped this step, and then damaged other stuff later. Please make this a separate step.

    Daniel Kiracofe -

    @daniel_kiracofe Can you give us an example of what instruction would you add into the description? Because it is clearly written “Place the Extruder-cover on the Extruder-body”, and the third photo shows it. Tbh I don’t understand what exactly is wrong/missing. O.o

    Martin L. - Official Prusa CS -

    Martin, thank you for your reply. I’ve given it some thought, and I would suggest these two specific wording changes:

    1st suggestion, in the sentence “but be careful, they are slightly longer, than the thickness of the entire assembly.” replace “slightly” with “1-3 mm” (or some other specific measurement). The problem with “slightly” is that is ambiguous. It could mean 0.1 mm, or 1 mm, or 10 mm. If you just say “slightly”, and I see my screw sticking out 12 mm, I do not realize that anything is wrong. but if you say “1-3 mm” and I see the screw sticking out 12 mm, I realize that something is wrong and I look harder at the picture to find my mistake.

    2nd suggestion, instead of “Again, make sure parts are properly aligned.”, write, “make sure all three parts are properly aligned.” This makes it obvious that the M3x40 screw is supposed to go through 3 parts, not just 2.

    Daniel Kiracofe -

    @daniel_kiracofe Those are very good suggestions, thank you sir!

    Martin L. - Official Prusa CS -

    Just got the kit and started assembling things and missed putting the extruder cover on as well. I think a problem is that the pictures are very small on the booklet and online, unless you zoom in. In the booklet there is a lot of space on that page, I think when you have assembly steps where a lot of people fail, it would make sense to add a zoom in picture of the problem area (showing the extruder cover and adding a text like: “Dont forget to put the extruder cover on before you screw in the 2 M3x40 screws. Once properly assembled the screws should only stick out about 3mm on the other side”

    Christian Staack -

    Check for interference between the PTFE tube and the Bondtech gear mounted on the extruder motor. If there is interference, remove the PTFE tube from the hot end and trim the end of the tube that is inserted into the hotend. Reinsert PTFE tube into hotend and check again for interference. Repeat until there is no interference between PTFE tube and Bondtech gear.

    Nelson Zambrana - Reply

    read all the comments, and then still managed to forget to add the cover part. went through a few more steps until I realized it, and by that time the plastic the nut is in, was totally destroyed by me trying to tighten the screw… had to dremel the screw, and now I dont know what to do with a part where the nut dont grip.. Prusa forget about the box, this part here is the one thats critical, and needs to be in bold red font. or maybe a seperate step.

    Johnnie - Reply

    good point, Nelson. I was going to just leave the excess there. Now I will take it apart and trim the PTFE tube until there is no interference with the Bondtech gear. Thanks!

    Richard Perkins - Reply

    Got to step 36 and realized I didn’t follow this step properly. Please show a direct bottom view facing the extruder nozzle to show that the heater block orientation. Basing the orientation on the wiring is confusing. Use the heater block + wire routing direction.

    Kevin Thompson - Reply

    Hello Kevin, the block’s orientation is clearly visible on all pictures. If needed, you can always click on the magnifying glass to enlarge them.

    Filip M - Official Prusa CS -

    Description says to have cables pointing down, but picture shows them on the side. Which is it? Please adjust the picture or the text to clearly indicate if the cables are supposed to be down or on the side.

    Lessian - Reply

    Hi, it does not matter if the cables are on the side or not, you can rotate the perspective as you wish and have them on the front or even rear of the box if it suits you. But for an easier manipulation, we recommend to have the cables hanging down as shown on the photos.

    Martin L. - Official Prusa CS -

    Turns out that ‘have the cables pointing down’ actually means ‘have the cables sticking out the side but pointing down’.

    Seriously, the more I work through this instruction manual, the more frustrated I get. Riddled with incorrect terminology, vague and confusing descriptions, poor quality control, and outright contradictions.

    Also, not being allowed to edit comments more than five minutes after being posted: very poor.

    Lessian - Reply

    That is correct, the cable bundle should be hanging down, but the heater and thermistor cables must be at the side of the heater block.

    Martin L. - Official Prusa CS -

    Having some issues with the IR sensor in the assembly. I’ve fully put together my printer but noticed that my IR sensor is always high, in the 1 state. After partially disassembling it, I’ve noticed that without the motor part of the assembly the IR sensor works fine. As soon as I add the extruder motor and the part attached directly to it, it shifts the FS-level ever so slightly - enough for it to consistently register 1.

    Any ideas for how I can rectify this? The printer is functional without it but the filament sensor is a big part of the feature set. Would be disappointing to not use it.

    Winston - Reply

    Hello, the motor should not affect the position of the lever in any way, if yours does, I’m afraid you have to go a few steps back and see if you have it all assembled correctly.

    Martin L. - Official Prusa CS -

    I can’t seem to correctly align the extruder body and the extruder motor. There's a gabp on the farther end from the cables where the tall magnet is. I’ve checked that the magnet is pushed all the way in. Will there be a screw later on that could fix this?

    Mila Santana - Reply

    Hello Mila, there will be a screw later on, but also there should be no gap - try to see what is creating it.

    Martin L. - Official Prusa CS -

    Martin L. - In a response to Lessian about the direction of the cables, you state: “it does not matter if the cables are on the side or not”. Does that mean that the orientation of the heater block isn’t important? I’m many steps past this and my hotend cables are pointing toward the back of the printer, which seems correct, but my heater block is at approximately a 45 degree angle, and I’ve been worried about this. Thanks for your help. - Reply


    No, to the contrary, the orientation of the heater block is crucial. Please see the photos and arrange it exactly as shown. The cable bundle should be hanging down (that’s what we are talking about in the green bullet point and that is what I was talking about in my response to Lessian), but the heater and thermistor cables must be at the side.

    Martin L. - Official Prusa CS -

    When I attach the X carriage assembly in step 16 there are no threads to tighten down the two M3x40 screws . Should there be?

    Terry - Reply

    Hi Terry, Step 5, green bullet point. You probably forgot to insert those hex nuts, or they fell out. :)

    Martin L. - Official Prusa CS -

    I don’t see where hex nuts go for the M3x40 screws that go through the extruder cover and into the X carriage assembly.

    Terry - Reply

    Hi Terry,

    These two screws go into the threading in the printed part directly and in the back of X carriage later.

    Marcin M. - Official Prusa CS -

    The hotend that came with the kit bent like the way shown in the photos. One cable was broken at the junction. I got a new pair of red wires. Chat agent told me not to bend the wires and just inserted the wires into the hotend straight. However, if I do that all four hotend cables point to the left of the printer. If I want all cables to point to the back of the printer, either I have to bend them or rotate the hotend by 90 degrees so that all four wires naturally point to the back.

    Shall I leave all four hotend cables point to the left of the printer and be done with it or rotate the hotend by 90 degrees without installing the fan-shroud in Step 35? Alternatively, if I have to do the way specified in this manual, how do I bend the red wires without breaking them? I heard that they are fragile.

    pcprusa - Reply

    Hello there,

    Bend the cables very slowly and carefully. Do not pull them. For sure do not rotate the hotend. The fan shroud is indispensable.

    Martin L. - Official Prusa CS -

    Thanks Martin. What is the best tool to use to bend the red cables?

    pcprusa - Reply

    Hello, no tool is needed, use your fingers. : )

    Martin L. - Official Prusa CS -

    Before bending, should the metal end of one red cable be above that of the other red cable? In other words, should the two red cables be lined up vertically on top of each other before we bend them both backward at the same time?

    pcprusa - Reply

    Hi, one of the cables should be on top of another, not on the same level - just like you can see on the pictures above.

    Filip M - Official Prusa CS -

    I only used 2 M4*40 screws till this step. I have 1 more prepared? Is it after this step or I missed a step to use the M3*40?

    Rain Li - Reply

    Hi Rain, that screw is needed later, its described in the comments above, someone else had that issue, read through them and you see where it goes, I think step 21 or so

    Christian Staack -

    I’m thrilled to see that I’m not the only one who spent some time scratching my head when I had 1/2” of M40 screw sticking out of the nuts…

    To prevent the two screws from Step 13 from falling out of their holes, it helps to cut a slot in the top of the box that you’re using to hold the assembly in this step. Then the screws can be fully into their holes and won’t go anywhere.

