Bob,
The cases, cylinders, and hub I started with were absolutely caked with gunk. I doubt the previous owner/s ever cleaned the oil and dirt off of them. I sat out in my shop gravel driveway with various cleaners, some heavy duty, and a little bit of WD40 spray. I spent quite a bit of time with tooth brushes, stiffer plastic brushes, and brass and steel wire brushes. I even used long small diameter screwdrivers to force scotch pads into the nooks and crannies. While I was able to get most of the gunk off the parts, the real small areas around the exhausts did not come clean. They were still dirty black looking but had no actual material on them and it didn't feel like there was any grease and oil. (My wife says I'm a little OCD about cleaning things)
The tumbler was happy with this level of pre-cleaning. He will give them back to you if they are too dirty. Apparently a lot of oil and dirt really messes up the tumbling media that he uses. The tumbling did clean the black stains that were left over from my efforts.
I don't think oxidation stains are an issue. It's just about the oil/dirt/grime that messes up his tumbling mix. I think I was told once that in an ideal world the parts would be blasted clean first. I don't know of any motorcyclist that has done this. Just make sure they're as clean as you can make them. Hope this helps.
Rowland
