To clean the commutator, use a strip of 320 or
400 grit sandpaper and pull it back and forth
like polishing shoes. Don't dig between the
segments because it will leave edges for the
brushes to go across and shorten the life of
the brushes. It's not necessary to clean the
metal part that spins inside of the field coils
unless they are really packed with dirt or a
thick layer of rust. Be careful not to break
the insulation on any of the wires (starter or
generator) because it may short out part or all
the windings. The main thing is to check the
front and rear bushings or bearings for wear.

Truck Tinker from Talking Rock, Ga.