U Bolt Tightening

THEREED · July 21, 2016

I have a 2014 CC AT and based on forum feedback decided to check my rear leaf spring u bolts. I tightened them to 59# and was amazed at how much I had to tighten a couple of them.

Drove it around today and I was amazed at how noticeable the difference is with the tighter bolts. A lot less clunk over big bumps and no more passenger seat vibration. The vibration before wasn't awful but seat headrest would vibrate at almost all speeds. This seems to fix it completely just by taking 10 mins with a torque wrench.

I'll consider this the second best thing I learned on this forum (turning off grade braking is number 1)!

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SkiDooNick700 · July 21, 2016

Agreed, I went under the truck one night with a torque wrench set to 50lbs and got like a half turn. My last trip to the dealer I asked my service writer about it he said they'll check it when its on the lift for the oil change. They torqued to 70 something and it really is amazing, and no more clicking sound on take off either.

Ron.s · July 21, 2016

You need to replace the U Bolts. Not only GM but all axel and spring guys will say the same thing. It's likely that they were torqued to spec at the factory and that results in the bolt and/or threads stretching. What caused them to loosen is anyone's guess but it may be the shims wore in or took a set. And the GM spec is torque and Angle (59/130*)-most of us haven't spent $975 for a digital torque wrench so can't torque to spec anyway. I went to a local shop and they up sized mine to 9/16" and torqued to 125#'s. Most shops make their own U Bolts-unplated 9/16" with rolled threads, grade 5 is what I have now. For $40 it's not worth the risk of using the old ones again, IMO.

Towerhog · July 21, 2016

Ok guys. The correct torque for the u bolts is as follows. Final torque is 59 ft-lbs plus an additional 130 degrees of rotation. Of course, this is on new u bolts. If you're stopping at 59 ft-lbs, then thats not nearly tight enough.

namanc6 · July 21, 2016

What's the benefit of turning off grade braking???

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kp52 · July 21, 2016

I just did mine at 60 and left them a few minutes ago, they few I checked were at around 40-45 ft/lbs before they moved.

Most likely the spring pack compresses over time and some preload is lost. I didnt have any noise or any other issues, was just checking the air in the spare and figured I'd have a look since I was under there. If everything was fine at 40-45 ft/lbs it should be good at 60.

gone_fishing · July 21, 2016

What's the benefit of turning off grade braking???

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http://www.gm-trucks.com/forums/topic/186752-interesting-progress-with-odd-trans-behavior-2015-6-speed/

bgweed · July 21, 2016

You need to replace the U Bolts. Not only GM but all axel and spring guys will say the same thing. It's likely that they were torqued to spec at the factory and that results in the bolt and/or threads stretching. What caused them to loosen is anyone's guess but it may be the shims wore in or took a set. And the GM spec is torque and Angle (59/130*)-most of us haven't spent $975 for a digital torque wrench so can't torque to spec anyway. I went to a local shop and they up sized mine to 9/16" and torqued to 125#'s. Most shops make their own U Bolts-unplated 9/16" with rolled threads, grade 5 is what I have now. For $40 it's not worth the risk of using the old ones again, IMO.

As long as you are replacing U-bolts, what about flipping them?

Zachlovesfood · July 21, 2016

If you've never taken the u-bolts completely off, but simply tightened them more, there's no need to buy new ones IMO. Especially on a half-ton truck...

My u-bolts were under torqued from factory. I probably stretched those bolts for the first time by tightening them. I went over 100 ft. lbs.

I like it tight!

I've had no issues whatsoever. If I had to take my current u-bolts off now, I would definitely put a new set on for reassembly.

I just don't see the point if they weren't stretched in the first place, unless you're upgrading them... Heavy duty u-bolts are probably a must for people who tow heavy and regularly.

Just my .02

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SouthernSilveradoGuy85 · July 21, 2016

If you've never taken the u-bolts completely off, but simply tightened them more, there's no need to buy new ones IMO. Especially on a half-ton truck...

My u-bolts were under torqued from factory. I probably stretched those bolts for the first time by tightening them. I went over 100 ft. lbs.

I like it tight!

I've had no issues whatsoever. If I had to take my current u-bolts off now, I would definitely put a new set on for reassembly.

I just don't see the point if they weren't stretched in the first place, unless you're upgrading them... Heavy duty u-bolts are probably a must for people who tow heavy and regularly.

Just my .02

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That's what I've always thought. It's not tightening them more, it's when loosening them when any bolt with high stress needs replacing. Usually head bolts and main bearing bolts for the crank. I TQed my u-bolts without loosening them when I first read about how people were finding the nuts not TQed causing the clunking. I think it was about 2 years ago and haven't had a problem with any clunking coming back and I don't drive it too easy.

Zachlovesfood · July 22, 2016

That's what I've always thought. It's not tightening them more, it's when loosening them when any bolt with high stress needs replacing. Usually head bolts and main bearing bolts for the crank. I TQed my u-bolts without loosening them when I first read about how people were finding the nuts not TQed causing the clunking. I think it was about 2 years ago and haven't had a problem with any clunking coming back and I don't drive it too easy.

It definitely fixed my clunking and vibrating as well. I think these GM bolts are highly underestimated. Once properly torqued, they're good to go!

And quite honestly, I've never ever read of someone popping a rear u-bolt on these new trucks. I can imagine it wouldn't be an easy task, even if you were trying for it!

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SouthernSilveradoGuy85 · July 22, 2016

It definitely fixed my clunking and vibrating as well. I think these GM bolts are highly underestimated. Once properly torqued, they're good to go!

And quite honestly, I've never ever read of someone popping a rear u-bolt on these new trucks. I can imagine it wouldn't be an easy task, even if you were trying for it!

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The way I drove and treated my '98 that would have been when a u-bolt would break. All the burnouts, going around corners too fast (while still being able to stay in my lane) and mudding/off road I'm surprised more didn't get messed up on it too be honest.

Zachlovesfood · July 22, 2016

The way I drove and treated my '98 that would have been when a u-bolt would break. All the burnouts, going around corners too fast (while still being able to stay in my lane) and mudding/off road I'm surprised more didn't get messed up on it too be honest.

Haha, that poor truck! Can't deny the fun though, good times are good times.

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Ron.s · July 22, 2016

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???

SouthernSilveradoGuy85 · July 22, 2016

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???

Yes of course tightening is what stretches the bolt, but we said it's when you loosen it after it's been TQed is when a bolt like that should be replaced. If all you do is re-TQ it then it shouldn't have to be replaced since it's just stretching a little bit more. Loosening it will make it contract so re-TQing the same bolt weakens the bolt. The bolt won't contract if you just TQ it tighter without loosening it. It might be a better piece of mind to replace the u-bolt anyways, but I honestly don't see how just re-TQing without loosening would make the bolt weaker. But I also guess we both will believe what we want to believe.