MMU2S Setup & Inspection

Updated 1 week ago ​by Martin Lexa

After successfully finishing the build of your Original Prusa MMU2S upgrade and going through the Preflight check chapter of the manual, there are still a few things you might want to check before putting your newly upgraded printer to use. This article focuses on the most common mistakes of the assembly and mechanical calibration of the Multi Material Upgrade unit, mounted on the frame.

Before you decide to install the Multi Material Upgrade on your printer, make sure your MK3S or MK2.5S is in perfect shape and prints without any problem if the filament is fed directly into the extruder without using the MMU.
-> Your extruder Bondtech gears have to be perfectly centered, tightened and clean.-> Your hotend and the PTFE tube(s) have to be clean (see this article if that's not the case).-> You also have to be using the latest version of our PrusaSlicer, have the latest firmware version flashed in the printer RAMBo board as well as in the MMU unit logic board.-> We strongly recommend using our Prusament filament, as it is manufactured with the diameter precision tolerance of only 0.02 mm compared to the industry standard of 0.05 mm. We can also guarantee its perfect ovality and provide other extra features related to the usage of the filament.
For more details about the Multi Material Upgrade components and their place in the assembly, you can consult our 360 Viewer.
better pic w/ opened MMU and captions


In the first section, we will go through the most common assembly related problems.

Idler body assembly

  • Idler bearings rotating

The five bearings that are protruding from the idler cylinder must be centered and spinning freely on their shafts. If that is not the case, you can always remove and inspect the bearing after pushing the shaft out using a 1,5 mm thick metal rod/wire or an Allen key that is at least 100 mm long.

if one of he bearings does not rotate or even appears to be damaged, this guide will help you replace it.
  • Idler rotating

The idler may sometimes be incorrectly centered, which means that it is hitting the idler body either on the left or on the right side - the gap between these two parts isn’t big enough (see the light blue spacers in the picture below). You can check that by rotating the idler by hand when the printer is off. The solution is to move the idler barrel a little bit away from the idler body and thus extend the gap. Around 1 mm of space on both sides is more than enough.

  • Securing the idler on the shaft

The idler cylinder has to be properly tightened by two M3x10 screws (that act as grub screws/set screws) on the MMU extruder motor shaft.

Be extremely careful during the tightening! If these screws are too loose, you will have problems with loading and unloading of the filament, and if you overtighten these two screws, the idler will crack.

Keep in mind that these two M3x10 screws might also get loose over time, so check the tightening every few dozen print hours. Another potential issue for you to verify is whether the two screws are tightened against the flat part of the extruder motor shaft. See the last bullet point of this step in the assembly manual.

this guide will help you to verify that.

Pulley body assembly

  • Hobbed pulleys alignment

The filaments are being pushed through the MMU unit using 5 hobbed pulleys mounted on a long pulley motor axis. The grub screws of these pulleys must be facing to the left, away from the Pulley motor. The pulleys must be perfectly aligned with the filament holes, as shown in the photo below, and well tightened by the small black grub screw against the flat part of the Pulley motor axis. If any of these 3 conditions are not met, the filament will be loaded or unloaded slower than expected.

Over time, filament dust may accumulate around in the teeth of the pulleys. The best practice is to clean them now and then with a mini-brush or a similar tool.
  • 19 mm PTFE tubes positioning

After the filament is grabbed by the hobbed pulley, it is pushed into one of the five 19 mm (0.75 inch) long PTFE tubes. These tubes are chamfered on one end, which provides some extra space for the unloaded filament, as the tip might be slightly thicker than the rest of the filament.

That is why the chamfered end must be facing out, away from the pulleys, as explained in this step of the Assembly manual.

  • Rear PTFE holder overtightened

Rear PTFE holder plate is the part shown in the photo below, you'll find it from behind where it holds the long PTFE tubes coming from the MMU. Before tightening the four M3x18 screws, make sure the half-circular grooves are perfectly aligned with the half-circular grooves of the pulley body. Also, make sure the screws are not tightened too much. The idea here is to avoid any deformation of the long PTFE tubes, which would cause unwanted friction of the filament.

If necessary, clean the little grooves with pliers or a small file.

Also please note that all of the long PTFE tubes have to be inserted all the way in. There is a circular hole for each PTFE tube in the pulley body, you should be able to push at least 0.5 to 1 mm (0.02 - 0.04 inch) of the tube inside the hole and thus the PTFE tube should hold in the MMU unit even before the rear PTFE holder is installed.

The corresponding steps in the assembly manual are these: Step 13 and Step 14.

Selector blade assembly

Make sure the little blade that is supposed to cut filament strings is secured in place with two M3x10 screws that are tightened enough. The blade also has to be placed exactly as shown on the photo below. Otherwise, the blade can slip out of its socket and block the movement of the selector. See this step in the manual.

If the blade keeps slipping out, a small drop of glue can help as well.

Spools and buffer positioning

As mentioned above, in case the PTFE tubes are squished or bent too much, it might cause excessive resistance to the filament moving inside. The spool holders shouldn't be too close to each other. Ideally, the spools should be positioned as described and shown in the handbook:

If you decide for any reason not to use the long PTFE tubes, or not to use the filament buffer, you have to make sure the filaments will not be tangling one into another.


Idler screws tension

Just like the extruder idler spring, its MMU2S counterparts need to be tightened just right. If the idler springs are over-tightened, it may cause the idler motor to skip. Over-tightening also causes the pulleys to be choked with filament. To find the perfect tension, tighten the screw about 0.1 mm to 0.4 mm under the edge of the hole on both sides of the idler body and then follow these steps:

  1. All 5 filaments have to be loaded to the hobbed pulleys position.
  2. Move the selector to the far right (5th filament position) and back.
  3. If you hear clicking, loosen the screws a little bit and try to move the selector again.
  4. Repeat until there is no more clicking.
If you have a permanent marker, you can make a thin line inside of the oval hole to mark the correct screw position.

If your MMU 2.0 makes a loud clicking noise as the selector changes position, and if you see that the idler barrel was unable to rotate into the correct position, the idler is likely over-tightened. Loosen the two screws (one on each side of the MMU2 unit) a little bit and try again.

FINDA calibration

See if the FINDA probe is correctly calibrated as described in this article. Always keep in mind the selector must be free of any leftover filament and dust.

IR sensor calibration

Since the 3.7.2 firmware version, the printer is able to test the functionality of the IR sensor by itself during the Selftest.

Check that the sensor is working properly. Open LCD menu -> Support -> Sensor info and you will see the IR value. If the filament is not loaded, the value should be “0”. If the filament is loaded in between the Bondtech gears, you should see “1”. If this is not the case, perform the calibration of the IR filament sensor.


If you are sure that all potential issues mentioned above have been avoided, feel free to try these testing g-codes (sliced for your MK3S + MMU2S upgrade):

2 filaments, 12 color changes, 25 minutes


3 filaments, 16 color changes, 25 minutes


4 filaments, 16 color changes, 25 minutes


5 filaments, 16 color changes, 25 minutes


The MMU2S upgrade is only available as a kit because, in order to operate it successfully, you need the experience from the building procedure.


Find out more troubleshooting tips here: Community Forum | Assembly Manuals | Youtube channel |

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