I don't think those are straight threads on that bolt, and it isn't a common thread either. I think it is a NPSF, that is a finer thread. It is used where sealing is important, but I am guessing. You'll know when you get that rascal out. Let us know when you get 'er done. I am curious

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When wrestling a grizzly bear, you have to keep at it until the bear gets tired, not when you get tired.
1948 Chevy 2-Ton