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U Bolt Tightening

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CounterMeasure Enthusiast

CounterMeasure

Posted
Posted

There's a TSB bulletin somewhere on this very problem that I can't find at the moment that the bolts were undertorqued at the factory. The bolts on mine were way undertorqued to only 35-40ft/lbs, causing clicking and popping on stop and starts. The dealership followed the TSB, which states to just torque them to 79ft/lbs. TSB doesn't say anything about replacing them. Doing this is fine, since you are cutting on new thread on the bolt by torquing down more. If you were to loosen them and torque, or re-torque to a nut position less than the original position, then I fully agree you should replace the bolts and nuts.

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Zachlovesfood Enthusiast

Zachlovesfood

Posted
Posted

There is some bad advice posted in some of these responses. It's your truck and you can do what you want but take some time to research torque and U bolts. The GM spec is intended to maximize clamping force and if done correctly stretches the bolt so that it should not be used again. This is standard advice from any suspension guru. It's customary to add a lubricant to the threads like never seize so that you create less friction. Friction creates resistance and can distort actual final torque. That's why torque for a plated bolt is lower than an unplated, the plated has less friction. The GM UBolt is 1/2" but measures close to 5/8" at the threads since they have cold rolled threads. The bolt torque spec is based on a 1/2" bolt and 100 lbs is way over max. It's the act of torquing that stretches the bolt not taking it loose. And how accurate is your torque wrench? My dealer has a Snap On Digital wrench-they are very accurate, measure torque and angle and cost about $975.

What happens if you over torque the U Bolt? It will likely fail in some way and it may not be apparent and may not happen tomorrow. In my situation I retorqued mine to 70#'s and felt uncomfortable knowing that it was a guess on my part. And I was uneasy wondering why they came loose from the factory!

This resolved my issues for a few months but things began to change back so I attempted to remove the nuts to lube and retorque them since one was loose again. The threads on one had galled and I could not remove without using my impact wrench. I'm assuming that the loose one was actually the bolt stretching, doubt the nut came loose. In between I had made two dealer visits and they were no help, "no problem found".

I went to a suspension shop and had them replace with a larger diameter UBolt and have a few thousand miles with no issues. They told me that a bolt stretched beyond spec will eventually break. Metallurgy can vary so some bolts are better than others. Their advice is to always replace since there is no way to know about the bolts integrity once it's been torqued to max. (Outside of a lab) I had mine replaced for less than $40 so why gamble???

Throughout this forum, I'm not the only one that torqued the bolts to over 100 ft. lbs. I've not found anyone yet with any complaints or new issues afterwards either. Broken u-bolts, ripped threads, stripped nuts... Not a word.

Since you ask, I borrowed a torque wrench from Cal-Tech Calibrations, right next to our fabrication shop, Gulf Coast Fab. Metallurgy is what we do, calibration to spec is what they do.

You said you were uncomfortable in the 70 ft. lb. range, well, as stated above, GM came out with a TSB that stated 79 ft. lbs...

My point, with my limited knowledge, is that these bolts are made to be stretched, and can withstand far more than assumed.

In my mind, I'm not gambling. I haven't a doubt in my mind that I'll have a failure, although I've been proven wrong before. I suppose time will tell. All I know is that is rather have a tight rear-end than a loose one!! LOL.

Everyone has their own solution to having peace of mind. There's no issue with upgrading u-bolts. It's what I'd be doing if I ever had to take the rear end apart! I'm comfortable with the ones I have though.

 

 

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bgweed Enthusiast

bgweed

Posted
Posted

Throughout this forum, I'm not the only one that torqued the bolts to over 100 ft. lbs. I've not found anyone yet with any complaints or new issues afterwards either. Broken u-bolts, ripped threads, stripped nuts... Not a word.

Since you ask, I borrowed a torque wrench from Cal-Tech Calibrations, right next to our fabrication shop, Gulf Coast Fab. Metallurgy is what we do, calibration to spec is what they do.

You said you were uncomfortable in the 70 ft. lb. range, well, as stated above, GM came out with a TSB that stated 79 ft. lbs...

My point, with my limited knowledge, is that these bolts are made to be stretched, and can withstand far more than assumed.

In my mind, I'm not gambling. I haven't a doubt in my mind that I'll have a failure, although I've been proven wrong before. I suppose time will tell. All I know is that is rather have a tight rear-end than a loose one!! LOL.

Everyone has their own solution to having peace of mind. There's no issue with upgrading u-bolts. It's what I'd be doing if I ever had to take the rear end apart! I'm comfortable with the ones I have though.

 

 

Sent from my iPhone using Tapatalk

 

Agreed, everyone has a solution, and at least people are leaning towards conservatism. I'd upgrade if I were replacing, but I wouldn't replace just to torque-up. A 1/2" SAE grade 5 bolt is quite comfortable being used and re-used at 70-85 lb-ft (dry, depending on if it's plated or not). If you take the same dry, unplated bolt and torque up to 100 lb-ft, you are just below 90% of proof load. This I would go ahead and replace if you had to loosen, but only because torque values are imprecise. You may be at 85%, you may be at 93%, it's hard to say without measuring the actual strain. If I torque a fastener to 90% or more, I'm going to replace it.

