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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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Sooo... Big picture for Diesel 4x4 Van:

  1. Reorganize Diesel Fuel Pallet (Skip for gasoline van)
    • Move fuel cooler in front of cooling stack next to factory aux transmission cooler (do all years have factory aux transmission cooler?)
    • Replace priming pump with lift pump (Kennedy or similar). Pumps must be mounted out of the way of both the torsion bars and future transfer case. High on the frame rail would do this, but then requires pump to pull fuel up from the bottom of the tank. Can be mounted low outside the frame rail in front of or behind diesel aux battery?
    • Relocate fuel filter (Need to look at where it can go)
  2. Convert front suspension from coil springs to torsion bars
    • For my 2006 van, I looked up 2006 pickup truck control arms. They look like they'll fit and it looks like that's what Quigley used. I do not know if vans can use 2011 and newer pickup truck control arms or if vans are limited to 2010 and older pickup truck control arms.
    • I will start out with the 1500 van torsion bars. If I need a higher rate in the front suspension, I can install pickup truck torsion bars which are much stiffer. For some years of truck, GM used the same torsion bar across 1500, 2500 & 3500 pickup trucks, so I don't think that using 1500 van torsion bars in my 3500 van is weird by comparison.
    • I will start out with the 1500 van torsion bar crossmember. Just like the torsion bars, the trucks use the same torsion bar crossmember from 1500 through 3500. The truck and van crossmembers are very similar dimensionally, so I'm confident the 1500 crossmember will be fine in my 3500 van. The van crossmember also fits easily between the van frame rails and only requires welding on mounting tabs.
  3. Install the front diff
    • The only way to install a front diff without cutting is with a 5+" suspension lift.
    • The 2011+ pickup truck front diff has less "stuff" poking out of it than the older diff, and looks easier to swap
    • The Quigley conversions put the bodies of the inner CV joints just under the lower surfaces of the frame rails. This is as high as the diff can reasonably be mounted in the frame. I need to take measurements from my 2011+ diff (as well as get the actual diameter of the inner CV joints) to see what will interfere with what when the diff is in place.
    • The bolt-in crossmember must be removed
    • The "corrugated" stampings that mount the bolt-in crossmember and the lower control arm rear pivots mount to must be cut extensively. The pockets to which the bolt-in crossmember mounts must be removed completely. The inboard "point" of the control arm pocket on the driver's side must be cut off.
    • The engine mount structure sits on top of the corrugated stamping. The mount structure consists of a "tower" that bolts to the corrugated stamping and supports the rubber mount ~4" off the corrugated stamping, as well as an arm that bolts to the engine block and reaches out to sit on top of the rubber mount. At least the tower and maybe both arm and tower will have to be modified or redesigned to allow clearance for the diff.
    • Once top-side clearance has been made, the 2011+ diff can be mounted entirely from below by custom built subframe or frame rails. Because of the torque involved, especially if using a low range T-case, the mounting structure should bolt hard to the diff mounting points and be rubber mounted to the frame
    • Once the diff is in place, the frame rails will have to be reconnected with a new crossmember to replace the bolt-in crossmember that was removed for diff clearance. This may carry rubber bushings for the diff mount structure, but should be bolted or welded to the frame and not be rubber mounted, as the original was bolted directly to the frame.
  4. Install the axles
    • The 2011+ diff uses an 8 bolt flange for the inner CV joint. The 2010- axles have a 6 bolt flange. I have not done any investigation to see if there is enough plunge in the inner CV joints to accommodate an adapter plate between the 8 bolt drive flange and 6 bolt inner CV joint.
    • The 2010- axles have 33 spline outer CVs. The 2010- pickup truck front hubs bolt into the van knuckles and preserve the 8x6.5" hub bolt circle that the 2006 van already has.
    • The 2011+ axles have 36 spline outer CVs that only work with the 8x180mm front hubs from 2011+ trucks.
    • I'm not sure what year the vans went to 8x180mm, but for vans built with the 8x180mm pattern, using the 2011+ truck axles and hubs would be the sensible course of action.
    • For 8x6.5" vans, maybe custom axle bars to fit the 2010- inner CVs and 2011+ outer CVs would be the most direct method.
  5. If you've made it this far, you should be able to figure out the transmission and transfer case of your choice.
 

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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..
Had a duh moment...
I'm not going to be pulling the body off my van to do this BUT, I can probably source the front frame section of a van from a local friendly junk yard in order to build a mockup and figure out what the clearance problems are going to be. Doing this would be a lot more motivating to me, because I can go through the relative drudgery of relocating the diesel fuel pallet and such while knowing that the end result will work. I won't be taking on that effort at risk.

There's actually very little flexibility in the diff position. The output flanges need to be centered between the frame rails. The diff needs to be high enough that the inner CV joints just barely clear the bottoms of the frame rails, and the fore/aft position needs to be such that it clears the swing of the steering linkage, but it doesn't have to clear by much. Within that very narrow positioning window, it's a matter of cutting as little as possible to get the diff where it needs to be. The fore/aft positioning <2010 diff was difficult because it had the forward bushing positioned exactly where it's most in the way. The >2011 diff does not have that, so should be easier.

I need to get some measurements, but the 8 bolt inner CV's are probably larger in diameter than the 6 bolt inner CV's, which means the diff will hang slightly lower if using the 8 bolt inners.
I'm wondering what the diameter and spline counts on the axle bars are. I'm probably not lucky enough for the 8 bolt inners to spline directly onto the same axle bars at the 6 bolt inners. Nor would I be lucky enough for the 33 and 36 spline outers to plug directly onto the same shaft... but that won't stop me from checking.
 

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Had a duh moment...
I can probably source the front frame section of a van from a local friendly junk yard in order to build a mockup and figure out what the clearance problems are going to be. Doing this would be a lot more motivating to me, because I can go through the relative drudgery of relocating the diesel fuel pallet and such while knowing that the end result will work. I won't be taking on that effort at risk.
I would get the entire front 2/3rds the frame, you will want to be able to solve all the problems from the back of the transfer case to the steering box..
 

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Does anyone know how similar the 1500 van frame is to the 2500 & 3500 frames? Can I snag a 1500 frame section for test fitting the diff and have results transferrable to the 2500 & 3500 frames, or is there too much difference?
 
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