The replacement bolts your dealer supplied are referred to as s-t-r-e-t-c-h bolts, because they are PERMANENTLY stretched when properly installed. When stretched they actually "neck down" or have a reduced diameter and area where the metal has been stretched. This reduced area has experienced Plastic Deformation, and will not hold the same torque (tension) if applied again, it will in fact stretch more, (loosing tension) and often fracture. This is why stretch bolts must ALWAYS be replaced, and never reused.
Stretch bolts are used by car manufacturers for several reasons, usually none of what is reported in the hotrod magazines. First with stretch bolts automated engine assembly is much easier as the tightening of the bolt can vary by as much as 1/2 turn in most cases and provide the same clamp force. This is due to the fact that when a fastener is tighten into plastic deformation, it will not change it's effective tensile strength, (and clamp force) by very much over a given amount of elongation, (stretch). So when the pneumatic machine tightens the bolts in your engine, it can vary up to 1/2 turn and still have the same clamp force... at least in theory.[...]
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