    Gandalf the Blue - Reply

    I missed the cover, even after reading people had missed it. My extruder looked right except for the length of the bolts sticking out. When I went to the next step I saw the printed part sitting there and realized I too had missed it.

    Ronald Smith - Reply

    my M3x40 screws were not longer, they fit perfectly, were could I have gone wrong to get this result?

    tudor - Reply


    Please ensure you have actually used the 40mm screws. If they are correct size, then perhaps something is blocking them.

    Filip M - Official Prusa CS -

    When trying to screw in the M3x40 screws, 1 of them seated nicely and the other turned very difficult. Attempted to back it out and the hole where the nut goes stripped and now the bolt and nut are locked and cannot be removed. What do I do? Only one of the screws is tight. Is that going to work?

    Brian Drey - Reply

    When looking at the end of the bolt that is locked in, it looks like it is damaged and has a large burr!

    Brian Drey -

    We got the old one out via cutting it and re-printing the bottom plate. Thanks to tech support for sending the correct file to print. All is back to normal.

    Brian Drey -

  15. For the following steps, please prepare: X-carriage (1x)
    • For the following steps, please prepare:

    • X-carriage (1x)

    • M3n nut (2x)

    • M3nS nut (4x)

    • IR-sensor cable (1x)

    Where’s m3x40 used for these steps?

    Eli - Reply

    It should be in one of the plastic bags. See the labels.

    Martin L. - Official Prusa CS -

    This is not a beginners printer. In no way is this a beginners printer. A beginner would look at the labels on the boxes for a picture of a cable. A beginner would not be looking for a diagram of plugs. Very poorly done.

    Lessian - Reply

    Hello Lessian,

    Our printers (and thus this manual with them) are used in primary/elementary schools across the globe. What do you suggest as an improvement?

    Martin L. - Official Prusa CS -

    My suggestion is to go through and rewrite the steps where people have expressed confusion about things. I have noticed a tendency for prusa staff to give helpful responses to people’s comments approximately half the time. Most of the helpful information comes from other beginners who have figured out an improvised solution and shared it. The problem seems to be that its a printer designed by advanced users, with a manual written by advanced users. Try looking at it from the perspective of a person who doesn’t know what they are doing but is trying to learn. My initial comment here is a perfect example.

    Lessian -

    I think they have done a great job, how difficult is it to read a label on a bag that clearly says what you are looking for?

    Esdras Alvarez -

    I think being a beginner at 3D printers is not the same as being a beginner at mechanical understanding, and knowledge of how to use tools and assemble things. To me it is rather obvious when the instructions say to prepare the “IR sensor cable” that they mean the box with the part labeled “IRsens cable”. If this is not obvious to you then you made a mistake ordering the kit and should have ordered the pre-assembled machine.

    Lee Kibbler - Reply

    I think this is the most obvious assembler instructions I ever read. And the comments give a sense of completeness as a whole. Great job, Prusa team!

    Frederico Brandt - Reply

    I wholeheartedly agree (and with Lee Kibbler as well)!

    Douglas Howard -

    Which bag is the IR-Sensor cable in? I can’t even find it on any labels…

    Erin - Reply

    Hi Erin, have a look inside the smaller boxes.

    Filip M - Official Prusa CS -

  16. For the following nut insertion USE A SCREW. THAT'S AN ORDER!!! Seriously, use a screw to pull the nuts in, both have to be properly seated in the X-carriage. Take both M3n nuts and using pliers (or screw) push them in the X-carriage, then using a screw from the other side, pull them all the way in.
    • For the following nut insertion USE A SCREW. THAT'S AN ORDER!!! Seriously, use a screw to pull the nuts in, both have to be properly seated in the X-carriage.

    • Take both M3n nuts and using pliers (or screw) push them in the X-carriage, then using a screw from the other side, pull them all the way in.

    • Don't forget to remove the screw.

    • Take all four M3nS nuts and insert them in. Ensure correct alignment using the Allen key.

    • From now on, keep in mind the nuts are inside, avoid rotating the X-carriage "downwards", or the nuts might fall out.

    I was 2 x square captive 3mm nuts short here. I suspect that I have put the missing nuts in the Extruder assembly, since there were some no longer needed screws there. I have raided the spares bag.

    David Maher - Reply

    Belay that last comment (I just found them, sorry).

    David Maher - Reply

    Is the 40mm screw referenced in step 16 just to pull the M3n nuts into position? I don’t see where it’s used in the next few steps.

    William Rigsby - Reply

    Hi William, it will be used in step 22.

    Jakub Dolezal -

    these nuts were really a bear for me to get in. i ended up taking a hammer and tapping on an m3x40 bolt with the nut threaded to get it to seat properly.

    Johnny Appleseed - Reply

    Next time, we recommend taking a smallter screw and use it to pull the nut into the socket by screwing it. See the Introduction chapter.

    Martin L. - Official Prusa CS -

    The M3x18 screw is perfect for getting the nuts in. I used the screw-pulling technique, and the 18 is just the perfect height to thread the nut onto after pushing it through. The narrow walls of the channel keep the screw from spinning and automatically align it as you tighten it down to pull it into place

    Dan Green - Reply

    The M18s worked perfectly for me as well.

    Keith Illg -

    I spent about 2 hours trying to get the nuts seated using the procedure outlined in the introduction. Very difficult!

    Hubert Fu - Reply

    Here’s what I used to get these in: I took the M3x40 and ran it through, threaded the nut onto it, and pulled it down into the “trench.” I gave it a firm pull to seat it into the opening that it needs to go into, then I unscrewed the M3x40 (since it is not fully threaded) and used an M3x10 to tighten the nut into it’s proper position. Worked like a charm.

    Nick Gawel - Reply

    The nuts wouldn’t fit in even pulling strongly on the M3x40 from the other side. Here is a quick super-efficient way (not sure it is approved by Prusa Officials though!). After taking the M3x40, run it through and thread the nut onto it (as instructed), heat the nut with a lighter so it gets real hot. Then pull down gently in place. Worked perfectly :) Warning : don’t pull too much or the printed part will be ruined!

    Sébastien - Reply

    Hello Sebastien,

    You’re right, although it’s quite creative solution, we would not advise it in the manuals, as using this method can be destructive to the printed parts irreversibly - it could be problematic if you were to do any reassembly in the future with the nut/screws permanently ‘melted in’ the plastic part.

    Marcin M. - Official Prusa CS -

    Use an m3x14 or m3x18, push it all the way in. Set the nut on top, hold with one finger, and turn the screw from the other side. Seated easily.

    Nathan Charlton - Reply

    I did the same. worked well. I just had to make sure it was turned correctly.

    Michael Nolte -

    I couldn’t pull the nut in enough with a screw from the other side, it would pop out as soon as I unscrewed the pulling screw.

    What I did before unscrewing the pulling screw, is I put another screw from the top the wrong way (so the wider part is touching the nut), and used it as a makeshift hammer, lightlghy tapping from the top with a real hammer until the screw got in enough so it stays put when unscrewing the pulling screw. - Reply

    I think I can clarify the screw pulling technique, or at least the way I do it; here, for convince of others:

    1. Put an M3x18 screw through the hole from the other side.

    2. Put the nut on the M3x18 and screw it down a few threads.

    3. Pull the screw from the back side and make sure the nut now is aligned and against its slot

    4. With a small allen key, keep the nut in the trench while you again screw the M3x18 all the way in

    5. Now, use an allen key to tighten the M3x18 screw. This is were people get confused I think. It is the tightening of the M3x18 screw that actually pulls the nut in. Just screwing won’t accomplish much. Tighten the screw and once the nut is in place, you may unscrew the M3x18 again.

    You’re welcome :-)

    Miguel Barroso - Reply

    Thanks Miguel, that helped!

    Tony Mastrangeli -

    thanks i wish there was some video (or link to a video some one else made)demoing this earlier on so many issues would have been quickly solved.

    mat -

    Thanks Miguel!

    Jon Scattaregia -

    Many thanks! It worked

    Daniel Weitz -


    Thank you so much for those instructions. The screw pulling technique finally made sense.


    Chris Baumgardt -


    Yours is the best version/description of the screw pulling technique. Outstanding.


    Jacob Lamberson -

    Why go to the trouble of the red text and red avatar if you’re then immediately going to say “using pliers (or screw)” ????

    Modify the directions to advise against pliers, they’re more likely to damage the printed part.

