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How To: Rebuilding A Jeep NP231J Transfer Case

Posted in How To: Transmission Drivetrain on January 12, 2017
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We pay a lot of attention to the engine, transmission, axles and suspensions of our Jeeps, focusing a significant amount of time and money on them with performance upgrades and maintenance. However, somewhere along the way, one of the most critical components of the entire drivetrain is often overlooked: the transfer case.

The T-case takes the brunt of the power distribution load and never rests, it’s internals churning along no matter the conditions, waiting to distribute power to the front and rear driveshafts at the driver’s command. They are robust and, for the most part, trouble-free under normal operating conditions.

Chain-driven NP231Js found under TJs, XJs, and YJs are like that. It’s a stout T-case that can handle a considerable amount of engine torque and operator abuse without missing a beat for years and years. Its biggest weakness, according to JB Conversions in Sulpher, Louisiana, experts in both New Process and Dana T-case rebuilds and upgrades, is the Jeep’s 27-spline output shaft breaking at the speedo gear, which is where the shaft is at its narrowest diameter.

Ditching the slip-yoke

The other weakness of the NP231J, according to JB Conversions, is the factory slip-yoke. On stock Jeeps that see normal highway and off-road use, the yoke slides back and forth across the splines of the rear output shaft within a narrow range of movement. The section outside this narrow area of is exposed to the elements resulting in rough spline surfaces covered in rust and grit.

Load your Jeep down with passengers and trail gear or start putting the suspension through its paces off-road, and the slip-yoke slides the output seal over those rough splines, cutting into the seal and causing the T-case to start leaking. Over time, that slow leak can drop the oil level to a point within the transfer case that a catastrophic failure occurs.

The factory slip-yoke also limits the amount of lift you can add to a YJ or TJ before the working angle of the rear U-joint gets too steep and is short-lived. JB Conversions’ fixed yoke not only allows the use of a longer rear driveshaft (4.5-inch on YJs, 6.5-inch longer on TJs) and more suspension lift, it also increases output shaft strength by about 50 percent.

A third reason to ditch the OE slip-yoke for a JB fixed-yoke is the latter allows you to be able to disconnect the rear driveshaft on the trail without fear of losing the oil in the T-case, driving out using the front differential. That’s a great safety benefit.

Our NP231J rebuild/upgrade parts from the JB Conversions Partial Master Rebuild Kit (PN 2033), Wide Chain Kit (PN 2032) and Slip-Yoke Eliminator Kit (PN 16-1200-1942) are ready for their new home in our ’89 XJ T-case.

Rebuild for durability

The need for a rebuild also comes with normal wear and tear, such as drivechain wearing and stretching over time; bearings and seals wearing from heat and contaminated fluid; and the nylon pads on the shift fork wearing down or falling apart. Those wear issues, and our plans for putting the T-case in a lifted, V-8 Jeep destined for hard trail use, are what prompted us to rebuild an NP231J pulled from an ’89 XJ. (Jeep started using the NP231J in 1988 Wranglers.)

Most NP231J rebuilds only require JB Conversions’ Partial Master Rebuild Kit (PN 2034), which has all the parts, bearings and seals for everything except those related to the rear output housing. Those items are included with the Slip-Yoke Eliminator Kit (our T-case, with the mechanical speedo, required kit PN 16-1200-1942).

We also ordered the Wide Chain & Sprocket Kit (PN 2032) to further beef up the NP231Js innards. The chain is what transfers the torque to the front driveshaft, and its 1-inch width is adequate for stock needs.

However, that OEM Jeep drivechain is suspect when handling the power of a V-8, which is why the more serious Jeep trail drivers upgrade to the 1 1/4-inch-wide chain typically found in Dodge 1/2-ton NP231Ds. That upgrade is easily accomplished with JB Conversions’ kit that includes the OEM Borg-Warner Morse chain, wider drive-sprocket, and shaft.

The other JB

Parts in-hand, we headed to Jon Barricklow at JB Custom Fabrication in Salem, Oregon, (no relation to JB Conversions) who specializes in building custom stainless steel shifters for both New Process and Dana T-cases.

Being close by, Jon was the natural go-to person for our T-case rebuild. He can rebuild T-cases in his sleep. It only took him a couple hours to rejuvenate our old T-case because JB Conversions’ rebuild kits include every seal and bearing, all neatly packaged, so no time is wasted on runs to the parts store to chase something down.

The instructions with each kit are well documented as well, complete with photos for those who find pictures easier to understand than words.

