The bearing I just received (S608z) seems to be of good quality (maybe 4 or 5). Seems tight and good ending. But the dimensions given do not correspond to the real ones. The bearings I just received were 31.5mm OD and 12mm shoulder width (10.43mm for the width of the rotating pulley). The shank appears to be closer to 0.25 inches (6.4mm). And the groove depth is 2.8mm. Maybe size doesn't matter (this is no joke), but I'm not sure why I'm listing sizes if they aren't. It is true.