    Wesley Boynton - Reply

    I managed to strip thee threads on the nut while pulling it through with a screw. Then damaged the printed part trying to rectify the situation. What are my options?

    Thanks n advance,

    Fred Turton

    Fred Turton - Reply

    Hey Fred, you can either glue the nut inside the hole, or re-print the part.

    If everything else fails, you can always purchase the part directly from us.

    Filip M - Official Prusa CS -

    Thanks for your prompt response, Filip. Where are the files for printing? Also how do I order a new part? I think I’d like to have original equipment.

    Fred Turton - Reply

    Hello, you can get the 3d model here: or search for the part in our e-shop. Keep in mind that in order to see the spare parts, you have to be logged into the account that you have used to buy the printer.

    Martin L. - Official Prusa CS -

    Thanks, Martin. I downloaded the file and ordered a new part.

    Fred Turton - Reply

    To seat the two hex nuts, I ran in one of the 40mm screws from the other side, and ran the nut down the threads about 20mm. Then pressed the end of the screw against a hard surface, to in turn press the nut into the pocket.

    Also, do NOT under any circumstance insert the square nuts at this point. They will fall out repeated in later steps. (I lost one…)

    Preston L. Bannister - Reply

    For the second step I would recommend putting some small pieces of tape over the holes that the square nuts went into so they don’t fall out if you accidentally turn your part the wrong direction as you don’t mess with them for a long time.

    William McCulloch - Reply


    You can also use a dab of dish soap to keep them lightly in place as well :)

    Marcin M. - Official Prusa CS -

    I recommend lightly threading some screws into the 4 square nuts until they need to actually be placed. These 4 nuts are a really loose fit, and easy to not notice if they fall out.

    Jeff Powers - Reply

    The four M3nS  nuts are a loose fit in the traps and regularly fall out. However, so long as they are installed just before mounting the X carriage onto the X axis bearings, they can be left out at this stage.

    Steven Woodbridge - Reply

    I agree, these four nuts could have waited until a later stage in the manual, keeping in mind all the different steps that is being done in between.

    Freddy Jay -

    Just put 4 M3x10 screws loosely into the captive square nuts. You’ll thank yourself for the next 44(!!!) steps when you _don’t_ drop the nuts out a million times. In Step 61, these are the “M3x10 screw (4x)” you’ll need.

    Robstar - Reply

    I would agree with Robstar and Jeff Powers. I ended up also lightly threading 4 M3x10 screws into the M3nS nuts because they were falling out. They will be used in Step 61. You’ll end up rotating the motor a dozen different ways before you get there. This will save you time from searching for M3nS nuts that will inevitability fall out if not secured.

    Ernest Yee - Reply

    @Robstar and @erndiggity made a really good point: Use the 4 M3x10 screws to fix the M3nS nuts!

    Jan Albiez - Reply

    I see only 3 places where the square nuts go. What am I missing?

    Stan Hurd - Reply

    Hello Stan,

    Please look closely at the second picture of this step. Yellow arrows.

    Martin L. - Official Prusa CS -

    I see where I missed it. Thanks. I didn’t realize the nut was missing until I finished the whole assembly and was putting the back on. Now I have to take it all apart to add the nut.

    This brings up another question I have. What kind of disassembly of the extruder is required to do maintenance such as changing a nozzle or adjusting the belt tension when it’s beyond the range of just rotating the motor? Do you have any instructions written or this? On my previous printer I made a moveable idler pulley mount for both X and Y axis belts so belt tension can be easily adjusted with no disassembly of parts of the machine. I am thinking of doing the same for this machine. I would appreciate any comments. Thanks.

    Stan Hurd - Reply

    To answer my own question: Read the directions in the handbook! Duh.

    Stan Hurd - Reply

    Super hard to get the screws in place. I finally managed by screwing a m3x20 in to the nut and then tightening it while simultaneously pushing hard on the top of the bolt (the threaded end) with a metal tool. That way i both pushed and pulled at the same time. It still was really hard work to get them seated

    FarbrorMartin - Reply

    The square nuts inserted on this step kept falling out for me, especially the top two. I added a tiny piece of masking tape over them which keeps them from falling out. This saved a lot of dropped nuts, screws are not inserted into them until Step 61. At this step, you can take the tape off.

    George Bell - Reply

    Impossible to get the nut seated into the x-carriage. I’ve tried both screw techniques. The tolerances need to be fixed

    Jane H - Reply

    Finally got the two nuts seated into the x-carriage. It was hard, here’s how.

    Step 1: Take a M3x18 screw. Pushit all the way into the hole FIRST with out the nuts. I did this to loosen it up, and cut down on some of the edges.

    Step 2: Take the nut, screw it onto the M3x18 first, and then push it into its position. Screw just enough so that the screw can reach the other side.

    Step3. Turn the part face down so the nail is facing the floor/surface (I placed a cloth on my desk), use the palm of your hand and slowly lean into it.

    There’s a risk you might break the part, but doing this finally got the nuts seated into the x-carriage

    Jane H - Reply

    Care is needed when using screw pull. I have one nut just spinning and is holding the screw. Can’t get it out. Luckily I have some black petg so I’m printing a new xcarriage

    win some loose some

    Neil Berkeley - Reply

    Three of my M3nS nuts fit into their slots firmly and stayed there, but one (the middle one) fell out whenever I flipped over the X-carriage. The second time it happened I just left it out, you have do a lot of flipping and fiddling with this part over the next ~30 steps.

    It looks like step 44 is where some of these slots will get inaccessible, so right before that step I put the nut back in. Worked out well enough for me! I would only recommend this approach if you’re sure you’ll remember to replace the nut. Otherwise the tape (or other) approaches recommended by others might be better.

    Ian Dunn - Reply

  17. Take the IR-sensor cable and locate the end with the smaller connector. Place cable in the X-carriage, use the small printed overhangs to keep the cable inside.
    • Take the IR-sensor cable and locate the end with the smaller connector.

    • Place cable in the X-carriage, use the small printed overhangs to keep the cable inside.

    • The distance between the connector and the X-carriage should be around 15 mm (0.6 inch). We will adjust it later.

    • Guide the cable through the slot, remember this path, we will use it for other cables as well.

    Given that some of the square captive nuts do fall out when you turn the X carriage over, this step would be better before the nuts are put in.

    David Maher - Reply

    Hi David, the cable will most probably fall out while assembling the nuts.

    Jakub Dolezal -

    They don’t fall out if you keep the part upright, where the nuts won’t fall out.

    Nathan Charlton -

    I use wee bits of blue tack.

    Miles Odonnol - Reply

  18. Following cable management is CRUCIAL for the extruder TO WORK PROPERLY! Read the instructions carefully.
    • Following cable management is CRUCIAL for the extruder TO WORK PROPERLY! Read the instructions carefully.

    • Start by making a small loop just below the extruder motor. Leave a slack about 2-3 cm (0.8 - 1.2 inch) This is comes handy for easier disassembly in the future.

    • Then guide the cable in the "channel" all the way to the back.

    • Slightly bend the cable down to form it around the edge.

    • Also, prepare X-carriage, both M3x10 screws (if you haven't used them already) and the longest Allen key with the ball-end, you will need it.

    DO THIS ON STEP 13. It’s easier that way, just slip them in the hole and let them sit till stip 19.

    “Also, prepare X-carriage, both M3x10 screws from earlier and the longest Allen key, you will need it.”

    Adam Bavuso - Reply

    Excellent point!

    Mihai-Andrei Stanimir -

    This probably would have been more helpful had you posted it as a comment in step 13.

    Douglas Howard -

    Also, when they say make a loop, don’t actually make a loop. It will take up too much of the slack, and you will get to the very final step like I’m at and realize that you have to take the whole thing apart again.

    Matt - Reply

    Hi Matt,

    It is possible to do a loop later, but you need to realize that the loop is necessary.

    Martini H. - Official Prusa CS -

    I had the same issue as Matt, don’t leave too much slack! Make a pretty small loop.

    Charlie Miller - Reply

    3 cm is about 1.2 inches, not 1.6 inches. I’m going to assume the metric is correct for the loop length.

    Matt Burrough - Reply

    Hi Matt,

    The exact size of the loop does not matter all that much, as long as it’s around 1 inch. But yes we’ll correct this.

