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.

[andrewwynn]

Very cool info

[oldskewel]

I took some pics yesterday that may make things clear.

This first pic shows the gap between two new Elring head bolts. BTW, I notice these are all Class 12.9, so they are extremely strong.
Held together at one end (with ends flipped so the heads don't get in the way), these new bolts have the same thread pitch so they fit almost perfectly and the light gap coming through is small and uniform. Any variance is likely to be due to the perspective of the camera.


Second pic should be like the first one except the bottom bolt is a new one, and the top bolt is used (original, came off my M54). The threads at the left end are held together, but the top (old) bolt is stretched causing the gap to grow along the length of the bolt. By the time you're at the far right end of the photo (BTW the old bolts had a bigger shoulder section on them), I'll estimate this mismatch has grown to about 1/3 of a full thread pitch, so call that about 0.5 mm. Trying to measure that using a caliper might be tough to do, but lining up old and new like this makes it easy to see.


Close up of the left section (new bolt on bottom, old one on top)


Close up of the right section (new bolt on bottom, old one on top)



And again, in case it was not clear earlier, the stretch of the old bolts appears to be pretty uniform. So if I held the right side together in the above photos, you'd see a mismatch on the left end.

And BTW, for those Physicists on the list, this seems like interferometry.

[andrewwynn]

Closer to a moiré that interferometry.if you had a long enough bolt the "beat" of interference would be evident and you could measure the stretch by the vernier.

[oldskewel]

Originally Posted by andrewwynn

Closer to a moiré that interferometry.if you had a long enough bolt the "beat" of interference would be evident and you could measure the stretch by the vernier.

LOL. Sure. I have more experience with interferometry, so that's what I thought of, but I'm sure you're right. Waaay back in the day, in my first real job after trade school, I set up a laser interferometery system to calibrate some high speed CNC machines. Pretty cutting edge stuff, both literally and figuratively.

Very primitive computing back in those days, but they basically counted as the laser wavelengths passed eachother to measure the mill moving. Similar to here, where "counting" the relative movement between bolt thread patterns was 1/3 of a wavelength I knew the bolt had stretched by that much without having to actually measure anything. Just counting.