Oh, I know about head sizes. On the railroad, a typical 5/8 bolt used a 15/16 wrench. But sometimes when taking apart an old freight car we'd have to go looking for a 1" wrench for the same threads. It was more fun when the nut was one size and the bolt head another.
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.
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