    Martin L. - Official Prusa CS -

    Loop length? I think it would be much better to say something like,

    Measure 1” (or 1.25” or whatever it would be) along the cable from where it exits the motor. MARK that location on the cable. Place the cable in the channel so that the mark on the cable is right where the cable enters the channel.

    THAT will assure that everyone has the correct amount of excess cable in the “loop” near the motor.

    Roger Garrett - Reply

  19. Following cable management is CRUCIAL for the extruder TO WORK PROPERLY! Read the instructions carefully. Before you assemble the X-carriage, check the nuts in the Extruder body are still in place. The upper nut sometimes falls out. Grab the X-carriage and place it onto the back of the extruder assembly as shown in the picture.
    • Following cable management is CRUCIAL for the extruder TO WORK PROPERLY! Read the instructions carefully.

    • Before you assemble the X-carriage, check the nuts in the Extruder body are still in place. The upper nut sometimes falls out.

    • Grab the X-carriage and place it onto the back of the extruder assembly as shown in the picture.

    • Make sure the motor cable follows the channel both in Extruder-body and X-carriage. In the X-carriage the motor cable will follow the path of the IR-sensor cable.

    • ENSURE NO WIRE IS PINCHED! Then use the M3x10 screw and Allen key with ball end to connect both parts together. If you are inserting the screw at this moment, it will be slightly inclined in the beginning, but it will "straighten up" after few turns. Don't tighten the screw completely, we need to adjust the IR-sensor cable.

    • Turn the extruder to the other side and if needed insert the second M3x10 screw. Don't tighten the screw, we need to adjust the IR-sensor cable.

    • Good job! Grab one extra gummy bear ;)

    After this step, I temporarily lashed the hotend wiring to the X-carriage with a ziptie to avoid bending the wiring around. I left the tail on the ziptie to remind myself to remove it again later.

    Brandon Bergren - Reply

    Step 19: Could not get the screws in once assembled so had to un-assemble, put the screws in before the extruder and then reassemble.

    DaveEP - Reply

    The two 10mm’s should be inserted on step 14 before the extruder.

    Adam Bavuso - Reply

    Using the ball end of the allen wrench the screws went in fine.

    ethan - Reply

    Agree that 2 10’s should go in earlier. Had to undo previous work.

    Niall Brown - Reply

    I think it would have been best if the nut was inside and the screw went in from the outside

    Vadim D - Reply

    100% agree with you Vadim. This part needs to be redesigned for sure. Hopefully Prusa can find a better solution.

    Michael Nolte -

    Hello Michael @master3dprints

    Actually this is meant to be the better solution as it allows the extruder to be easily disassembled. Keep in mind that you sometimes need to disassemble it at least partially for maintenance or upgrade purposes.

    Martin L. - Official Prusa CS -

    Using the ball end on the allen wrench, the 10mm screws went in at an angle at first, and then engaged the nuts and straightened out, just as outlined in the step.

    William Rigsby - Reply

    Yes, as many have said before, insert the 10mm screws in Step 13. Way easier! They also help to align as you put the pieces together, which is one less thing to worry about since your hands are very busy keeping the wires in the right positions.

    Dan Green - Reply

    I agree with Vadim D, maybe the best approach would be just invert in the project the nut and screw position and the screws would be easily tight from the back.

    alexandreberg - Reply

    that might be true, but the goal here is to make the potentianl disassembly of the estruder as easy as possible.

    Martin L. - Official Prusa CS -

    Also, the square nuts fell out several times during this step.

    Preston L. Bannister - Reply

    Y’all should tell us to not eat the extra gummy bears when we divide them up ;)

    Mila Santana - Reply

    I stuck a random screw in the square nuts and kept them there ‘till they needed to be used.

    Penjim - Reply

    In a step the screws are tightened. I tightened them here because the ir cable moves freely after they are tightened so there is no need to wait for a later step. The cable can be adjusted whether they are tightened or not.

    Robert - Reply

    Agreed Robert.

    Mathew Boon -

  20. For the following steps, please prepare: Prusa IR-sensor (1x) M2x8 screw (1x)
    • For the following steps, please prepare:

    • Prusa IR-sensor (1x)

    • M2x8 screw (1x)

    • M3x40 screw (1x)

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

    • Your package includes IR-sensor with black or red PCB. The red is a new version with some hardware optimisations. Filament detection is the same on both.

    I don’t see the M2x8 screw on the Cheat Sheet. Fortunately, I figured out it was the smallest one in the bag.

    Ken biles - Reply

    Hi Ken, the M2x8 screw should be on the bag label, the cheatsheet will be updated soon.

    Jakub Dolezal -

    You might want to point out in the diagram what part of the sensor is the “black PCB” that isn’t supposed to be touched. With the little chip-board itself being black it wasn’t obvious to me.

    Scott Mutton - Reply

    Hey Scott,

    Thanks for the heads up, we’ll make it more clear.

    Filip M - Official Prusa CS -

    Why in this step does it show that black clip on the board and then it disappears in the other photos as I can’t complete step 42 now with it! Help!

    Sarah - Reply

    Hello Sarah,

    I’m not sure what problem do you have with that step. Please contact our tech support for advice, or have a look at step 21.

    Filip M - Official Prusa CS -

    I had the same question as Sarah. Like her, I installed the board upside down at first. :) Sarah, eventually the filament will travel through the black “clip”. For me, it was counterintuitive to install the board with the chips facing down. Fortunately it is easy to turn over and fix the problem!

    Laurel W - Reply

    Is there anyway to override the IR sensor and run the machine if ours does not work? I accidentally touched mine during assembly and I am concerned about the functionality

    James - Reply

    Hey James,

    Sensor is the last thing checked during the selftest, so you can simply ignore it and run XYZ & first layer calibration manually from the Calibration Menu.

    Filip M - Official Prusa CS -

    I didn’t read ahead to notice not to touch the chip board, so mine has been touched as well. Is there a way to safely clean it or order a new one?

    Darel Beghtel - Reply

    Hi Darrel,

    Just continue the assembly and see if the printer passes the Selftest. If not, you can order a new sensor in the online store. (You just have to be logged in).

    Martin L. - Official Prusa CS -

    Watching the Self Test take place I see nothing about the filament sensor being checked. My printer won’t autoload the filament so I suspect there’s a problem with the sensor. Could you advise what sort of voltages I might expect to see on the sensor pins in the on and off state please? Thanks!

    Chris Norton - Reply

    Hi Chris,

    unfortunately, the voltage on the sensor is not a good indicator. Update to the latest firmware and then go to Support - Sensor info, to see how the filament sensor reacts. Then contacts at and send us a photo of the screen.

    Martin L. - Official Prusa CS -

    Thanks for that Martin, should’ve gone for the obvious solution I suppose. It looks like the magnet loaded arm is sticking slightly and not retracting properly when the filament is removed. A fun hour of dismantling ahead! Appreciate the very rapid response.

    Chris Norton - Reply

    My pleasure to assist you : )

    Martin L. - Official Prusa CS -

    I have it working now and appreciate how much easier it is to get to the extruder with the new design! I found that putting a thin nylon washer behind the FS lever (step 6) helped with its free movement.

    Chris Norton - Reply

    The reason we should not touch the sensor is to protect it against static discharge?

    I have a anti static strap from my PC build :)

    Ofer Krupka - Reply

    Yes, that is the main reason.

    Martin L. - Official Prusa CS -

    My Filament sensor didn‘t worked I figured out that My Filament sensor had to be nearly lose.

    Now it works

    Paul - Reply


    Just purchased MK3S and it comes with the black IR sensor. What sort of optimization are between both versions and should’t new purchases come with the red IR sensor?

    Thank you

    Antonio - Reply

    Hello Antonio, the new sensor is slowly being introduced. It’s mainly optimization, the sensor functionality doesn’t change.

    Filip M - Official Prusa CS -

    for those doing MK3S kit with MMU2S go to step 3 3B. MK3S/MK2.5S - extruder upgrade

    Brendan Hall - Reply

    Hi there,

    This kind of shortcut is possible, but we still strongly recommend to read the entire MMU2S assembly manual, at least to check if you didn’t miss some step.

    Martin L. - Official Prusa CS -

  21. Place the IR-sensor on the top of the  Extruder-body and secure it with the M2x8. Make sure the black plastic "u-shaped" part is facing down. Tighten the M2x8 screw, the sensor shouldn't be able to move, but be careful the PCB isn't indestructible ;) Connect the cable, mind the correct orientation of the connector and wires.
    • Place the IR-sensor on the top of the Extruder-body and secure it with the M2x8. Make sure the black plastic "u-shaped" part is facing down.

