I had an experience where a large piece of machinery had to be removed from an -embedded steel base plate foundation- and save all of the threaded holes. All of the bolt heads had mushroomed from rust to become many sizes once the rust flakes were removed. I ordered a set of six-point impact sockets by 1/32 increment sizes from 1.5" to 2.5" for the task. We used needle guns to remove the flakes of rust and drove the nearest size socket that was closest to a particular bolt head-on with a heavy hammer. Then an impact hammer removed them. As I recall there were about thirty of them and they all came out.