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Check your U-bolts!!

I had the same thing happen to my truck right after i changed my leaf springs. it was scary as hell.
 
I usually zip em down by hand, then snug em in stages in a cross pattern so the pull is equal on all four legs. Critical to tighten them up with full weight on them.

Once I've gone as far as possible with a 1/2 " ratchet, I break out a 30" breaker bar and go to town on em. I check em a few days later, and haven't ever needed to tighten them more, and haven't had a set come loose on me.
 
rampage said:
Just curious, did you end up having to loosen the u-bolts and install different angled shims? ... and if so, are you concerned now?
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Well, if GM pre-stretches them, I'm not sure what the problem is re-using them if they are in sound condition and you didn't have to mangle the threads to get the nuts off. I'll be replacing these for sure since who knows what forces they were under when they were loose.

The U-bolts didn't fail. Is there any reason to believe re-using u-bolts causes the nuts to loosen more easily?
 
Mastiff said:
Well, if GM pre-stretches them, I'm not sure what the problem is re-using them if they are in sound condition and you didn't have to mangle the threads to get the nuts off. I'll be replacing these for sure since who knows what forces they were under when they were loose.

The U-bolts didn't fail. Is there any reason to believe re-using u-bolts causes the nuts to loosen more easily?
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There is actually. If when they are originally torqued and the threads deform in the slightest, the next time you torque them, your torque wrench will reach its value before the bolt has actually been torqued to that value.
 
Is there any special torque spec on the center pin itself? It's not a huge thing, so I didn't go crazy on it. If that loosened up I bet bad things could happen. On the driver's side though, I do still have the center pin but the head must have popped off since the shim is gone and the pack is offset from the perch...

Fun fact: if your rear driveshaft is out, you can't shift into park with the engine running (assuming you're in 2WD or front hubs disengaged). Without the driveshaft, the tranny will still be turning with the brakes on. Well, you can shift into park, but it makes a terrible sound.
 
You could shift the T-case into neutral ,wait a few seconds,then put it in park--but it wont hold the vehicle from rolling either!...



I learned that fun fact in vocational school,when we had a teacher's '67 Buick Lesabre up on the lift to diagnose a strange noise when the car was driven at various speeds--turned out to be a rusted u-joint that had no needles left in one cup..

I was in the car with it running while it was on the lift...teacher tells me to put it in drive and accelerate it slowly...got it up to about 60 mph...then he says "OK,I know what it is--shut it off"....

I turned the key off,then tried to shift into Park,and "BLAGHHHHHHHHHHHHHHHHHHHHHHHT"...he screams NOOOOO!..:yikes:..

"Put the BRAKE on "..soon as I touched the pedal,the tires went from 60 mph to zero in about 1 millisecond!...

I bet that took a little life out of the park pawl on that TH400!..oops..:doah:
 
After reading through this yesterday I went out to check the U-bolts on the Suburban. About 600 miles ago I swapped in a 14bff, using Dorman 9/16" fine-thread U-bolts with 1" tall nuts. These replaced 9/16" coarse-thread U-bolts with 1/2" tall nuts (holding a 10-bolt). The K10 appears to have the same coarse-thread bolts installed on its 10-bolt. I presume (but cannot prove) that 9/16" was the factory size for 1983 1/2 ton trucks and 1984 1/2 ton Suburbans.

Either way, the new U-bolts were a couple inches longer than needed, so I wasn't able to tighten them with an impact wrench (for lack of a deep enough socket). I tightened them with a doubled-up wrench (25" or so) as tight as I could tighten them (my Chilton manual said 145 ft-lbs, IIRC) Yesterday I found that the passenger side had loosened up a bit (it required only about 50 ft-lbs to turn :eek1:). The driver side was still tight.

So I'm glad I checked them! Somehow I had completely forgotten to do so after the first test drive (400 miles)... :doah:
 
If anyone here should be having the rear end drop out of their truck,it should be mine!..

