You misunderstood me.

I was saying that you prob wouldn't reach yield until about 160 N·m where 122 is spec. (that could be off a fair amount it might not be a direct ratio).

Reality check;

M10 10.9 spec is 66 N·m and ratio to yield: 88 N·m. In testing, it was right about 90 N·m where the bolt went to plastic and the torque stopped climbing. BTW: a very very weird experience to turn a bolt 45° and no change in torque!

So: a 12mm should handle 140-150 N·m before damage. You should be able to torque to 122 limitless times.

So back my original stance: no problem at all to reuse, i would throw on thread lock if I had handy but wouldn't go out of my way if I didn't.

At anytime you're fed up with this thread, just stop responding.
As a matter of curiosity, how did you arrive at 88 N.m as the torque at yield. Shouldn't we consider Proof load rather than yield.
Proof load for 10.9 steel is 830 MPa. Effective tensile area (at the root of the thread) is about 50 mm2. So 830 x 50 is 41500 N (41.5 KN Clamping Force) Similarly for M12, it works out to be about 53 KN. Anything above this will cause permanent deformation. So we're at the limit already.
Agree?

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).

Great information thanks. Although my bolts look great & have no sign of rust, do you think I should still replace them with new ones, from the local bolt store, because of the age and mileage: 2004 model with over 160,000 miles? Thanks

I would definitely re-use them on my car. I recently did that actually, on my 2001 with 203k+ miles. I'm in CA with no rust issues.

An additional thing you can do to give yourself confidence that no plastic deformation has occurred is to see if you can easily thread things together with your fingers. Plastic deformation will be uneven, making that impossible over a long depth.

I was referring to your comment above:

"but re-using would surely logically depend on corrosion, age, mileage, etc. as well, but that is not given here"

and my high mileage and age of the car

good catch I used tensile vs. yield.

The correct number would be 122*94/78=147 N·m for yield. (20% higher than proof).

The proof strength is 0.83 of yield not 0.75.

https://uploads.tapatalk-cdn.com/202...01c98110a7.jpg

That's a very good test. Especially when you can use a new nut on an old bolt.

Suspension bolts are usually on the order of 2x safety factor. A 12mm bolt has a clamping force of 5T. Two means 10T. Just shy of 8T@ sheer force each so the brake caliper can withstand 16T of sheer force that is a lot more than the tire can apply in perfect conditions.

https://www.bmwoffairfax.com/order-parts-online.htm

Just reuse the bolts as the next brake job will be end of life.

Yes, replace them. That's the answer you want to hear, right? :dunno: