Replacing Torsion bars and Leaf Springs

08lmm72mm

Member
May 13, 2019
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Saskatchewan, Canada
It's time to replace both of these items. Looking over the torsion bar replacements I have 2 choices code GK which has a max torque of 8615 and code GL which has a max torque of 8782. I'm not sure what comes on a 2008 2500HD 4x4 (non z71) but I was leaning towards running the GK with a lighter torque rating to possibly give a better ride.

Am I thinking correctly here? Truck has a cognito 4-6 and I've noticed I had to turn the torsions higher than I had them years ago to clear the tires.

Truck has 162,000 miles on it.

I also noticed there is alot of lighter torque rating torsion bars for these trucks but I'm assuming they are probably too light to run in a diesel application?

Edit
I searched up my VIN on a gm parts site and it only shows the heavier GL as listed torsions for my truck.
 
Last edited:

08lmm72mm

Member
May 13, 2019
139
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Saskatchewan, Canada
I didn't think there would be much difference. I found in the glove box on the list of codes the torsion bar ones are on the very bottom right corner. Mine are 6GL and 7GL.
 

08lmm72mm

Member
May 13, 2019
139
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Saskatchewan, Canada
Maybe I'm second guessing myself but does anyone think replacing the tbars will improve the ride at all? Once they start to sag and I put more preload back into them to retain the same amount of height wouldn't that effectively make them 'stiffer' or would they still basically work the same as new? I've researched this a ton today and haven't found any solid information on it.
 

juddski88

Freedom Diesel
Jul 1, 2008
4,431
21
38
Chesterfield, Mass.
It does improve ride quality. In my experience up here in New England, with relatively crap roads compared to the rest of the country, stock springs and torsion bars show signs of needing to be replaced around 200k miles or 10yrs old.
 

08lmm72mm

Member
May 13, 2019
139
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Saskatchewan, Canada
Good to know! It'll cost me $1000 up here in Canada just for the torsions, hate to do them and nothing improves.

Drivers side torsion, can feel a pretty good ridge at the top going into the LCA.

Less than 1" of room at the rear of the leaf.

Pretty much together at the front of the leaf.
 

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Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Phoenix Az
Maybe I'm second guessing myself but does anyone think replacing the tbars will improve the ride at all? Once they start to sag and I put more preload back into them to retain the same amount of height wouldn't that effectively make them 'stiffer' or would they still basically work the same as new? I've researched this a ton today and haven't found any solid information on it.

adding adjsutment back into the bar doesnt increase spring rate, all it does is add "preload" to the bar but the effective spring rate the truck needs to achieve desired ride height stays the same. on a torsion bar, the spring rate is based on the amount of twist. so if you have 20* of twist in the bar between where it attaches at the LCA to where it attaches to the frame mount while at ride height, that degree of twist will stay damn near the same no matter how much you change the adjustment screw. instead, the truck moves up and down.

now on a sagging bar, whats happening is the spring rate is changing. Why? because the truck is getting lower but no adjustments have been made. So the only change is the torsion had now gone from 20* to 25* between end to end. eventually this leads to spring distortion and a high loss of spring rate. So effectively, you are riding on softer torsions, we just dont know by how much and they will continue to degrade quicker and quicker.

putting new ones on will help the ride depending on shocks. if you dont have enough spring rate, the shocks meant for the high spring rate will allow too much suspenion compression and can feel bouncy. too much and it feels like you are riding a brick. id rather valve a shock to a torsion bar than a torsion bar to a shock though but when trying to get back to factory specs/ride, this is how you would look at it.

hope that helps shine more light on it? if not, ask away!
 

08lmm72mm

Member
May 13, 2019
139
19
18
Saskatchewan, Canada
now on a sagging bar, whats happening is the spring rate is changing. Why? because the truck is getting lower but no adjustments have been made. So the only change is the torsion had now gone from 20* to 25* between end to end. eventually this leads to spring distortion and a high loss of spring rate. So effectively, you are riding on softer torsions, we just dont know by how much and they will continue to degrade quicker and quicker.

putting new ones on will help the ride depending on shocks. if you dont have enough spring rate, the shocks meant for the high spring rate will allow too much suspenion compression and can feel bouncy. too much and it feels like you are riding a brick. id rather valve a shock to a torsion bar than a torsion bar to a shock though but when trying to get back to factory specs/ride, this is how you would look at it.

hope that helps shine more light on it? if not, ask away!


