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AA Slip Yoke Eliminator for the Jeep NP231 Transfer Case

When I ordered the Pro-Comp Stage 2 YJ Suspension (Coil Conversion) Kit, I knew that because of the amount of lift and the already short rear driveshaft in the Jeep Wrangler, I was going to need to put a Slip Yoke Eliminator (SYE or Fixed Yoke) Kit in my NP231 transfer case. These kits eliminate the slip yoke section on the rear of the transfer case, which allows for a longer rear driveshaft (reducing driveshaft angles). SYE kits also solve a very common source of problems on the trail (breaking the slip yoke and releasing all of the transfer case lubrication) .

The Advance Adapters' Slip Yoke Eliminator Kit.

I looked at all the various offerings on the market and decided to go with the Advance Adapters kit because it came with a heavy-duty 32 spline shaft, and because I live near their shop, so I could save money on shipping and get a tour of the place! After looking at the instructions a few times, it became obvious to me that I was not going to be able to do this without some help. A neighbor of mine (whom I had never met before this point), stopped by one day while we were working on the lift kit to see what we were doing. We showed him our progress on the lift kit and then I started discussing the transfer case kit that I was nervous about installing. Turns out he runs a mechanics shop and does just this sort of work for a living (Note to self: meet all other neighbors and find out what they do. You never know when it will come in handy.)! He also happens to own a very nice CJ-7. He offered to help me install the kit at his shop, and there was no way I was going to turn down a great offer like that (thanks Jess!)...

The skid plate removed.

We started by draining the case and then supporting the AX-15 transmission and the NP231 with jack stands

This allowed us to remove the cross member/skid plate. Then we removed the six nuts that attach the NP231 to the AX-15, and removed all the electric and vacuum lines attached to the NP231. Because we were installing the lift kit at the same time, the front and rear drive shafts had already been removed.

The NP231 Transfer Case after removal.

Once the nuts were off, we simply slid the transfer case off the transmission and took it to the shop. The instructions provided by Advance Adapters were good, but the pictures were not of the best quality. However, we were fortunate that we rarely had to refer to the instructions, as Jess had worked on many transfer cases before.

First Jess removed the front output yoke. Then he removed all the bolts that held the tail cone to the tail housing, and removed the tail cone. Then he removed all the bolts that held the tail housing to the front case half, and removed the tail housing. The Advance Adapters kit comes with a new custom tail housing, so the stock one can be discarded along with the tail cone. Then Jess removed all the bolts that held the case together and pried the case apart.

The slip yoke (cone) has been removed. The tail housing has been completely removed. The transfer case open and ready for modification

 

He removed the front output shaft and the chain, and then I took those pieces (and all the bolts and the front case half) to the parts cleaner and gave them a good scrubbing. While I was doing that, Jess removed the main shaft and disassembled it.

Transfer case parts The old transfer case shaft. The new shaft and the old shaft side by side.

Jess took the drive sprocket over to the bearing press, removed the old bearings and installed the new ones included in the kit. Next, he reassembled the drive sprocket, mode hub, shim, and retaining ring onto the new main shaft. We then reinstalled the front output shaft, new main shaft, and chain back into the case, making sure everything was well lubricated. The front case half went back on next, with a film of RTV applied to seal it. We reinstalled the bolts and torqued them down according to the instructions.

The finished transfer case

At this point, according to the instructions, we needed to shift the transfer case into all modes and make sure the shift rail did not protrude more than 1" from the case. If it did, we would have had to shorten in (the new tail housing only provides 1" of room). In our installation this was not a problem, so we lubed the new tail housing and installed it with a thin bead of RTV.

The modified case back in the Jeep

We then reinstalled the front yoke, and installed a new C.V. rear yoke. The last step was to reinstalled the speedometer housing. The total time was about 3 hours. I am sure that if had tried it myself, I would not have had the luxury of a shop with all the right tools, and I probably would have tried to install the kit while the transfer case was still attached to the vehicle and I am also sure I would have taken a lot longer.  

The HD CV driveshaft installed.

The next day, we reinstalled the case to the transmission and we measured the distance from the new C.V. yoke to the yoke on the rear differential (In this particular installation the rear driveline length came to 16 1/2", but this is a function of the new lift kit and load weight, so all measurements should be made individually on a each vehicle).

With these measurements in hand I was able to order a top of the line heavy duty driveshaft from Tom Wood's Custom Drive Shafts and installed it when it came in a couple of days later. If it had been necessary to drive the vehicle before the rear shaft was done (perhaps to a local driveline shop) it can easily be done by simply driving in front wheel drive mode.

 


Contacts:
Advance Adapters
4320 Aerotech Center Way
Paso Robles, CA 93446
Phone 1-800-775-4407
Fax 1-805-238-4201
Tom Woods Custom Shafts
306 East 31 Street
Ogden, UT 84401
Phone 1-877-4XSHAFT (497 4238)
Fax 1-801-393-4592