Tonight I took apart the thermostat housing. As expected, the studs were thoroughly rusted and stuck in the aluminum. I snapped two off, but it was no big deal- once I separated the two pieces I was able to remove the studs with a pair of vice grips.
For reference, these studs are M6x35mm, and I will be replacing them (as well as the nuts and washers) with new studs.
I threw a small batch of other hardware into the vibratory tumbler I bought from Eastwood. Using the green pyramid media, it seems to take a long time to get the rust off, but it does eventually work. I think I may have had too much media in there to start with, so things weren’t tumbling very effectively.
Once the rust was gone (several days, all told, running it a few hours here and there), I switched the parts over to the “dry shine” media. I let it run all day while I was working, and the bolts came out nice and shiny.
I’ll be experimenting with this process some more over the next few weeks. I’m also looking into how to get these pieces plated, so they don’t just rust again. I’m inclined to try to find a shop to do it, rather than use a Caswell kit. Depends on how much it costs though.