This is the confirmation to my suspicions I've been looking for. When I lifted, the cognito 4-6 NTBD came with Bilstein 5100s (silver) and we turned the torsions up to raise the front 1/2 to 3/4" to clear the 35s. Truck rode like a dream, better than stock actually. Then I swapped out the Bilsteins to Fox Remote reservoir 2.0s, wasn't bad at first but the truck turned into a steam boat. I've since put a new set of the exact same Bilsteins in and I had to crank the tbars higher and the ride still isn't nearly where it used to be, most likely due to plenty of very rough roads for work I'd assume and the bars are going on 14 years.

I'm thinking since they are sagged I have even more preload then I was in the first place and they are running out of twist or getting soft as you mention and I'm banging against the bump stop on rough road?
 

juddski88

Freedom Diesel
Jul 1, 2008
4,431
21
38
Chesterfield, Mass.
This is the confirmation to my suspicions I've been looking for. When I lifted, the cognito 4-6 NTBD came with Bilstein 5100s (silver) and we turned the torsions up to raise the front 1/2 to 3/4" to clear the 35s. Truck rode like a dream, better than stock actually. Then I swapped out the Bilsteins to Fox Remote reservoir 2.0s, wasn't bad at first but the truck turned into a steam boat. I've since put a new set of the exact same Bilsteins in and I had to crank the tbars higher and the ride still isn't nearly where it used to be, most likely due to plenty of very rough roads for work I'd assume and the bars are going on 14 years.

I'm thinking since they are sagged I have even more preload then I was in the first place and they are running out of twist or getting soft as you mention and I'm banging against the bump stop on rough road?


The torsion bar is just not infinitely flexible. Metal reaches a point where it resists flexing (twisting in this case)
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
18,299
488
83
Phoenix Az
This is the confirmation to my suspicions I've been looking for. When I lifted, the cognito 4-6 NTBD came with Bilstein 5100s (silver) and we turned the torsions up to raise the front 1/2 to 3/4" to clear the 35s. Truck rode like a dream, better than stock actually. Then I swapped out the Bilsteins to Fox Remote reservoir 2.0s, wasn't bad at first but the truck turned into a steam boat. I've since put a new set of the exact same Bilsteins in and I had to crank the tbars higher and the ride still isn't nearly where it used to be, most likely due to plenty of very rough roads for work I'd assume and the bars are going on 14 years.

I'm thinking since they are sagged I have even more preload then I was in the first place and they are running out of twist or getting soft as you mention and I'm banging against the bump stop on rough road?

You just don’t have a spring rate that matches shock valving.

On a over sprung vehicle (our case with heavy trucks, limited suspension travel and torsion bars), you run light compression valving and heavy rebound valving in the shocks. As the bar becomes weaker, you would technically need to up compression valving and lower rebound. Since that didn’t happen, it leads to the tire bouncing uncontrolled, suspension packing and you get the feeling you have.
 

08lmm72mm

Member
May 13, 2019
139
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Saskatchewan, Canada
Finally got both leafs delivered. Each one came from a different vendor off Amazon, both are supposed to be Dorman 929-126 . Can you spot the difference?
 

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gmduramax

Shits broke
Jun 12, 2008
3,773
51
48
Nor cal
Those the little metal plates? Factory springs have them I would pull them out of your leaf pack and install in the new one
 

08lmm72mm

Member
May 13, 2019
139
19
18
Saskatchewan, Canada
Those the little metal plates? Factory springs have them I would pull them out of your leaf pack and install in the new one

Just measured it, each metal spacer is 1/8" thick for the 1st 3 leafs and the last one is 1/16". There is only 1/2" of threads on the top where the nut is. If I were to try and put the ones off my stock ones in I'd only beable to maybe get the nut on for 1 thread, I don't think it'll work, unless there is something else I can do?

This is one of the things that ordering off Amazon can be a pain with.
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Phoenix Az
Use your stock center pin as well unless it’s broke

But that other pack also is missing the plastic sliders that keep the springs from gouging into the keepers near the end of the leafs.

I’d send that wrong back and see if you can get the right one like the other. I wouldn’t doubt if spring rate is different too