I have printed quite a few of these, and with the correct setup printer it's a perfect fit - for a normal 608 bearing that's about 6.97mm wide you should be able to push it and the M8 bar into the slot with some force, but by hand. So it's possible you are extruding too much plastic if the bearing does not fit at all. If you need a sketchup file, take a look at my Bowden extruder, that has the file for this Idler.