Originally Posted by oldskewel

You can definitely safely reuse those bolts if you don't see any signs of degradation.

Attached is a BMW spec on bolt torques, since you seem to be interested in drilling deeper into this than most.

Attachment 81532

Some complicating factors, just to point out how the mechanics / physics / metallurgy is not as simple as one might like (and why the "class 10.9", etc. exists to try to remove these complications from things people need to worry about every day):

• thread pitch matters;
• friction (and surface prep / lubrication) matters a lot;
• different parts of the fasteners will see different stress, so some level of yield at some locations may occur and be acceptable;
• when tightening a fastener, there is a stress due to torque resisted by friction, in addition to the stress due to elongation, and those combine, and they depend on friction, thread pitch, etc. at which point seemingly arbitrary parameters get introduced to the equations;
• that BMW spec (I think) has a disclaimer in there about re-using up to 3 times but no more;
• ... but re-using would surely logically depend on corrosion, age, mileage, etc. as well, but that is not given here;
• and this is BMW's spec, reminding us that anyone can make their own spec, which may be different.

If you're looking for more information and flames in a related topic, search on here for advice on whether to re-use the stiffener plate bolts (the 6-bolt aluminum or steel plate under the engine).