My understanding of the question at hand goes like this: the grade 8 bolt
while "stronger" than the grade 5 is at the same time more brittle, and in a
shear application would tend to break under load where the grade 5 would
bend before breaking. As for mixing grades, bolts are held tight by streach.
gr8 bolt with gr5 nut: tighten to gr5 spec, the bolt is not streched enough,
tighten to gr8 spec, likely to strip nut before torque is reached. If you
are looking for bolts for the garage door and are concerned, I would get out
the drill and go up a size or two, and buy new bolts, not what you've
collected from the bin. Carrroll Smith in "Prepare to Win" goes into a bolt
discussion, but it is about mostly aircraft stuff.
Jim
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