The existing 626 bearing based idler may not fit the timing belt pulley on the Printruder. This is a redesign based on the narrower 688 ball bearing (4mm depth) that will fit the pulley.Thanks to Zaggo for the awesome Printruder IISubstitute the 688 ball bearing for the 626, use a M8x20 bolt.I have also included a sketchup template of the idler block in case you want to try other sized ball bearings. I have also added a thin 1mm wall to the bearing end of the idler so that it prints better, and provides additional reinforcement.