SummaryBased on my original bearing ( http://www.thingiverse.com/thing:1023433 ), with a few improvements 1 - smooth pulley - i found with the toothed pulley the belt occasionally jumped over a tooth 2 - I used bigger ID ball bearings, the new bearings are 13mm outer diameter, 5mm wide & 6mm inner diameter - makes for a much stronger axle on the pulley. 3 - Added Mk4 4 - with bearing retainer clips (the outside circles) I also included a pulley with the original 4mm ID, if you want to use the original size 4mm ID ball bearings