Reinforced MMU2S Extruder Idler

update: This part was changed in February 2021 to increase the arm length. If you have that newer variant, you should print the 2021 version. 

The shaft that comes with the idler gear is about 2.3mm too short (see cover photo - shaft is recessed the same amount on the other side) and so has a tendency to distort its seat in time. This slow distortion resulted in failed filament changes (IR filament sensor got out of calibration and stopped detecting filament). I replaced it twice before finally creating this fix. If you are experiencing filament change failures, remove your idler door and look in the hole where the idler shaft goes. In my case the idler shaft was almost 50% sunk into the seat.

To address this, I enlarged the holes to a full 4mm so a brass sleeve can be inserted (found at the local hardware store). This strengthens the assembly in two ways: 1) larger diameter so more surface area to distribute the force and 2) effectively extends the shaft so, again, more surface area to distribute the force. #2 could be accomplished simply by buying a 22mm shaft to replace the 19.7mm that comes with the kit but this is working for me.

This is a remix of this:
https://www.thingiverse.com/thing:3609270
which adds the brass only to the pivot point (which has never failed for me but I figured may as well while I'm at it).

Print instructions

I increased the number of perimeters to 3 to increase strength. I used 50% infill but there really isn't any infill to speak of in the places where we are concerned once you increase the perimeter count.

My gcode comes out better than the image suggests. I gouged the corners a bit while I was cleaning out strings from the observation window (been very humid here lately).

Getting the brass inserts to size is a bit tedious. I first cut with a copper pipe cutter then used a sheet of sandpaper on a flat surface to sand to final dimension. You need three: 2 short for the idler seats and one long for the pivot point. To help grip when sanding the two short pieces, I used a spare screw from the Prusa kit with two nuts locked together at the desired height. Then using a circular motion, sanded down until I started hitting the screw.