Rock tumbler idea has variations - rocks inside small gas tank and shake shake shake to knock out rust (on a tank you can't replace), and the commercial version that cleans up alternators and starters, cast metal parts.

 

Or a bench grinder to clean the bolts themselves.

 

Bench grinder with a wire wheel attached

 

Yeah. but when you see how well Purple Power works you will be tempted to put small aluminum bits in it. Don't.

I use it in a large tub to put things like the motor mount brackets and other steel parts in.