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Definitive Guide to Idler Reassembly and adjustment?  

Trusted Member
Definitive Guide to Idler Reassembly and adjustment?

Hi, I'm having a hard time with this part of the process of opening my idler door.  I've found a few articles about opening the door, but I'm having trouble with the part about closing it 🙂

(if this would be something to send directly to Prusa as a suggestion, please let me know.)

This is especially concerning as my problem with extrusion may have come from the choices I made while tensioning the idler.

So far, I have found this thread (2018) with some back and forth about which is the "best" way: Extruder Idler Adjustment – Hardware, firmware and software help – Prusa3D Forum (

My understanding is that originally the guide said to use the physical position of the bolt head (flush with the plastic).  

Then there is this video (2018) which is simply titled: "How to improve the quality of your 3D prints...".  Going by the title, I wouldn't have found it through a search.  The Idler information starts at 54 seconds.  (56) How to improve the quality of your 3D prints on the Original Prusa i3 MK3? - YouTube

This video (from Prusa) says to start extruding and tighten the tension bolts until they catch.  (is there a way to tell the system to just start extruding?  Right now I run the "autofill" process and I keep having to tell it the color isn't right.)

So there is some mixed information out there, and the information that is in that video, I would like to see that a knowledgebase article.  

The advice about the screw heads came from Step 17 of the Extruder Assembly AFAIK: Prusa Knowledge Base | 5. E-axis assembly (spiral wrap) (

So: it seems like idler tension CAN be a problem, as it is covered in the YouTube video released by Prusa.  If this is true, I think it would be good to have it included in the knowledgebase.  

Here is an incomplete list of articles where the idler door is opened, but there is no reference to how to re-close the door with the correct tension: 

Prusa Knowledge Base | Removing filament from extruder manually (

Prusa Knowledge Base | How to replace a PTFE tube (MK3S/MK2.5S/MMU2S) (

Prusa Knowledge Base | Checking/re-aligning the Bondtech gear (MK3S/MK2.5S) (

Posted : 27/10/2020 1:05 am
Illustrious Member
RE: Definitive Guide to Idler Reassembly and adjustment?

There's no magic way to set it, as far as I know. It will vary based on your assembly, e.g., how tight you cinch the door hinge screw is a major game changer and can make the entire thing not work. 

The door should swing pretty freely. Any friction will affect the idler tension.

Did you lubricate the idler bearings? No? Do it now. Get the intergear mesh teeth while you are at it.

Did you place the set screw on the motor shaft flat?

Did you really align the gear teeth to the actual filament path? I mean centered, within 0.05 mm.

If you did all that, the extruder will like you more, and will be kinder to you.

Do you have an enclosure the printer sits in? Well - look up filament melting above the PTFE tubes on this forum.

Now -- close the door, and preload the tension screw (or screws on a Mk3) until the spring just starts to compress.

Preheat the nozzle to 190  to 200 (intentionally cool), then load some fair quality PLA. 

Once loaded, use the MOVE AXIS control to extrude filament 50mm at a time. If you hear any clicking or filament isn't loading and coming out the nozzle, increase the tension by turning the screw 1/2 turn. 

As soon as the clicking stops or filament is smoothly flowing out the extruder, you have it set correctly for that filament.  

Then, as you print your first few prints, sit by the printer and watch and listen. Adjust tension as needed. You'll get better with experience. 

Other filament spools may need fine adjustment. Anything FLEX will probably need less tension, some harder filaments (PETG or PC) will need more tension.

This post was modified 12 months ago 2 times by --
Posted : 27/10/2020 6:34 am
Trusted Member
Topic starter answered:
RE: Definitive Guide to Idler Reassembly and adjustment?


This is great.  Please add to knowledgebase!  🙂

Seriously, that's what I'm talking about, having this information documented, even if the method isn't precise, include some advice.

Posted : 27/10/2020 7:33 am
Trusted Member
Topic starter answered:
RE: Definitive Guide to Idler Reassembly and adjustment?


I added your advice to the comment section for the MK3 E-Axis Assembly here:

Prusa Knowledge Base | 5. E-axis assembly (textile sleeve) (

Thank you again.

Posted : 28/04/2021 2:35 am