A very good Friend of mine taugh me an old NASCAR secrete that worked great with cast iron heads. We would take a counter sinking tool and cut a 45 degree cut around each head bolt on the head deck surface. It would spread the pressure across the head instead of directly under the head stud bolt area.
that trick worked because many times, if not enough champher is at the top of the bolt or stud threads, when torqued, it will pull the threads up a tiny amount, causing the mating part to rise from the surface by a thousandth or two [or more] effectively causing the clamping force to concentrate at the thread boss instead of spreading the load as designed. when adding the champher to both mating surfaces [this time the head surface], this allows any minute thread pulling a place to go, thus helping to allow the parts to mate as designed. all holes i produce in just about anything gets a nice champher on both sides of the through hole, and an equally good champher on a blind hole regardless of the parts intended use.