 

That being said, a good way to get in trouble would be to lube the bolt up, then use a torque spec for a plain, dry bolt. You could be adding 30% more bolt stress than you think you are.

TxTruckMan Senior Enthusiast

TxTruckMan

Posted
Posted

What's with all the super expensive torque wrenches? $900? I have a pair of old Craftsman torque wrenches for $100-$150 each. Larger one is accurate to 4%.

emeryz28 Enthusiast

emeryz28

Posted
Posted

I have the Rough Country Ubolts that came with the 2.5 leveling kit. I torqued them to 59#'s as that is the GM spec and the RC direction said torque to factory spec. I still have a little noise on the driver side. So this has me wondering if I should retorque?

 

 

 

The dealership followed the TSB, which states to just torque them to 79ft/lbs.
bgweed Enthusiast

bgweed

Posted
Posted

What's with all the super expensive torque wrenches? $900? I have a pair of old Craftsman torque wrenches for $100-$150 each. Larger one is accurate to 4%.

If a torque wrench is verified to be within 4%, then it is fit for service. Like I alluded to earlier, torque requirements are an approximation for achieving a particular tension and stress in a stud / bolt. To calculate a torque that gives you the proper preload / tension on a bolt, you have to assume a value for the friction between threads (including the pitch of the thread) and even the bolt head / nut against the mating surface. These qualities can be simplified into a single value (called "nut factor"), but it's still an approximation. Here at my job, when I need something tightened to a very precise amount, we don't bother with a high-dollar torque wrench... we hydraulically stretch the bolt to achieve the exact tension / stress we require, then spin up the nut until it contacts the surface. This removes the approximation issue completely. For what it's worth, part of my job here has been to evaluate situations where a torque wrench was found out of calibration and to determine if the applications where it was used need to be re-torqued. Very rarely do I need to have them re-done, and so far in the life of this plant we've had no fastener failures or leaks at these connections.

 

I have the Rough Country Ubolts that came with the 2.5 leveling kit. I torqued them to 59#'s as that is the GM spec and the RC direction said torque to factory spec. I still have a little noise on the driver side. So this has me wondering if I should retorque?

 

What is the finish on the RC u-bolts? Is it about as slick as a plated bolt, or more rough like dry bare metal? If they are stock diameter, you should have no trouble going up to 79 ft-lb per GM TSB. If it's a rough finish, you could go even further if you need.

emeryz28 Enthusiast

emeryz28

Posted
Posted

they are slick, but I recall them being possibly fatter and more heavy duty feeling than the stockers when I made the swap

 

gm-lift-kit_1305a-base_3.jpg

bgweed Enthusiast

bgweed

Posted
Posted

I would take them to 79 ft-lb, and while you're there, take note of the bolt size. If you notice any issues driving, check again to see if they have slipped any, and you could re-torque based on the previously verified bolt size.

emeryz28 Enthusiast

emeryz28

Posted
Posted

So I torqued the u bolts to 70 ft-ib as I was still a little concerned to go to 79, although the u bolts are larger and appear to be more heavy duty than stock. Still did not solve my problem. So while I was under there with the torque wrench I found the lower shock bolts were too loose. Not to loose to be visually noticeable or noticeable if I was looking under and had someone bounce on the bumper, or noticeable if I grabbed the shock. But when I put on the torque wrench they were not up to spec. I tightened them to spec, which I believe was 85 and PROBLEM SOLVED!

TxTruckMan Senior Enthusiast

TxTruckMan

Posted
Posted

I tightened my U bolts last night to 79 lbs. They were definitely loose. Probably took 2 complete revolutions. This appears to have taken care of my slight spring clanking that I've been experiencing.

 

Thanks to all the info in here!

bgweed Enthusiast

bgweed

Posted
Posted

So I torqued the u bolts to 70 ft-ib as I was still a little concerned to go to 79, although the u bolts are larger and appear to be more heavy duty than stock. Still did not solve my problem. So while I was under there with the torque wrench I found the lower shock bolts were too loose. Not to loose to be visually noticeable or noticeable if I was looking under and had someone bounce on the bumper, or noticeable if I grabbed the shock. But when I put on the torque wrench they were not up to spec. I tightened them to spec, which I believe was 85 and PROBLEM SOLVED!

 

Good to know. Glad you figured it out!

TxTruckMan Senior Enthusiast

TxTruckMan

Posted
Posted

I don't know about thread size, but if you're asking about the nut size mine were 21mm. I have the 6.2L with larger axle. I assume they're the same size nuts.

  • 3 weeks later...
HawkeyeSierra Enthusiast

HawkeyeSierra

Posted
Posted

Thanks to the OP, for posting this topic. At my first service appt, I had them torque the u-bolts and now the tiny shudder that I once had on upshifts seems to be gone. Maybe it's just my imagination, but it seems smoother to me.

  • 5 months later...
GMC2015 Enthusiast

GMC2015

Posted
Posted

I know this is an older thread, but has anyone else tightened their u bolts and gotten positive results?

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