Originally Posted by oldskewel

Well, I think you asked lincoln, and I answered. I'm not sure about BMW's reason, and don't know if he is.

If the bolts + nuts deform, it may be detectable by seeing how easily they thread back together before reassembly. So if they thread back together easily along the full length of the bolt with finger torque, there is little or no plastic deformation, and I would re-use them as if they were new. (full torque + angle)

If they don't thread easily, they are deformed.

If you were able to re-install them to the exact same strain (the same full stretch they had before, but no more, which would be more plastic deformation), you could do it almost indefinitely. But you cannot just blindly follow the Torque plus (same) Angle repeatedly. It very well could be that due to tolerances and bad luck, it would fracture on the first re-installation.


Or it could be that if things are done carefully and things are in spec, the bolts go right up to the yield stress, but no further. In that case, with no yield, you should be able to reuse indefinitely.

Experiments could get a better answer on this - it depends on the bolts as well as the things that are being clamped (the stiffening plate, washer, tube, etc.).

But if you re-install with no extra plastic deformation, you will get all the clamping force, and no inching closer to fracture.

Do you know the feeling when you're torquing a bolt and can feel the threads strip? Like the torque is increasing linearly with increasing angle, and then when they strip, it's like the angle increases without any increase in torque (or in the case of threads, it would be a reduction in torque as the threads break away)? That's an indication of yield (plastic deformation), and is exactly the feeling when the whole bolt deforms in a TTY scenario. If you pay attention while torquing, you can try to feel and hopefully not detect any of this. If you don't plastically deform it any more, then you'll still be in the elastic region which was extended due to work hardening.

I know some applications will call for things like a pulley bolt to be replaced upon replacement of the pulley, even if it seems fine. I see that as a different story, and would replace it if they say so. One main difference there is that there is a single point of failure. These stiffening plates have 6 bolts, so if one or more breaks, it will not be catastrophic like a pulley bolt failure would be. And will not require an engine teardown like a TTY head bolt would.