This replaces the smooth pulleys where the belt teeth instead of the smooth back run against them. Motion is smoother and quieter than before with these. I couldn't find a commercial with the right dimensions, so I made my own.

I printed mine in PLA at 0.2 layer height, without supports. Eight are required in total, but print one first to check fit and tolerances.

To install, first loosen both belts. Each stepper has four bolts in slots that hold their bracket in place.These only need to be loosened 1/2 - 1 turn. Next loosen the bolt on the end of bracket that applies pressure against the stepper.

Each pulley is held on with a single both and washer. Be careful when removing the stacked pulleys. There is a small spacer between the two that can stick to the bottom of the top pulley when you remove it. This space is important to ensure the bolt only applies pressure again the center hub of both pulleys.

I remove the bearings with a small he key pushed in from the opposite side. Gently push around the bearing's outer edge to nudge it out. Be sure to save and reuse the small aluminum spacer in the middle. Remove the second bearing the same way.

To assemble the new pulley, press in a bearing into one end. Then drop in the spacer and press in the second bearing. Reinstall the pulleys, making sure to replace the small spacer between the stacked pulleys.

Reinstall and tension the belts, and test.

The original bearings are 686-2RS 6x13x5mm if you need to replace them.