    • Tighten the M2x8 screw, the sensor shouldn't be able to move, but be careful the PCB isn't indestructible ;)

    • Connect the cable, mind the correct orientation of the connector and wires.

    • Leave a slack behind the sensor like in the picture. Don't create too big loop as it might interfere with the frame. If needed adjust the length by gently pulling/pushing the cable.

    • Ready? Ok, check once more no cable is pinched and tighten both M3x10 screws installed previously.

    Here is an easy tip to use. Cut open the anti static package that contains the PCB sensor (DO NOT REMOVE IT FROM THE PACKAGE). By holding it inside the open edge of the package you can hook it up to the wire connector and then by holding the connector you can put it in place without ever touching the PCB sensor at all. I hope this can help others.

    John H - Reply

    super-like this advice, John H.

    Alan Ostroff -

    Extremely usful and easy! Thanks a lot! :)

    Valentina Mladenovic -

    Consider using an anti-static wrist strap for this step.

    Also, it’d be nice if the directions suggested a method for ensuring cables are not pinched. For me, it was as simple as checking the free movement of the cable sliding through the channel before plugging it into the IR sensor, but the instructions given might be more nebulous for some people who do not have any experience with cable management.

    Wesley Boynton - Reply

    I noticed the end of the X-carriage is designed a little bit longer to prevent damage to the IR sensor if placed with that face downward. Just want to say thanks for that Prusa!

    Heimdall - Reply

    I found that this a great time to verify that the Bondtech gear and the PTFE tube are aligned, mine was not. You can still adjust the gear position.

    Koen Verbeeck - Reply

    Good tip. Thank you.

    Miles Odonnol -

    Prusa: Consider adding the FS-cover assembly step straight after this or after the Extruder-idler assembly. It makes no sense to leave a sensitive component exposed and likely to be damaged when you could cover it sooner

    Rich - Reply

    Hi Rich, there was a reason for this approach. I will update this chapter soon.

    Jakub Dolezal -

    There was trouble engaging the M2x8 screw with its threaded hole.

    What worked: remove the IR sensor. Insert screw only and push down firmly while driving it home. Remove screw. Install IR sensor board.

    Miles Odonnol - Reply

    Be careful not to tighten the screw holding the IR-sensor too much. After I finished building the printer the filament autoload function didn’t work. Using the diagnostic screen I noticed that the state of the sensor was stuck and didn’t change when inserting the filament. Once I unscrewed the screw holding it in place a little bit it started working properly and the value started changing properly between 0 and 1.

    Avi Schwartz - Reply

    Thanks for this tip :) It was exactly my problem, and now it’s solved. So the screw holding the sensor should be barely tightened and the plastic cover comes on the top should hold it in place. Otherwise the opposite side of the PCB (opposite side of the connectors) is lifted up and the sensor always triggers, even if no filament is inside. Now it’s working correctly. Thankfully this part is easily accessible after assembly by undoing two screws on the extruder, so no re-calibration is necessary.

    István Nagy -

    I had the same problem, after building, the filament sensor was always on. I just loosened this tiny screw less than 1 turn and all is well. Don’t stress out over how much to tighten this screw because it is easily accessible on the finished build, you just need to take the FS Cover off (Step 42).

    George Bell -

    I had the same problem, with the sensor stuck on 1. The ball cage was clean, the ball and FS lever were moving freely but something was still blocking the light path through the sensor. In my case the solution was to raise the entire sensor slightly by putting a small piece of card between the sensor and the body before screwing it down.

    I think the root of the problem is that the FS lever is not quite the right shape, and partially blocks the light path through the sensor even when it is fully retracted.

    Richard Herring -

    Hello, @rherring ,

    That is a good solution, an alternative one is to check if the FS-lever does not have an elephant foot or some defect on it, it can sometimes print with a larger area and then it’s blocking the sensor even in 0 position.

    Martin L. - Official Prusa CS -

    In order to test the assembly, I used the Arduino UNO3, as before many of others has struggled with unresponsive filament sensor functionality. So, I used to interconnect 5V pin, the GND, and connected the output of IR sensor to the digital 7 on the Arduino board. Here is the code:

    void setup() {

    pinMode(7, INPUT);



    void loop() {

    Serial.print("IRSensor: ");




    Without filament: IRSensor: 1

    With filament: IRSensor: 0

    Rolands Zuters - Reply

    for those doing MK3S kit with MMU2S go to step 3 3B. MK3S/MK2.5S - extruder upgrade

    Brendan Hall - Reply

    Hi again,

    This kind of shortcut is possible, the MMU2S extruder is different. But we still strongly recommend reading the entire MMU2S assembly manual, at least to check if you didn’t miss some step.

    Martin L. - Official Prusa CS -

  22. Finish the sensor assembly by inserting an M3x40 screw.
    • Finish the sensor assembly by inserting an M3x40 screw.

    • Make sure all the gaps are gone.

    Jesus, that whole assembly was so tense. Glad I finished that with confidence that I correctly followed the directions.

    Wesley Boynton - Reply

    I love that everything is 3d printed. It gives me all kinds of ideas on how to reinforce projects and integrate fasteners.

    Joseph Finan - Reply

    I rip the red wire while screwing during this part…

    I think it’s good idea to add warning, what should I do now?

    Ofer Krupka - Reply

    Hi, sorry to hear that! You can either try to repair the wire if you know how, or you can purchase a new one in the online store:

    Martin L. - Official Prusa CS -

    Somewhere in these instructions, and here might be a suitable place, I think there needs to be a line in neon letters for those people following the instructions to upgrade their filament sensor. It needs to say loud and clear that, after they have added the new sensor, they now have a Mk3S and so must remember to upgrade their firmware to the 3S version.

    I know it’s obvious with hindsight, but I struggled for days with the ‘check wiring’ error during re calibration. I read dozens of pleas for help and they all received ‘check your wiring, make sure it’s connected to the lower pins on the Einsy board or you probably have a faulty sensor’ etc.

    Eventually I found this :

    and bingo, after upgrading the firmware, everything worked perfectly (as did the first sensor I had that I assumed I had inadvertently killed resulting in me buying a second one).

    Andy Timmons - Reply

    Mr Timmons, thanks for your suggestion, we will look into this.

    Martin L. - Official Prusa CS -

    Tightening the screw “too much” made the IR sensor on all the time (when filament not in). This made the autoload filament not to function. After completing the printer complete assembly, via the support menu in the LCD panel you can check the IR status.

    Ganesan Doraisamy - Reply

    I had gaps in the hot end zone of my build and I had to screw the 2 M3x40 (from step 14) screws all the way to make them disappear. Is this alright?

    tudor - Reply

    Shouldn’t the FS-cover assembly (Step 42) be done now to avoid accidental touching of the sensor?

    Sebastian Quiros - Reply

  23. For the following steps, please prepare:
    • For the following steps, please prepare:

    • Hotend fan Noctua (1x)

    • M3x14 screw (3x)

    • Make sure you are using the correct screws.

    • Keep the cable in a bundle for now.

    I have a totally different looking fan in my kit. the fan I have is enclosed and has a shoot coming out of one end.

    it has the following printed on it: PRSUA Research Print Fan 5v

    I will send a picture of it, because it does not fit in any location that I put it. I suspect that I have been

    sent a different fan.

    Mearl Wood - Reply

    Sorry, So Sorry, I found the correct fan motor. I did not explore the box thoroughly and overlooked the second fan.

    I do not need another fan.

    Sorry for wasting you time with my stupid mistake. I’ll try to do better.

    Need more gummie bears!

    Mearl Wood - Reply

    Yes, there are two of them : )

    Martin L. - Official Prusa CS -

    Could (probably is) be my mistake, but looks like I’ve only got two of the M3x14 screws left from the bits for this step. Grabbed one from the spares bag.

    Wesley Boynton - Reply

    Same here with the M3x14. Thanks for the spares, guys!

    Gawain Reifsnyder - Reply

    Also missing 1 M3x14 - looks to be common thing

    Marek Bujalski - Reply

    Hello Marek,

    Check the Spares bag n. 9.

