Inliners International Forums General Discussion Engines Manifold studs will not easley thread into head

1966 194. had to have the seals renewed on my head. I got it back from the machine shop, with the two studs removed. Nothing looks damaged. a 3/8-16 hardware bolt goes into both stud locations in the head just fine. the studs which are 3/8-16 by 3/8-24 (I think) will not easley thread into the head, or a 3/8-16 hardware nut. It feels like damaged threads at the tip of the course side of the stud, or a crossthread, but its straight on going in.

has the 3/8-16 side of these studs been factory machined in a way so they don't back out easley. I find it unlikely they are both damaged as either stud is the same in either hole or a hardware nut, and either bolt hole readily accepts a 3/8-16 hardware bolt.

Am I correct or has coincidence beat the odds? Thanks