I use a lapidary tumbler, burnishing shot, and plain old dish washing
liquid.  It does a great job removing grease and light to medium
corrosion from all metal parts.  It's saved me countless hours doing
slot machine restorations.

Your initial reaction may be that it will damage the parts, but that's
not the case -- jewelers use this method to polish silver and it does
a great job.  Buy the largest tumbler you can find because you will
soon be using it for larger parts.  If you want more details please
write to me privately.

Hal O'Rourke