Just a technicality, but the bolt is not actually weaker as long as it isn't plastically deformed past it's peak strength. For example, if one stresses a bolt to point "C" on the graph above and releases the load, the bolt will be plastically deformed, but upon reloading, will reach a new higher yield point than originally. This is the concept of "work hardening."The bolts stretch therefore they are weakened and a second use will do even more the head bolts on LS motors are done the same way it's the material they are made from after market head bolts are not same material. This is short Verison as to why.