As far as damage to threads in my tumbler, I run 200 grit silicon carbide for about three hours. No damage to threads, my tumbler rotates about 20 rpm, and everything comes out clean and pretty. I wouldn't use this for bolts that would especially show, like on the engine, but for mounting parts to the frame it does great.
Tim

I am currently digging back in to a 1953 3800 (one ton) with a nine foot bed. I've owned it since 1979, and drove it until 1982 (or so). My wife got me involved in restoring it back in 2002, got the body removed and the frame redone, then things came up. Now I am retired and starting again. If anyone is interested I have photos on Imagur ( https://timwhiteblues.imgur.com/ ). I live way back in the woods in the Ozarks on 40 acres at the end of a 2 1/2 mile private road.

Tim