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 bearing fits a 8mm rod and has an outer diameter of 22mm.Notice though, that I DO NOT recommend to use any bearing at all. The friction is much too small with the extra bearing and your spool might end up unrolling too easily! Instead you can try the same bushing without any bearing and a 9mm diameter hole that fits a 8mm rod nicely. Have a look http://www.thingiverse.com/thing:19015Check out http://www.thingiverse.com/thing:19016 for the 8mm rod mounting thing.Print two of those bushings. Two bearings needed as well.