This spool bushing is based on Thing #3783.I changed it to fit to the spools I got from reprapstore.com (3mm filament); they have a bigger (54mm diameter) hole. The bushing fits a 8mm rod nicely and can be used without any extra bearing. I recommend it without any bearing (for reasons of better friction of the spool). If you still want to use metal bearings, have a look at http://www.thingiverse.com/thing:19014Check out thingiverse.com/thing:19016 for the 8mm rod rack mounting thing. Print two of those and enjoy.