    Martin L. - Official Prusa CS -

  24. For easier fan installation, remove temporarily the black twist tie from the cable bundle and release at least one loop. Then tie the bundle back, see the picture. Before you move to the next step, take an Allen key and GENTLY PUSH the motor cable to the channel to create space for the fan cable.
    • For easier fan installation, remove temporarily the black twist tie from the cable bundle and release at least one loop. Then tie the bundle back, see the picture.

    • Before you move to the next step, take an Allen key and GENTLY PUSH the motor cable to the channel to create space for the fan cable.

    I can’t speak for the Queen’s English, but I know in the USA where I grew up, we called these fasteners “twist-ties.” In this case, you might want to call it a “tie” or something similar, since the distinction between “wire” and “cable” is kind of vague.

    Wesley Boynton - Reply

    We can all understand what they mean and I hate to say it but I don’t think “twist-tie” is proper English. Plus, it’s not like there are two of them on there. No offense meant in any way, just saying I do not think that it is necessary to revise the manual to change that. :-)

    William McCulloch -


    Twist-tie is a thin wire encased in a narrow piece of plastic casing, you often get those with newly purchased electrical equipment to hold the cable together, whereas zip ties are a toothed strips of plastic with a clamp to lock it in place :)

    Marcin M. - Official Prusa CS -

    +1 for “twist-tie”. don’t want anyone removing the black wire from the bundle!

    Robstar - Reply

    Also call them “cable ties”. Agree that “wire” is misleading

    Gerard Sharp - Reply

    Hello, thank you, we will check the terminology here.

    Martin L. - Official Prusa CS -

    Actually, “cable tie” is confusing because of the plastic “zip-tie” ™ - styled cable ties used in a few steps time for cable management.

    So I would suggest either wording like “so gently loosen the black twist tie holding the bundle and release at least one length”…

    Or “gently loosen the wire tie holding the bundle and release at least one length, noting this is the short plastic-coated wire holding the bundle together, and we are not separating the fan wires from the heatshrink”…

    But that is clumsy and way too verbose!

    Gerard Sharp - Reply

    Hi Gerard,

    Thank you for the suggestion, we’ll look into re-wording that step.

    Marcin M. - Official Prusa CS -

  25. The fan has two sides, one has a Noctua sticker. Make sure, this side is facing inside the extruder. First, create a loop on the cable. Make sure the black protective wrap is close to the edge of the fan. See the picture. Place the fan on the extruder and proceed in the following way:
    • The fan has two sides, one has a Noctua sticker. Make sure, this side is facing inside the extruder.

    • First, create a loop on the cable. Make sure the black protective wrap is close to the edge of the fan. See the picture.

    • Place the fan on the extruder and proceed in the following way:

    • Start by placing the fan's cable in the upper channel

    • Slide the fan close to the X-carriage and GENTLY PUSH the cable in using an Allen key. Before you push the fan all the way to the left, place the cable in the X-carriage channel.

    • FINAL CHECK! The fan is oriented with the cable facing up, then the cable goes through the upper channel all the way to the X-carriage. In the X-carriage don't forget to use both channels. Make sure the CABLE ISN'T PINCHED along the way!

    I can’t seem to fit the fan cable in the side channel, the motor cable is taking up all of the space. I’ve tried pushing it in with the Allen key, but it isn’t accomplishing much. Any suggestions would be appreciated.

    Tyler - Reply

    Hi Tyler,

    Try to twist the motor cable to make the bundle more thin. Also don’t be afraid to squeeze the cables into the channel. :)

    Martin L. - Official Prusa CS -

    I found the cables easier to insert when I placed the fan as in the second picture then while holding the wires in place rotated the fan completely out of the way and rested it on the motor. Then I was able to easily use my flat headed tweezers to push the cable into the channel and then rotate the fan back.

    Daniel Friesen - Reply

  26. Use the three M3x14 screws to fix the fan in place. Don't over tighten them, you can break the fan's plastic casing. Also make sure the fan can rotate freely.
    • Use the three M3x14 screws to fix the fan in place. Don't over tighten them, you can break the fan's plastic casing. Also make sure the fan can rotate freely.

    • Note that the screws are "self-tapping" in the printed parts. There are no nuts.

    • Leave the last hole empty, for now.

    Pretty sure I stripped one of these, but had to disassemble for other issues and it grabbed better the second time around

    Kevin - Reply

    Hi Kevin,

    Perhaps there was some excess material or a flash in the threading which cleared itself out. If you encounter any further issues, please let us know :)

    Marcin M. - Official Prusa CS -

    It would have been helpful to me if you would have said “These screws are just screwing in to plastic so don’t over tighten” rather than saying “Don't over tighten them, you can break the plastic casing. “ I was thinking that there was a nut behind these when I am now pretty sure that it is just a plastic hole that the machine screw is grabbing. Maybe not the best choice of hardware? One of my screws started to spin (much to my surprise) rather than snug up. I took it out and managed to get it to grab adequately on the second attempt.

    Ken.Lotts - Reply

    Hey Ken, we’ll add a note about those holes and not using nuts.

    Filip M - Official Prusa CS -

    Are you sure it is m14 and not m10? There doesn’t seem to be enough depth for the m14

    Tim Harper - Reply

    Hi Tim, it should be m3x14 - double check the holes for debris, perhaps something is blocking the screw.

    Filip M - Official Prusa CS -

    Yeah, thanks. I had mistakenly switched m3x14s for m3x20s. It is definitely m3x14

    Tim Harper -

  27. For the following steps, please prepare: Extruder-idler (1x)
    • For the following steps, please prepare:

    • Extruder-idler (1x)

    • Bondtech WITHOUT the hole for the grub screw (1x)

    • Bearing (2x) might be stuck inside the gear

    • Shaft (1x)

    • M3n nut (1x)

    • M3x40 screw (2x)

    • Idler spring (1x) place the spring on the screw

    I couldn’t find the bearings anywhere.

    Then I figured out that the next step (28) had already been performed prior to shipping out. Should adjust these instructions to accomodate for that.

    Per Farny - Reply

    It was at this step that I discovered there was an additional (spare?) lock screw for the locking bondtech gear in the bag. I’m not pleased with all of the spares popping up outside the spares bag.

    Wesley Boynton - Reply

    Hello Wesley,

    That grub screw is not used in this step, therefore it is not mentioned here. The screw is used in the other bondtech gear that you put on the motor shaft. However, we will look into adding an additional note here.

    Marcin M. - Official Prusa CS -

    At this step I found a tiny black screw in the bag. Like the one used to fasten the extruder pulley. I hope this is a spare. Or did i miss a step?

    Christoph - Reply

    No worries, there is indeed one spare grub screw in the bag.

    Martin L. - Official Prusa CS -

    The shaft seems about 4 mm too short. I had to centralise it carefully so that each end of the shaft was the same distance (2 mm) inside the outer faces of the supports.

    Mark Standeven - Reply

    Any idea where to get a replacement bearing? I dropped one of mine and it bounced off the edge of the earth.

    Steve Faletti - Reply

    Hi Steve,

    Oh, now you need Heracles and ask him to bring it back for you! Or, head to our online store. To see the spare parts, you just have to be logged in to the account that you have used to purchase the printer (same email address).

    Martin L. - Official Prusa CS -

    I could not find the shaft unless it is already in the bearings.

    Paul - Reply

    Hi Paul, the shaft could be hidden inside one of the gears.

    Filip M - Official Prusa CS -

    for those doing MK3S kit with MMU2S go to step 11 3B. MK3S/MK2.5S - extruder upgrade

    Brendan Hall - Reply

    Hi there,

    This kind of shortcut is possible but bear in mind that failure to follow the full MMU2S assembly manual will lead to problems with the calibration and a lot of frustration.

    That is why we recommend gaining experience with the printer without the MMU first and upgrading afterward.

    Martin L. - Official Prusa CS -

    On the second picture there should be a Turquoise (?) arrow pointing at EACH of the two screws. Plus, the instruction about the spring should say

    Idler spring (1x) place the spring on the ONE OF THE screws

    The way it is now it looks more like there’s ONE screw (to the left) and then to the right what THAT screw should look like AFTER the spring is added to it. And, yes, I know that the text says TWO screws. But a change to the pic and the text about the spring would just make it that much clearer. Also, a few steps later, Step 31, someone noted their confusion about the screw with spring. So I’m not the only one who was a bit confused. In fact, it doesn’t seem necessary AT THIS STEP to mention placing the spring on the screw. Leave that until the screw-and-spring is actually used in a much later step.

