Assembly step 2

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These are the instructions for the SO1 and are here for historical reasons only.

If you are building an SO2, please see the instructions at

(click the picture to download the PDF file)

Smooth Idler Assembly:

The Idler Wheels align the belts so that they can grip the pulleys. When assembling the wheels a precision washer (1mm thick) goes between the bearings. The precision washers are specced to be 1mm thick and provide a surface for the races to contact, keeping everything tight.


  • Insert a bearing into the open side side. These are force fit tolerances so you might have to push hard.
  • Press the bearing until it’s seated against the inside lip of the idler drum.
  • Slide an M5 bolt through the bearing (outside in) — you’ll use this to align the precision washer and the other bearing.
  • Slide the precision washer over the bolt and let it slide all the way down until it is touching the bearing you just inserted
  • Slide the other bearing over the bolt and let it slide down until it hits the rim of the drum.
  • Press the second bearing into the drum just as you did the first until it is tight against the precision washer
  • When the assembly is complete, you can remove the M5 bolt

Note: do not forget the precision washer!

When mounting the idlers onto the motor mount plates, mount them in the "up" orientation shown below. This will give you the proper spacing between the plate and the center of the idler. The proper spacing allows the idler to align with the pulley, so your belt runs in a straight line:

Mount idlers in the "up" position for proper spacing

Bill of Materials

For one Idler Wheel Assembly (referred to as MSK01-06A). Four total will be needed.

Sortable table
Item Qty. Part Num. Title
2 2 MSK01-07 5mm x 16mm x 5mm bearing
3 1 MSK01-08 Precision (Shim) Washer (1mm thick)
4 1 MSK01-06 Smooth Idler Drum

User Submitted Notes

(This is the section to add your own notes pertaining to this sub-assembly) If you run into a problem or find yourself asking a question, please post the solution here to help the next person who has the same problem/question)

Inventables build images

Required parts
Use a bolt to line up the washer and second bearing
Front and back view of completed pieces

Next: Assembly step 3