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if you want a good read on designing a 4x4 conversion, this thread is worthwhile...he did it on an older sprinter but much of work would apply to any GM IFS 4x4 conversion: Sprinter 4WD Conversion Idea, GMT-800 IFS.
Thanks! I'll take a look. I guess it's time for an Expedition Portal membership.
Here's the other side of the crossmember:

 

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2008, 2005 & 2015 Torsion bar crossmembers compared.
Note the curvature of the 2008...
In going back through this thread, I found this post, which shows that the pickup truck torsion bar crossmembers are also mounted on rubber bushings. Interesting.
 

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In doing a little more part number research...

The 2001-2005 pickup trucks and 2000-2013 SUVs of 1500, 2500 and 3500 varieties all use the same torsion bar crossmember: 15769710
The 2011-2020 2500 & 3500 truck crossmember is 84149360
Both crossmembers are rubber mounted, as noted above. The mounts are Dorman 905-506 being used in '99-'07 mostly 1500 pickup trucks, per the Dorman website. The similar 905-509 is used in 2500 & 3500 pickup trucks from 2001-2010.

As a result, I'm not too worried about using the 1500 van crossmember in a 3500 van, since GM used the same crossmember from 1500 through 3500 pickup trucks in '01-'05. I may want to verify how thick the steel is in the 15769710 crossmember and compare that to the van crossmember.

Mounting the crossmember might be as simple as (thoroughly!) welding a couple of mount ears on each frame rail and bolting the crossmember up... then switching to torsion bar suspension is just a matter of bolting in parts.
 

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I got under a friend's GMT800 2500 CCSB 4WD pickup. The torsion bars are 1 7/16" (36.5mm) within the tolerance of a pair of calipers.
The pickup's crossmember is stamped from 0.160" thick steel.
The van crossmember is stamped from 0.168" thick steel.
Both units have similar windows stamped in the middle.
Both units are mounted using the SAME rubber bushings.

Having seen the way GM built the 2500/3500 pickups and comparing that to the 1500 van crossmember, I'm completely fine with using the 1500 van crossmember in my 3500 van.
I have a junk yard van torsion bar on the way. It was supposed to get to my dad's Friday, but didn't, so I'll have to wait until next week to play with it.
 

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fabricating the torsion bar mounts won't be much work, you will have to pull your fuel system components out (they have to be relocated anyways). The transmission crossmember will have to be modified. The real work comes when you install the front differential, designing a mounting system to package it around the engine and staying clear of the steering will be a lot of work. It would me much easier to do with the body off..
 

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fabricating the torsion bar mounts won't be much work, you will have to pull your fuel system components out (they have to be relocated anyways). The transmission crossmember will have to be modified. The real work comes when you install the front differential, designing a mounting system to package it around the engine and staying clear of the steering will be a lot of work. It would me much easier to do with the body off..
Oh yeah, thanks for bringing that up! I knew I'd have to move the diesel fuel pallet off the trans crossmember in order to mount a T-Case and front drive shaft. If the torsion bars go above the trans crossmember like they do on the pickup trucks, then I'll have to reorganize the fuel pallet out of the way right at the beginning in order to convert from coil springs to torsion bars.

My photos of the GMT-800 torsion bar crossmember:





I realized I need to snag one from the back side of the crossmember to show how the torsion bars are located in the crossmember on the far side.
 

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1500 Van torsion bar is 1.350 diameter... so it's way too soft to use in a 3500 van. Torsional stiffness of a cylinder goes as the 4th(!) power of the diameter, so a small diametral or cross sectional difference turns into a very large stiffness difference.
Pickup truck torsion bars it is!
 

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Got to play with the van torsion bar over the weekend; also picked up a 2011 truck 3.73 front diff for $150... gotta watch, the deals are out there.

One thing that I noticed about the 2011 diff vs. the older units--and this may have been brought up in this thread, I don't remember--is that the distance from the forward extremum of the diff cover to the axle centerline is going to be be smaller on the 2011 units than on the earlier units with the clamshell case. This means the diff can be mounted with the axle centerline further forward without interfering with the steering linkage.
 

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Yes, the newer configuration is much more "swap friendly", that said; if you want to make your life easier you should plan on building in a lift to your diff mounting (dropping the diff down 3-5") which will buy you pitman arm, motor mount, front driveshaft and oil pan clearance which are all problems with a duramax van IFS4x4 set up that is "stock height".
 

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The torsion bars give easy ride height adjustment, especially with aftermarket lever arms.
I'm expecting that I'll jack the diff up into place and see what it hits along the way, then come up with a best compromise for where to put it that results in good choices at all the interference points.

