Yield bolts - observing plastic deformation

[oldskewel]

The stiffening plate bolts on these cars are slightly controversial regarding reusing them. Many do, especially since they are not buried deep, like, for example ...

The cylinder head bolts are also said to be torque-to-yield. The stiffening plate bolts are (I think) said to be torque-to-angle, and I think people infer that they are actually TTY as well as TTA. Probably right, in my opinion.

Since the cylinder head bolts are buried deep, I'm replacing them as part of my head gasket replacement, etc. I did a very easy test to see if they yielded - I just held two bolts together, one new one (Elring, OEM), one used one (original), with their threads touching eachother along the full lengths of the bolts.

By looking at the daylight shining through the gap between the two bolts' threads, it was easy to see that they did not match up. They are both nominally M10-1.5 x 110mm Torx E12 bolts. Holding the threads at one end tightly together, I could then see a linear progression of the threads mismatch along the length of the bolts. So over a span of about 100mm, the old bolt showed to be stretched by about 1/3 of a full thread pitch, or about 0.5mm.

Nice, and interesting that it was pretty uniformly / linearly deformed (as it should be) along the full length.

I always re-use the stiffening plate bolts (reinstalling carefully), but it occured to me that one of you rich guys that can afford $200 for a set of bolts that you may or may not actually need might be able to do the same test with an old bolt vs a new one for the stiffening plate bolts.

Just hold the old+new bolts together with the threads together, hold them up to the light to see how the gap varies along the length. I'm very interested to see the results. And if you actually don't detect any difference in thread pitch, it probably means they did not plastically deform and it is fine to re-use the old ones.

Yeah, I guess I should just take a photo to make this a zillion times clearer. Sorry.