I found some small wire brushes about 3/8 inch dia that could
be chucked into an electric drill and slowly rotated in the threaded
hole to clean the crud out of the threads.

I pump compressed air into the cylinder through one plug
hole while picking the crud out of the threads of the other. This makes
sure the stuff doesn't lay in the cylinder and get into places it
shouldn't. Got to wear goggles while doing it, and a respirator is a
good idea, since the dust has lead in it.
Chunks of the stuff have caused problems when they break
off in flight: they can foul a plug or get under a valve and reduce
power. Not a good thing if you're just lifting off.

Dan