Zr2 front suspension

[Chuck FB]

21 minutes ago, kylant said:

they are an m18 bolt. 110+ is pretty standard for many aftermarket u-bolts. i have used 3/4" bolts that called for 175-200 ft/lbs

The thread may very well be an 18 mm x 2.5 mm thread pitch but the material either is or would be close to a 16 mm bolt, bolts are typically a machined thread size but these ubolts are a rolled thread so the thread diameter ends up being larger than the actual material the threads are being formed onto. That's why I call it a unique and weird item because their thread sizing vs the ubolts material stock diameter size don't match each other, or another words don't match a normal bolt chart. 

[Chuck FB]

1 hour ago, GMCnewbie said:

Damn!  I had these on my list of torque specs and didn't even catch this one.  I totally looked past this as it called out the rear spring "unit bolt" rather than u-bolt.  Oh well.  I googled the torque spec for the u-bolts on the 2024 GMC 2500 u-bolts and the AI response said 110lb ft along with a number of search results.  I went with 110.  I don't think I damaged anything as I did a check of the stock bolts on both sides before I removed them.  Both sides appeared to be tightened to roughly 110lb ft based on my torque wrench.  This is by no means scientific, but it was what I had available when I was working in my truck.

 

Edit:  forgot to add I switched to the 3500 u-bolts due to the spacer added for the AT4X setup.

Even AI isn't above this one to figure it out, I expect its search is sorting through all the questions and answers others have thrown out there taking a stab at what makes sense. From my looking at charts and what I experimented with a while back, I am rather confident to say that you have not gone beyond any reasonable torque, certainly not if you had not lubed them and even if you had I still doubt you still have begun to reach their max potential torque value, with a 5/8 ( probably 16 mm diameter material stock ) with even larger diameter thread, 110 ft lb is a pretty soft ball number for what should be a fairly high grade of material used to make them out of. After you have used your truck more, no harm in checking them again. 

[GATORB8]

Rear ubolt sequence and values from Alldata:

[Chuck FB]

25 minutes ago, GATORB8 said:

Rear ubolt sequence and values from Alldata:

That's interesting, so I wonder why the other spec was listed ( assuming that was correct ) and then for the ZR2 its calling for 96 lb plus a half turn. What I question here is the fact that its showing an upper overload pack like the 3500 single wheel or dually is equipped with as standard. Can a ZR2 have that option like a standard diesel 3/4 ton if equipped with the diesel so it can have the special higher gvw rating ?. I wish it would have said on that photo as to which trucks this spec is applicable to. 

[GATORB8]

5 minutes ago, Chuck FB said:

That's interesting, so I wonder why the other spec was listed ( assuming that was correct ) and then for the ZR2 its calling for 96 lb plus a half turn. What I question here is the fact that its showing an upper overload pack like the 3500 single wheel or dually is equipped with as standard. Can a ZR2 have that option like a standard diesel 3/4 ton if equipped with the diesel so it can have the special higher gvw rating ?. I wish it would have said on that photo as to which trucks this spec is applicable to. 

This isn't ZR2 specific, it is the only value listed for the 24 2500HD.

[Chuck FB]

2 minutes ago, GATORB8 said:

This isn't ZR2 specific, it is the only value listed for the 24 2500HD.

Oh sorry, I saw the ZR2 listed at the bottom of that post and now that you point it out, that is listing your own truck !. Ok so that clarifies the issue and being that a 2500 can have the upper overload if ordered a certain way which ends up basically being like the 3500, they used that generic drawing including the overload. This still unfortunately does not take away from the fact that its a goofy spec to work with to recheck the torque of a fastener, however its the spec one has to work with through GM. 

[GMCnewbie]

Thanks for sharing that.  I feel a little better knowing I torqued them to 110.  Looks like Google AI wasn't too far off on this one.  

[Chuck FB]

1 hour ago, GMCnewbie said:

Thanks for sharing that.  I feel a little better knowing I torqued them to 110.  Looks like Google AI wasn't too far off on this one.  

I am almost tempted to loosen one of my ubolt nuts and then perform the 96 lb plus a half turn. As I mentioned before after looking at torque charts I experimented with mine by increasing the torque as they were sitting around 100 before I touched them and all I will say is I increased that number considerably but unfortunately did not mark the nuts to see where I landed in rotational degrees. Being that your hardware is new and not rusty I'd encourage you to go through the process and by the time you get around to it the nuts may not be torqued much beyond the 96 pounds and might be able to start from where they sit now or slightly back them off and then torque to the 96 to feel confident about then marking each nut so you don't loose track and turn them all another 180 degrees. Then at that point play with the torque wrench to see what torque they generally are sitting at. I will take a wild guess now that it may end up being in that 140 to 150 range. 

 

Update, as I suspected I low balled my numbers to be on the safe side LOL, never the less will be interested to hear what you come up with when you have a chance to mess around with your truck.