I have a couple of projects in front of that, though. Putting a Cadillac Northstar V8 into a Pontiac Fiero and a Jeep 4.0 powertrain into an AMC Eagle, with an Eaton E-Locker in the Eagle Dana 30 IFS front.
 

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This is an Awesome read,,, but wanted to confirm some things from say the first 15-20 or so pages..

My 2cents regarding Quigly, is Did Quig figure out a way to make a 4x4 conversion without doing a frame off, and maintain stock ride. For sure they did.. some say Hackjob or Hack job because of the ugly rusty cuts.. They could of cleaned that up a bit i agree.

I am going to assume anyone wanting to pursue this 4x4 conversion as a DIY'er has no reservations on having to pull entire front clip, Motor, and trans, that's what we do with the diesel swap Suburbans. that's what we do in the Frank section here.

The torsion bar cross member in the 1500-2500-3500 classic trucks/SUVs GMT-800-900 is very simple. press the key against some steel and use a bolt.. all the stress is on those puny bushings.
Mounting torsion bar cross member and a separate transmission cross member is as easy as drilling some holes. if you have ever had to replace those bushings, you have to air hammer out the rivets and use threw bolts in the holes in frame.

Someone mentioned using a off the shelf lift kit would make the space required to do the fab, did anyone confirm this? by adding said space, could we use/weld in HD truck Motor mounts free up further space and potentially use the OEM front diff aluminum banana (for lack of better name :)) on the passenger side.?

I know this thread started with someone wanting 4x4 but with next to no lift... if a lift gets the job done easier in a Express without spending 5K USD on the DIY cross members from Weldtec I'm game ? I am extremely interested this thread as i want to build a Class C motorhome (GM cutaway Express/Savanna) with 4x4 and Duramax. i am sourcing a parted out a 06 2500 Express van to use the frame and and a HD truck donor as guinea pigs!

Lets keep this going Fellas!
 

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Someone mentioned using a off the shelf lift kit would make the space required to do the fab, did anyone confirm this? by adding said space, could we use/weld in HD truck Motor mounts free up further space and potentially use the OEM front diff aluminum banana (for lack of better name :)) on the passenger side.?

I know this thread started with someone wanting 4x4 but with next to no lift...
I am extremely interested this thread as i want to build a Class C motorhome (GM cutaway Express/Savanna) with 4x4 and Duramax. i am sourcing a parted out a 06 2500 Express van to use the frame and and a HD truck donor as guinea pigs!
If I had gone the IFS route, I would have definitely started with a truck lift that included a differential drop structure (then just modified that to my liking).
The van frame is really too flat to do a "zero lift" 4x4 conversion (with a duramax). You just don't have the necessary realestate between the diff housing and the oil pan and the drivers side motor mount solution is very tough to work through.
If your intent is to use the entire truck drivetrain(including allison) you really need to consider getting a newer truck donor(07.5 or later LMM would be good) so that will allow you to use the newer van dash wiring and cluster(2010 and later) and solve a bunch of other issues related to BCM communication with the ECM/TCM.
if you are going to stick with the 4l85e then it's much easier..
 

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The van frame is really too flat to do a "zero lift" 4x4 conversion (with a duramax). You just don't have the necessary realestate between the diff housing and the oil pan and the drivers side motor mount solution is very tough to work through.
And even beyond that...
The 2011 diff measures 26 3/4" across the CV joint flanges (slightly less because the tape bends over the diff cover, but that's what the tape reads)
The van frame rails are 29 1/2" inside where the CV joints would go (slightly less because the tape bends around stuff, but that's what the tape reads)
That means that the inner edge of the frame rail falls about 1 3/8" out from the face of the diff output flange. This means the body of the inner CV joint will conflict with the frame rail.
The diff then has to hang pretty darn low to run the inner CV joint under the frame rail. That puts the center of the CV joint about two inches below the bottom of the frame rail, which is, I think, below the hub centerline. I'll take some more measurements of that this weekend to figure out how low the bottom of the diff hang would hang.

Of course Quigley did the same thing and the torsion bar control arms have more axle clearance, so it can't be that big a deal.
 

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Here is the end result of the 2006 Duramax 4x4 van I built a few months ago, using a 2006 Duramax van and a 2007 Quigley 4.8 gas van chassis. Several modifications were needed to clear the engine oil pan to front differential, and 1" and 0.75" body pucks at the first two rows of body mounts to clear the doghouse.

636961
 
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