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3b,

Slight difference of opinion, but a grade 8 bolt is not going to fail before a grade 5 bolt, -unless it is not torqued properly-. That's the whole point of a grade 8 bolt. It has a greater tensile strength, and a greater shear capacity than an equivalent grade 5 bolt. Before you could stress that grade 8 bolt to the point where brittleness could come into play, the grade 5 bolt would have already yielded and failed.

One reason not to use a grade 8 bolt however is where you cannot torque it properly without damaging the assembly. Looking at a torque chart, a grade 8 bolt requires about 50% more torque to be "tight" A bolt is tight when it starts to stretch but is still elastic. It acts like a spring. If you don't torque a bolt enough, it can experience fatigue failure because it is being loaded and unloaded as working forces are applied to it.

Anyway, I would be leery about using a grade 8 bolt on a manifold because the manifold might crack if you actually tightened the bolt as it should be, not because the bolt would fail from being brittle.

But it should also be said that if you use a grade 8 bolt on a manifold and don't tighten it properly, at some point it may experience fatigue failure.

In general the grade 8 bolt may fail by breaking catastrophically rather than by stretching beyond the elastic limit, as in a lower grade bolt, but this failure will happen at a much greater load.

Kurt