If it's a pipe thread and you've managed to get a 7/16" bolt in it it surely didn't fit well, to much difference in thread pitch for any possible combinations that might sort of fit.

Try an 1/8" pipe plug, see if it fits nicely?

Grigg


•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-