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Just to close this out, I had a friend drill it out and retap it to use a 5/16-18 thumbscrew. Works perfect, on to the next problem!
Thanks again everybody.

Well, I do have a thread gauge, it seemed like 18 but I can't see enough in that hole to tell for certain, so I bought a couple thumb screws of 5/16-18. But no go, jams in the threads as did the...

Thanks fellas...i didn't think it was metric either ChiefTiff, and I will look into what you said about 5/16" (I thought I tried that, though) and the other thread types.
Geoff, also a good idea to...

A friend gave me an old 6" bit brace. It is in good shape, but no markings. The problem is the thumbscrew is missing. I've tried a 1/4" (too small), a 3/8" (too big), and a couple metric in that...