Modifying Your GMT400’s Transfer Case for Use with a 4L80E
Mike Aguilar
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The 700R4 transmission that came with your GMT400 truck from the factory has a 27-spline output shaft. You can use that 700R4 with a 4.7 LS swap, but most professionals will tell you that you should use a 4L80E if you’re swapping in anything bigger, especially if you’re building it up for more power. The problem is, the 4L80E has a 32-spline output shaft. RacingJunk is going to walk you through the steps of replacing the 27-spline input shaft on your transfer case with a 32-spline input shaft that will mate up perfectly with your new/rebuilt 4L80E. As an added bonus, we’ll also let you in on the secrets of how to make your truck All-Wheel-Drive instead of Four-Wheel-Drive.
A Quick Walk-Through of this Project
Replacing the input shaft on your transfer case can be broken down into three basic steps:
1. Transfer case removal
2. Input shaft replacement
3. Transfer case reinstallation
Obviously, each of these steps will need to be broken down even further. Read on, intrepid wrench-spinner, as we do our magic. We’re going to go on the presumption that the transmission and transfer case are already out. This first portion will also presume that your transfer case is the standard NP241 found on most GMT400 trucks.
Remove the four bolts (arrows show three. One is hidden) on the rear output shaft cover and use a rubber mallet to loosen and remove the cover. Use a large pair of snap ring pliers to carefully remove the snap ring on the output shaft. Be careful not to break this snap ring. It is tricky and strong.
Next, remove the four bolts securing the tone ring cover and use a prybar and rubber mallet to separate the cover from the main body of the transfer case. The output shaft bearing will come out with the cover. Grab the snap ring pliers and carefully remove the snap ring securing the tone ring, then remove the tone ring. Finally, remove the last snap ring securing the planetary assembly in the case.
Remove the 11 bolts securing the two halves of the main transfer case body. You will most likely need a breaker bar or impact wrench to break these free. Use a prybar and rubber mallet to separate the two halves of the case. Remember, even though you (hopefully) already drained the transfer case prior to removal, some will remain and go everywhere unless you are prepared for this to happen.
Turn the transfer case so the rear section is facing up and lift the rear (now upper) half of the case off. Be careful to not damage the oil pump or the pickup during this step. The chain connecting the rear output shaft to the front diff output shaft is now exposed.
Another snap ring/C-clip, this time on the front output shaft gear. Removing it allows you to lift the rear output shaft assembly on one side and the front output drive gear on the other side as a unit. Again, removing this snap ring can be a “bear.” Once those are out and carefully set aside, the shifter mechanism can be removed and set aside. Now you can move on to removing the input shaft.
To get the input shaft out, we need to remove the input shaft cover. Four bolts secure the input shaft cover to the rear cover of the housing. Flip the housing half over using a block of wood (pretty much anything will work) to support the input shaft inside the housing and remove those bolts. Again, a breaker bar or impact gun will probably be needed. Pry the cover up and off. Another snap ring secures the input shaft in the housing. Once removed, the input shaft and planetary gear assembly can be carefully tapped out with a rubber mallet.
Remove the Input Shaft from the Planetary Assembly
Another one of those infernal snap rings secures the input shaft to the planetary gear set assembly. Carefully removing it allows you to remove the input shaft.
Reassembly is, as you would imagine, a reversal of the steps outlined above. Be sure to match up the replacement input shaft with the one that came out to make sure it will fit the planetary gears in your NP241 transfer case. When reinstalling the covers, be sure to line up the dowel pins and use a very light bead of silicone (I prefer RTV Black as it’s more oil resistant) on one side before mating them up and bolting them together. Make sure you clean all mating surfaces completely prior to reassembly or you will have a leak. Also remember, the NP241 uses ATF, not gear oil.
Don’t overdo the sealant when closing things up.
Above: Mechanical lockout with set pins for full time four-wheel-drive. Not recommended for high power applications.
One problem with the 4WD setup on the GMT400 is that it uses a front diff that disconnects the passenger side wheel when the NP241 is shifted into 2WD mode. This isn’t good when we’re upgrading to an LS engine with more power than the wimpy TBI305 they came with. One way around this is to raid the NV241 and full-time locked front differential out of a 1999-2006 Cadiallac Escalade or Yukon Denali with All-Wheel-Drive.
The NV241 is a viscous-coupled transfer case that divides torque 62 percent to the rear and 38 percent to the front. The front differential from these trucks is externally identical to the one your GMT400 came stock with, but it doesn’t have that weak link that can break when you pump more power through it.
If the NV241 you yank sat behind a 4L60E/4L80E, great. If not, you’ll need to swap the input shaft. The process is pretty similar to the one described above, but since there is no shift mechanism, there is no linkage to remove. The snap ring on the output shaft is accessed through a window in the case (see image) and you only have to separate the two halves of the case; there aren’t multiple covers to remove.
If you go the way of installing the front diff from the Escalade/Denali, be aware that you will need to fab up a passenger side mount for it, as the older diff mounts lower than the newer one. I’ve got pictures of that above as well. The driver side mounts are identical, as is the track width between the CV joints.
Note Well: Make sure that as you’re reassembling the transfer case, you fully seat the snap rings in their grooves, or you will have big problems later.
