OEM head bolts are reusable, and are not torque to yield throwaways. As long as they have not been overtorqued, they are good for reuse. This is clear in the Helms and factory manuals, which do not mention TTY degree specs for the bolts or note that bolts should be replaced. Though one assumes the manuals are working on the assumption the engines being worked on were factory assembled or otherwise assembled to spec.

It is important to note, because a lot of guys will tell you Honda head bolts must be thrown away once removed. I think this is due to the use of OEM TTY bolts on a lot of other engines with aluminum heads (Ford, Chevy, Europeans...).

The best way to see if an OEM bolt has been overtorqued is to measure it against a new one. If it has been troqued beyond 75 percent or so of its yield, it may not spring back to the OEM length, and will be just a bit longer. If so, don't reuse it.

This also can happen to ARP studs. Once any bolt or stud is stretched too close to its yeild limit, it may lose its elasticity.

In addition to stretch due to overloading, look for torsional stress. If you think someone has rebuilt the motor, there is a chance the bolts were torqued without the threads being lubed, or with dirty threads, which could cause twisting or torsional stretching.

I use a magnifying glass to look up and down a bolt for signs of twisting, which shows up in the grain on the surface. When a head bolt breaks, it is usually due to a torsional stress twisting it in half.