    Roger Garrett - Reply

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

    If you find that a bearing is missing check inside the gears. They can work their way inside during shipping.

    agregorie - Reply

  29. Take M3n nut and place it in the Extruder-idler. Use the screw pulling technique. Insert the pulley in idler as shown in the picture.
    • Take M3n nut and place it in the Extruder-idler.

    • Use the screw pulling technique.

    • 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. The shaft should be flush with the surface of the printed part.

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

    be sure shaft pin is fully seated equally , if its not, the gears may slip and material will not be properly fed to hotend.

    clicking and slipping may occur.

    Andreas Stevens - Reply

    I’m thinking an adjustment “anvil” is in order. A flat base with a small “bump” that you can press the axel against, to get perfect centering. I’ve got about 1/32 of an inch depression on either side. (Sorry, it’s been so long since I’ve used my calipers, that the battery is dead, so I can’t convert to mm.)

    Ken biles - Reply

    How do I know the shaft pin is fully inserted? It is a very tight fit and doesn’t seem to be in the “right side” (according to pics) deep enough.

    Dan Thompson - Reply

    Hey Dan,

    You can put the shaft “on top” of the part, this will let you see how far it should go before you insert it. Alternatively just compare it with the picture. You can also try pushing it further down with an allen key.

    Filip M - Official Prusa CS -

    So when I proceeded to step 31, I could see/feel the shaft pin impacting the side of the extruder body, so I took the extruder idler assembly back off and forced the shaft pin in deeper until there was about 1mm relief on both ends.

    Dan Thompson - Reply

    I used an Irwin Quick-Grip hand clamp to get the shaft into the idler. It worked really well.

    Keith - Reply

    Excellent idea!

    Paul alvarez -

    Yes Dan about 1mm under flush on both sides of the pin is right.

    David Maher - Reply

    was anyone able to do this with their own hands. I had to use mallet in the end. it broke the part and had to reprint it in PLA and even that was hard to get in. i used the mallet on that too and it fit.

    Kirby Downey - Reply

    Mine slid in with minimal force pushing against a table side. Then, I used an M3 screw with a nut as a stopper a few mm from the end to push it into the correct centre position, i.e. so a few mm from flush from both sides

    Ante Pettersson -

    I do not recommend using a mallet at all for this assembly!

    Heimdall -

    I was able to work the shaft most of the way by hand. To center placement of the shaft, I clamped an allen wrench in my workbench vise to use as a punch. Then gently tapped the opposite flange with a light nylon hammer.

    Kevin Danenberg - Reply

    I used a screw to push through both holes a bit to clean them out so pin would slide in yet still be tight. The threads on the screw act as sand paper to help with this. then used calipers to insure pin was centered

    Dean George - Reply

    “Use reasonable force” usually means “a notable amount” rather than “with restraint.” I’d adjust this to say “use a cautious amount of force” instead, or even better just “be careful not to over-tighten and break the printed part” like usual.

    Wesley Boynton - Reply

    I pressed the shaft against the workbench to get it all the way in, and then used an M3x40 screw with a rubber mallet to tap the shaft just slightly, so that it was sitting the recommended 1mm inside the opening of both ends.

    Wesley Boynton - Reply

    Turns out that while my method was really good, it doesn’t eventually work out if you’re an idiot and put the gear on backwards.

    Don’t put the gear on backwards. Removing the shaft without breaking the part is really hard. Fortunately, this part can be glued back together without much issue — it’s not a “moving” part, in the sense that it’s basically just a door hinge. I’ll still print a replacement once I’m up and running, perhaps in another color as a nice reminder every time I look at my printer to RTFM :)

    Wesley Boynton - Reply

    Be careful with this step. The shaft is a very tight friction fit. Attempting to push the rod into the printed part actually put a hole in our table.

    Jeffrey Foley - Reply

    Mine was impossible to get in, I put the pulley in the printed part without the bearings and used a rubber mallet with the other side against a table (shaft up going through the non-flat side to the flat side) and hit it a few times to get it just poking out the other side. Then, removed the pulley and put the bearings in and kept doing the same thing using the pulley as an insert to stop the part from breaking. Once I got to the end I used a hex key to get it situated in the middle. Worked like a charm, could also be done with a small trigger clamp, it just kept going in at an awkward angle as the force was so great that it kept warping the rubber on the clamp so it pushed sideways a bit. They need to be made to not be so tight but if you get a tight one, this works IF YOU ARE VERY CAREFUL!!!

    William McCulloch - Reply

    Update,- Better yet, use a small gauge round file to file the tops and bottoms of the holes as those are the problem areas, I put it in the wrong way around the first time and had to use the hex key and a mallet to get it out as the clamp kept losing grip. Just file it if you can, much easier BUT only small amounts at a time. Either way works but the latter is the easiest if you have the tools.

    William McCulloch -

    I too had a lot of trouble getting the shaft in, in such a way that the space to the end was about equal on both sides. What I ended up doing that worked beautifully was to snip off a tiny 1mm piece from the filament I had to cut off in an earlier step. Putting this on my table, I was able to push the rod down, and it seated perfectly.

    Daniel Bonomo - Reply

    Hello Daniel,

    It’s a nice way to do it, but that said, the rod doesn’t have to be necessarily put in the idler equally on both sides. As long as you are sure it will stay in place securely, you will be fine :)

    Marcin M. - Official Prusa CS -

    I used an M3-40 screw as a round file and cleaned the insides of the holes with the threaded end. After cleaning the holes the shaft went in fairly easily. I held the idler as I pushed in the shaft to help support the printed flange. It worked well.

    Andrew Cimo - Reply

    I did the same thing as Andrew Cimo and it works great. I also started by threading a (very) small amount and then pulling it out to rip a little at a time out at the beginning. Then you can use the M3-40 screw as a file until it slides in and out smoothly on both sides. There will still be friction but you can push it in with your finger.

    Joseph Finan - Reply

    yep. broke the part trying to seat the pin. Looks like my build is on hold until I can get a new one

    Jack - Reply

    Hi Jack, Here is the link to the online store: : )

    Martin L. - Official Prusa CS -

    Can I just say here, how much I HATE whoever it was who made this SHAFT! It’s exactly the right size to be impossible to seat correctly.

    Is there a reason this thing is 2mm shorter on both sides, it would be SO much better, if the shaft was the SAME length as the the distance from outer edges of the idler. Now I’m left trying to force the shaft PAST the outer plastic, but not TOO far, lest it poke out the other side!

    I’m taking an early sweet, I’VE EARNED IT!

    Christine - Reply

    So good to see that I was not the only one having trouble getting in the shaft properly. I’ve used a small round precision file to smooth the hole after that I was ready to push the shaft in with reasonable force ;-)

    Meta de Mul - Reply

    If you are in the USA, you can use a #31 drill bit (.12 inch) to clean out the bearing shaft holes. Just do it carefully by hand with a pair of pliers holding the end of the drill bit. The shaft will fit with a snug friction fit.

    Mark Gaboriault - Reply

  30. Let's use this opportunity to check once more the proper alignment of the filament and the Bondtech gear.
    • Let's use this opportunity to check once more the proper alignment of the filament and the Bondtech gear.

    • Push the filament from the top, through the Bondtech into the PTFE tube.

    • Check the alignment and if needed, adjust the position.

    • Tighten the screw, but be careful, you can easily strip it.

    • REMOVE the filament.

    How far in the PTFE tubing should the filament go?

    Zeshan - Reply

    That dos not matter. Important is to have the Bondtech gear aligned to it, as shown on the picture.

    Martin L. - Official Prusa CS -

    It should be made clear in the instructions about depth!

    Mine only makes it in by 0.4mm ?

    Gavin - Reply

    I had the same worries when assembling my two S-upgrades. But then I stopped thinking about what is not written in the manual, focussed on what is there and both printers are now working well. ;)

    Martin L. - Official Prusa CS -

    My filament doesn’t flow easily into the tube. It actually gets stuck on the edge. I’m not sure what steps to go through to fix this since everything is together. It looks like the shaft is slightly over the opening.

    Is this okay? Is the filament supposed to easily flow down into the extruder tube?

