Deel, I probably pound a GMC harder than most and I have never had a timing cover bolt loosen using the 3/8-16. They have never leaked either. I was wrong on the header bolt. The head is a 3/8" socket with a shoulder. Very easy to get a wrench on. I use Permatex #2 for a sealant. I have also found using a exact 3/8" drill on those 2 lower holes also helps align it. GMC's and Chevys are the same.
