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I'm missing something here Ray. If the head bolts are designed to stretch when tightened, wouldn't they stretch over time with heating and cooling cycles? That would result in less than the designed compression of the head gasket and leaking or blown head gaskets after a few years.
Also I would think that reusing the head bolts would result in warped heads due to the above uneven compression. The effect would be the same as applying the correct torque on only some of the bolts. But I don't see how it could cause cracks through an exhaust valve seat.