anybody know the bolt size for exhaust manifold on a '58 371? One of mine is broken off in the head. I've drilled it out partially but am concerned about going too far -- can't be sure the drilled hole is centered as I'm doing it blind. Can't get extractor to pull out remains of drilled bolt - at least not yet. Hole is already more than 1/4 dia.

I'm figuring bolts are 5/16 but not sure if thread is SAE versus machine? Bet someone out there knows!!

 

If you are using a 9/16 wrench on the bolt head, it will be a 3/8 body.

 

thanks...simple rule of thumb I should have thought of! Appreciate it.