Does anyone know the bolt size for a mini rear rack?
If you mean the four threaded top inserts for mounting baskets/platforms, then M5-0.80 in whatever length you need.
Edit: The thread depth of the welded inserts is about 1/2" (12.7mm for those still using the metric system).
Thank you for the help.
I know this is an ancient thread but thought I would contribute to the discussion. The threaded rack mounting point near the rear axle on the Mini is actually M6.