Tech Question - Re- Installing U-Bolts

Dear Mike,

I would like to ask some questions regarding comments at your web site stating to not reuse U-bolts. I'm really hoping you can give me some feedback based on real world experience.

I replaced all the rear axle leaf spring U-bolts on my 1991 Chevy K1500 pickup yesterday with brand new, quality aftermarket U-bolts, high-nuts, and thick hardened washers. I also replaced the top-plates that the U-bolts go over and the bottom anchor plates that go under the axle housing, at the same time.

The top plates and anchor plates were new GM factory parts. After torquing the U-bolt nuts to factory specs of 81 foot pounds with a hand torque wrench and following the recommended tightening sequence, I noticed a piece of rust around the spring pack center bolt, right between the top-plate that the U-bolts go over, and the top of the spring-pack on the drivers side. I had not driven or moved the truck yet, so I loosened the U-bolts on the drivers side, removed the rust, and re-torqued the U-bolt nuts to factory specs of 81 foot pounds. I thought the rust was causing a small space between the center of the top-plate and the top of the spring pack, but it turns out the space was caused by the *very* small arc of the springs between the U-bolts.

I basically removed the brand new drivers side U-bolts for nothing, and then found out that you are not supposed to re-use U-bolts, if that is actually considered re-use since the vehicle was never driven or moved. I even took the time to paint the U-bolts with epoxy paint and a good enamel top coat to prevent rust. The weight of the vehicle was on the springs the whole time I did the work, it that matters at all.

When I re-installed the brand new U-bolts and nuts on the drivers side, the nuts screwed down easily all the way to the anchor plates. I noticed no cross threading or any problems. The U-bolt nuts also seemed to torque down fine to 81 foot pounds the second time.

1. Since I had not moved or driven the truck yet is it OK that I Loosened and re-torqued the brand new U-bolt nuts once on the drivers side? I would hate to have to buy another set of U-bolts for the drivers side and re-paint an redo everything unless it is necessary. It seems to me that as long as the U-bolt is not stretched too or beyond the materials yield point, then it should return to it's original length after the U-bolts are loosened. I do not understand why the U-bolt would apply any less clamping force the second time it was torqued down unless the tension in the U-bolt created stresses that were near the bolts yield point. If the U-bolts are stressed too or beyond their yield point when torquing the bolts, then that would seem to prevent them from reliably holding tension or spring clamping force. Stressing a fastener near the yield point of the material does not seem like a good design to me. If I had used an impact to crank the nuts down like most shops probably would, then I would wonder if the bolts had been stretched too far, but I used a hand torque wrench and followed the correct tightening sequence.
2. Are cold formed aftermarked U-bolts generally stresed near there yield point when torquing the nuts to factory specs?
3. Is the problem with reuse & stretching of the U-bolts mainly from driving or does this occur the moment the U-bolt high-nuts are torqued? As long as the nut torque does not cause a bolt stretch that causes stresses that are at or exceeding the yield point of the U-bolt material, why would they not spring back to their original length after the bolts are loosened?
4. How important is it to re-torque the U-bolts after a certain amount of driving? I had read about this recommendation but it occurred to me that the U-bolts of new vehicles are never retorqued. Perhaps they retorque them on the assembly line (I doubt it) but they are certainly not retorqued after any significant amount of driving. I would guess the new car manufacturers just torque them and ship them. My only guess is that retorquing is necessary in aftermarket bolts because factory U-bolts are hardened and most aftermarket U-bolts are cold formed and unhardened, making them inferior to factory U-bolts, less reusable, and more cumbersome to work with since they require retorquing. Is this the case?
5. In a sense, isn't retorquing the U-bolts once or more after a certain amount of driving similar to reusing them? Most stretching will have probably occurred by the 50 to 100 mile point and re-torquing them would seem fairly equivalent to re-using them, except that you're just stretching the bolt a second or more times without re-loosening them first. Your site recommends to check the torque on newly installed U-bolts three times, once at 50 miles, again in another 50 miles, and then again at 500 miles. However, if you are torquing the U-bolts on three separate times plus the original time you installed the nuts for a total of possibly four times, this seems basically equivalent to reusing the U-bolts, the only difference being that you are not loosening the nuts first before re-torquing them. Why would loosening the nuts before re-torquing hurt anything?
6. If the main problem with re-using U-bolts is with reduced clamping force, approximately how much is the clamping force reduced when Using a U-bolt for the second time? If nobody knows then how do they know it's a real problem or the extent of the problem?
7. If the main problem with reuse is with the possibility of the high nut backing off, can I just run another nut down onto the original high nuts (used like a jamnut) to prevent loosening? This would be done after driving and retorquing as you recommend.
8. I guess the main question is, if this were your vehicle would you purchase new U-bolts for the drivers side again, and redo the job, or would you just re-torque at 50, 100, and 500 miles of driving and not worry about it?

I would be very grateful for your answers and feedback to my questions and concerns above at your convenience.
Thank you.
Sincerely yours,
John

John,

Quality aftermarket u-bolts are made to SAE standard J429 using SAE1541 steel. 1541 provide a 150,00 PSI minimum tensile strength, and 130,000 PSI minimum yield strength and a 120,00 PSI proof load. So stretching, or exceeding the yield is not something one needs to be concerned with.

The reason u-bolts should be replaced after they have been torque is because the threads are rolled, not cut.

Because of this the threads are distorted when torqued. This distortion will not allow the torqued to be maintained when the nuts are loosened then retightened.

Because loose u-bolts are the main cause of broken springs, we suggest that they be re-torqued. While the springs are new and the u-bolts are new, the rest of the suspension is aged and will have wear. It is common that the new parts receive additional stress than the older parts.

Re-torqueing continues to move the nut up to new threads.

Bottom line is we recommend that the u-bolts be replaced once they have been torqued to spec. Whether one does so or not is their choice.

-Mike