    Kristian Boruff - Reply

    Hello Kristian,

    On which edge does it get stuck exactly? In this step we are aligning just the the teeth of the bondtech gear with the inserted filament. After finishing your E axis assembly, the filament should get centered above the PTFE tube after closing and tightening your idler hatch.

    Marcin M. - Official Prusa CS -

    “You can easily strip it” is referring to the screw’s head rather than the screw’s thread?

    Mark Wheadon - Reply

    Hey Mark,

    It’s referring to the head.

    Filip M - Official Prusa CS -

    it seems like the bondtech is pressing into the tube that the filament is supposed to go through… is this correct? if not, how do I fix it?

    for reference:

    Julian - Reply

    Hey Julian,

    You can contact our tech support - we might have sent you PTFE tube that’s a bit too long.

    Alternatively you can trim the end of the tube a bit.

    Filip M - Official Prusa CS -

    My tube looked exactly like Julian’s as well. I took the spare PTFE tube, cut 1.7mm from the bottom and re-shaped it. It fits perfectly now. My previous tube was mangled at the top because of this and unloading was not working most of the time without force.

    Clay - Reply

    I got hung up on this step. My PTFE tube was maybe 0.5 - 1 mm too long so that the bondtech gear pressed down a little bit on the rear side. Because it was not much length, tech support advised me to scrape down the end of the tube with a craft knife until it was not touching the gear. After that, I found that the filament still did not go into the tube - it seemed the front to rear alignment (not the lateral alignment described here) was wrong. But tech support pointed out that installing the idler in the next step will resolve this, so don’t worry about it.

    Laurel W - Reply

    Did the installation of the idler gear work? I am having this issue as well and am concerned about spending another couple of hours just to have to take it back apart again for a bad, misprinted part.

    Brad -

    I want to provide a follow up to my comment in case anyone is here with the same problem. I had the issue where the filament was pushed too far forward go down into the tube at this step. It was aligned with the gear, but pushed the other direction (no adjustment in this direction) and hit the outside lip of the white tube. Tech support assured me it was fine and to proceed. So I did. When it came time to feed the filament during initial calibration/setup, I was nervous that I’d be tearing it all back apart. To my joy, the filament fed smooth as butter and all was well. The Extruder Idler did it’s job as support said it would and pushed the filament into place.

    Brad -

    The gear is not rotating freely, even when I’m trying to rotate with my finger it’s still not easily rotating. What could be the reason for this and will it be a problem ? Could it be because of over-tightening the screws ? When I putted it on the motor on the motor I checked it and it was rotating freely. Also the filament doesn’t go inside the tube more than 2mm.

    Valentin Chernaev - Reply

    For everyone who’s facing the same problem and it’s wondering as well, and since I haven’t got any reply in 3 days, here’s the update:

    I have completed the assembly and I passed the calibration tests and printed my first miniature figure. Everything looks fine, I think I just need more precise calibration, but there is no issue with the fillament. Later in the next steps you’ll add second gear that helps this one to push it to the extruder and it’ll be fine

    Valentin Chernaev -

    Hello Valentin,

    Indeed, the gear is not supposed to rotate freely. The motor shaft rotates a little, step by step. And the gear is supposed to be firmly tightened against the shaft.

    Martin L. - Official Prusa CS -

    Tighten the screw, but be careful, you can easily strip it.

    It would be nice if this tip was present in the earlier steps when we are tightening this screw since the tolerances are quite small.

    Miguel Barroso - Reply

    Hello Miguel,

    good point, thanks for the suggestion.

    Martin L. - Official Prusa CS -

    Tighten the screw, but be careful, you can easily strip it. This is important. ;-)

    I think I stripped mine a bit, however I managed to get i thight again. How will I be able to tell if it is thight enough? The gear seems to rotate with the shaft just fine.

    Christoph - Reply

    Hello Christoph, if the gear rotates with the shaft and does not slip. it’s all good.

    Martin L. - Official Prusa CS -

    My PTFE tube seems to be a bit too short. It does not even barely touch the Bondtech gear, as shown in the picture of this step. When using flexible filament, this is always leads to filament tangled around the driven Bondtech gear. The filament i am using is Treed PureFT (Shore A-D: 95-35).

    Kai Fricke - Reply


    The shorter PTFE tube is not a big problem. If the filament is properly fed into the PTFE tube without blocking the movement, it is fine. The problem could be with flexible filament - there we recommend

    - Reduce the print speed - off 40% or more

    - Reduce the press on the springs on hobbled pulleys.

    Martini H. - Official Prusa CS -

    Thanks, that did the trick!

    Kai Fricke -

    I’m not sure , if I’ve done something wrong or the part is flawed, but filament doesn’t go into PTFE tube (for the record, not in the axis that can be regulated by Bondtech gear, it goes out in axis perpendicular to that axis). It stucks at its side, with no amount of gear rotation able to push it in, partially because of bend that gear is making on filament. Is it possible that something is just not aligned, if everything is tightened together? I think that gear is too close to the tube, leading it off center.

    Adam Kaczmarski - Reply

    Hi Adam, perhaps the gears haven’t been properly aligned with the filament feed line, or they are too loose/too tight.

    You can also open the idler and manually feed the filament for the first time, then unload and load through the LCD.

    Feel free to contact our livechat for assistance if you won’t be able to find the solution.

    Filip M - Official Prusa CS -

    I will dismantle and mount extruder again, to make sure it’s all aligned. As for the manual feed part - I’ve cut filament in V-shape to help it go in, but as soon as it went into a tube, it stucks. I am not sure if that is intentional, but I assume it should go at least 1-2cm into a tube, right?

    Adam Kaczmarski - Reply

    Hello! For now, don’t worry about it. If it goes just 2 or 3 mm in, that is OK. Focus on the alignment here.

    Martin L. - Official Prusa CS -

    As mentioned in my comments on the previous steps relating to this. The filament needs to be in line with the grooved inset bit near the bottom of the bondtech gear, not the larger teeth at the top. This should put the top of the bond tech gear approximately in line with the end of the motor shaft.

    Lessian - Reply

    Hi, I could insert the filament from the top and through the gears. However, the tip of the filament can only goes as far as the top of the milky white colored funnel-shaped component. I cannot push the filament further even with force, and even after I have completed Step 32. Do I have to worry about this?

    pcprusa - Reply


    Perhaps there is some small debris stuck in your tube? You can see how to take it out in this guide. Remove it and check for any stuck pieces inside. How to change a PTFE tube - MK3/MK2.5

    Marcin M. - Official Prusa CS -

    The screw keeps loosen, now the motor runs but because the screw is lose it doesn’t catch the flat part of the motor rod. At this point the thread is stripped. How can I order a new screw?

    Victor Centeno - Reply

    Hello Victor,

    head to our online store and log in to see the pare parts section.

    Martin L. - Official Prusa CS -

    My filament fits in and aligned very well, but when it gets to the PETG tube, it hits and won’t go in anymore. Is that fine, will the motor fix the problem?

    Rain Li - Reply

    Hi Rain, it needs two gears to properly enter the tube, otherwise it can kink too much. It’s better to let the motor do it :)

    Filip M - Official Prusa CS -

    I can’t get my filament to go into the tube. It keeps hitting the top edge. Is this ok? Can I continue?

    Jennie Nayton - Reply

    Hi Jennie,

    At this stage, it is OK. Just make sure the hobbed groove of the gear is in line with the PTFE tube.

    Martin L. - Official Prusa CS -

    Cut your piece of filament with a beveled end on the part going into the extruder and it will slip right into the bowden tube, even though this is just testing the gear itself and not the filament going into the tube. Remember it forever, loading filament, bevel the filament end.

    Ronald Smith - Reply

    We approve this message.

    Martin L. - Official Prusa CS -

    I am also having a difficult time getting the filament to slide into the ptfe tube. It’s aligned well, but it seems to just get stuck or stops feeding through when it gets to the tube. I hope that after installing the next pully, it will help with the alignment of the filament. Please let me know if this is a problem, and if so, what I can do to resolve this issue.

    Thank you!

    Max Steinman - Reply

    Hello Max,

    In this step, we are only aligning the filament path with teeth of the Bondtech gears. At this point, you don’t have to worry about anything else :)

    Marcin M. - Official Prusa CS -

    JUST IN GENERAL: It would be nice if there were a checkbox next to each instruction so that we could check each one off as we complete it. Similar checkboxes on the printed manual would be nice as well.

    Roger Garrett - Reply