I noticed something when I took my intake off. The intake manifold holes are not threaded all of the way.....at least the couple that I looked at. I wonder if the valve cover holes are the same. This might partially explain why so many people have broken bolt heads off......Over-tightening for sure will do it, but it is much easier to do if the hole is not threaded all of the way down.

Just something to think about....

Cheers,
KrzyFast