Edited June 23, 2025 by Chuck FB

[GATORB8]

48 minutes ago, Chuck FB said:

Oh sorry, I saw the ZR2 listed at the bottom of that post and now that you point it out, that is listing your own truck !. Ok so that clarifies the issue and being that a 2500 can have the upper overload if ordered a certain way which ends up basically being like the 3500, they used that generic drawing including the overload. This still unfortunately does not take away from the fact that its a goofy spec to work with to recheck the torque of a fastener, however its the spec one has to work with through GM. 

I think it's the unfixed lower plate that creates an issue with a straight torque value, unlike most standard fastener torque ratings.  These don't appear to be torque to yield.

 

I try to always use a tell-tale like a paint pen to watch for loosening fasteners.  Install and torque then mark across the bolt/nut/surface.

 

I use these:

https://www.amazon.com/dp/B09D9M2R5R?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_5&th=1

 

Edited June 23, 2025 by GATORB8

[Chuck FB]

I like the idea of the product you pictured, I have tried using a shake type paint marker and maybe it was the brand or age of it but I wasn't thrilled with what it would do but have seen mechanics use the shaker marker with success though. 

 

I just went through my ubolts and while I did not loosen them all I did experiment with a couple of them by backing them off and going to 96 and then the additional half turn and at least with the condition my ubolts are in ( a few months old from time of truck manufacture ) , with that course thread they use the torque ramped up even more than I thought it might to get the extra half turn. They are tight now and again rather shocking as to how much lower the tension was on them with relatively little use of the truck and never with a load or pulling a trailer to help work/stress the suspension. Its no wonder I have seen comments of truck owners talking about their springs creaking when having a load on it such as a slide in camper that is rocking from side to side as the ubolts became way too loose and the springs are shifting around under use, that is not good or safe. 

 

As far as this method GM uses to reach the ubolt stretch value they are after, they would have their reasoning of course but as to how the plate is loose/floating and so forth, I doubt that is it as like I said spring shops just go straight to a torque value their hardware they handle has a spec for and no messing around but keep in mind they do it in stages to torque them up evenly due to the nature of ubolts and how they work. Then either bringing the unit back at some point or rechecking yourself after some use with the spec they give. I've had a variety of ubolts custom made at a spring shop over the years and everything from pickups to semi's and I get the spec from them and then assemble and do the initial torque and after some use go back over them to retorque, of course with the larger trucks its 500/600 ft lb on the front axle and around 1000 on the rear which for me means hammer the nuts on good with a 3/4 impact on the rear ones. 

[newdude]

3 hours ago, GATORB8 said:

Rear ubolt sequence and values from Alldata:

 

2 hours ago, Chuck FB said:

That's interesting, so I wonder why the other spec was listed ( assuming that was correct ) and then for the ZR2 its calling for 96 lb plus a half turn. What I question here is the fact that its showing an upper overload pack like the 3500 single wheel or dually is equipped with as standard. Can a ZR2 have that option like a standard diesel 3/4 ton if equipped with the diesel so it can have the special higher gvw rating ?. I wish it would have said on that photo as to which trucks this spec is applicable to. 

 

 

The spec GATORB8 posted is correct and is right from the GM manual (which was screen capped from Alldata).  ZR2 HD or regular HD, does not matter, same spec.  

[Chuck FB]

11 minutes ago, newdude said:

 

 

 

The spec GATORB8 posted is correct and is right from the GM manual (which was screen capped from Alldata).  ZR2 HD or regular HD, does not matter, same spec.  

Excellent, I knew you would know but then to mire through all of our chit chat to set us straight, thanks for the confirmation !

[GATORB8]

20 minutes ago, newdude said:

 

 

 

The spec GATORB8 posted is correct and is right from the GM manual (which was screen capped from Alldata).  ZR2 HD or regular HD, does not matter, same spec.  

Just as a point of clarification since it may not jump out, there is a sequence in addition to the stepped torque procedure, see the numbers on the nuts:

 

Each of the steps would be done to the four nuts in the numeric order above before moving onto the next step.

[Chuck FB]

Yes that is a good point as not everyone has had prior hands on with ubolts for leaf springs and won't realize every move that is made affects the outcome of the job. Its trying to get the front and rear of the leaf spring within the clamping area sucked down evenly as well as side to side. I do many steps throughout the clamping of one leaf spring wheel end, making sure its all touching the axle mount plate ( the welded on perch ) and part turns of each nut in that sequence and raise up the torque evenly is what it amounts to. Even though it doesn't sound like much as per the half turn after the 96 pounds is established on all four nuts, it is in torque terms because of the course thread. That's why I would recommend doing a mark on each nut and continue the pattern as they lay out to evenly get to the 180 degree final point on each nut as it may take another 4 if not more short pulls on the flex handle or torque wrench ( which ever is used at that point ) to reach what amounts to 3 more flats of each nut as per 180 degrees of rotation. Spoiler alert, plan to expect the final torque to be reaching in the ballpark of double of what the initial torque set is, the torque ramps up that much with this course thread, it would all depend if lube had been put on the ubolts or they are very dry etc as to what it happened to be when done.

[revrnd]

Anyone have a U bolt nut kicking around that they can thread a known metric bolt into?

 

I can crawl under my truck tomorrow and check.