Exhaust manifold 1998 Chevy Astro

Weld a nut onto it, then after it cools for about ten seconds, sprinkle a little water to shock it. If it feels like it is going to twist again, spray some Mopar Rust Penetrant on it and let it sit for 20 minutes. It will bubble and sizzle. It's about the best rust dissolver you can find.

To weld the nut, it works best to heat the bolt first with an acetylene torch, then set the nut in place and weld it. The bolt is going to suck the heat out of the weld so it won't penetrate very well. If you heat the bolt AND the nut at the same time, the nut will get too hot from welding and it will melt. By heating just the bolt, the molten metal from the bolt will flow nicely just as the nut melts too. Stop welding for a few seconds if the sides of the nut start to melt. You need the nice sides for the socket to slip on easily.