I put 4 used (but like new) U-bolts meant for a truck with a 2" lift in the rear,and the threads were about 3" away from the spring plates...being broke and desparate,I used them ,and I used a STACK of 5/8" washers to take up the "slack" on the drivers side u-bolts,on the other side I used 4 of those long threaded rod couplers for 3/4" rod as spacers to take up the extra space..

I tightened them with a 1/2" breaker bar and a 3 foot cheater pipe,until the felt like they were either going to bust off,or started bending the spring plates...
I had used 3/8" grade 8 bolts for the center bolts too--both were rotted and busted ,and the original spring plates were rusted SO thin they actually separated and broke--I was lucky the rear end did not come out from under the truck driving it that way--never noticed they were that bad,until the right rear spring shackle busted in two one cold winter day while I was plowing my driveway..:eek:..

I'd better check them and see if they are still tight--I didn't like using the "too long" U-bolts,but as long as they seem OK,I am going to run them..the guy at the inspection bay said "eh--I have seen much worse!"..he said these should be much safer that the rotted original parts I was riding on it had before..I agreed,but still feel it's not really "right"..

I know a guy who had a IHC Scout and he used 1/2" threaded rod for u-bolts when his busted on a sunday afternoon wheeling--and his axles were above the springs,so the u-bolts carried ALL the weight of the truck,unlike a GM where they basically just hold the axle housing to the springs..he never bothered to put "real" u-bolts on that truck either,and they never failed...
 
cybrfire said:
There is actually. If when they are originally torqued and the threads deform in the slightest, the next time you torque them, your torque wrench will reach its value before the bolt has actually been torqued to that value.
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How is this any different than lug nut studs?

I don't intend to drag this off topic, but u-bolt re-use and thread deformation comes up all the time...yet there is only one spec given by the OEM (GM in this case) for lug nut studs, which they know will be repeated many times, and based on fastener diameter, are near the maximum torque applicable for that size fastener.

If there is a dissimilarity in those two applications, I'm not seeing it. :confused:

That, of course, assumes people are actually torquing both fasteners as they should be.
 
The difference is if you design a "torque to yield" application. Items like head bolts and u-bolts fall into this category. Lug studs do not. The engineering stress/strain curve has a linear section where the loading can be applied, relaxed, applied again, over and over with no change in the strength of the material (ignoring fatigue stress that takes thousands of cycles). Some applications the fasteners are stressed just above the linear region to the point where the material stretches. At this point, when you relax the load, the material retains a fraction of the stress and the stress/strain curve shifts. However, the next time you load the fastener you are at a higher level of stress and closer to the failure point.
 
We took the jimmy out today for a scenic run and while getting ready I thought of this thread, so I broke out the torque wrench. About 1/2 of the U-bolt nuts were a little loose I set the wrench to 120 ft lbs and checked them all. Then up to 150. About 1/2 were slightly below the 120.

Since I had tools out I decided to crawl under and see what the tick tick noise was that developed in four wheel drive during the last trail run I was on. That was Eagle Rock.

So I started looking at the front driveline at the axle and worked my way up. I quickly found one bolt missing where the CV bolts to the flange and 3 others that were 1/2 out. I turned the 3 back in about 3/8 of and inch worth of threads. So I tightened all 3 up then took each out one at a time inspected the threads and put two opposite each other back in and drove to the hardware store and bought two new ones. Put those in and tightened them up and went on the scenic drive. No more tick tick.

If it wasn't for thinking about the thread I wouldn't have even looked under the rig. So another save from CK5.
 
Here is a youtube vid on engineering/stress strain.

 
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F-ingrob said:
i would blame the shims
thats what happened when i had the same problem many years back
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Ive had steel shims on my front axle for over a year with plenty of dune running and have had zero issues....
 
F-ingrob said:
they shot out from the between the axle and spring pack
i now use the ones that bolt to the springs
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The ones I had were bolted to the spring pack too. The center pin sheered somehow and the shims spit out.
 
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