Parts in hand, Jon showed us how to quickly make our tired old Jeep T-case into a strong, reliable power splitter that will happily do its job behind the torque of a V-8 day in and day out, year after trail-running year. We hope you find the same path a good investment.

The first thing to find and remove from the T-case is the data tag, which tells you the year, model, and gear ratio of the unit. If the tag is left on when you clean the case in solvent, it’ll remove the paint, making it hard to read. We learned that lesson, as you can see from our tag on the right!
Remove the vacuum switch for the front differential and related hardware before cleaning. Plastic doesn’t like solvents! An 11/16-inch deep-well socket gets it off easily.
Remove the speedometer gear. If you are not going to do the slip-yoke eliminator upgrade, mark the position of the gear with the T-case housing before removing so it can be indexed properly when reinstalled.
When removing the yoke, use a 1 1/8-inch socket and puller so the dust cover isn’t dented. A dented dust cover can be hit with each rotation and sound like a driveshaft problem.
Carefully remove the rear housing extension/seal retainer. There are no gaskets used on the NP231, just RTV sealant, which does a really good job holding pieces together as it ages. This part will be replaced by the slip-yoke eliminator kit.
One of the best tools you can have for working on T-cases is a great pair of snap-ring pliers. The snap rings in most T-cases are strong and require a lot of leverage to spread open.
A 18V impact makes short work of removing the 10mm bolts holding the retainer to the rear case half.
An invaluable tool for working on NP transfer cases is an indexing pry bar like this one from GearWrench (PN 82210). It fits nicely between the pry tabs and housings. The NP231 housing faces are machine-fit, so gouging them while trying to pry them apart with a screwdriver or traditional pry bar is something to be avoided at all costs.
The case halves are held together with bolts of varying lengths. Make sure they go back into the same locations from which they were removed.
Inspect the oil pump and housing carefully and note how the pump aligns with the case. The feed tube can be a little difficult to get back into place during reassembly.
Pay close attention to the oil pump filter. If there are any signs of metal particles in the filter, inspect all bearings and cages in the unit very carefully. If you find metal specks, it’s caused by wear in the case somewhere.
The mainshaft and front output shaft have to be removed together because the chain isn’t flexible and doesn’t stretch. They are replaced the same way. We are updating the chain and output shaft assembly with a wider version for greater strength.
Don’t forget to remove, inspect, clean, and replace the magnet at the bottom of the main case. Jon says this is often overlooked or left out on rebuilds.
Here you can see the difference between the stock output shaft (top) and JB Conversions’ shaft that is part of the Slip-Yoke Conversion kit. The heavy-duty shaft (bottom) has an 18 percent larger spline diameter and 32 splines instead of the 27 on the smaller shaft. The JB shaft can handle 54 percent more torque than the OEM version.
Remove the synchro assembly and carefully inspect it for any signs of broken teeth, which can be a common occurrence on T-cases that have seen a lot of off-road abuse. Also note how the synchro is oriented. It’s easy to put in backwards.
Pay close attention to the shifting sector for pin wear or other damage. It’s also good to take a photo of this before removal so it can be put back together the same way.
The old shift range (mode) fork in our NP231 had non-replaceable nylon pads. The little pads were worn out, so we upgraded to a new fork (left) from JB Conversions with the replaceable pad design.
The mode fork shift rod in the stock T-case of ’88-’89 YJs (and other Jeeps that have a 10.2-inch long rod) will not work when changing to the fixed rear output shaft yoke. It has to be cut down to 9.380 inches.
If you aren’t good with arithmetic, a 10.2-inch shift rod will need to be shortened .820 inch to make it work with the JB Slip-Yoke Eliminator kit.
The best way to shorten the range fork shaft is to put it in a vise and use a cut-off wheel. Wearing leather gloves and safety glasses is prudent when doing such work. Smooth and bevel the cut end of the shaft when finished.
Most NP231 rebuilds don’t need to go through the extra steps to pull the planetary assembly to get at the rear output bearing and bushing. If yours needs those parts replaced, get the JB Master Rebuild Kit (PN 2034.), which includes those related items. We pulled ours apart and found everything was fine.
The slip-yoke eliminator kit uses of a much shorter output shaft housing (left) than OEM, which means a longer driveshaft can be used along with a 1310 U-joint. It also removes the oil seal leaking issue common to Jeeps.
A very important step in the rebuild process is lubricating all the new parts as they are re-assembled with the same lubricant that will be run in the T-case. ATF is a good choice because it’s the same fluid that is factory spec for the NP231J and what the oil pump is designed to run.
Replace the gears on the new shafts, install the new wide-chain, and then drop both shaft assemblies back into the case as one unit. The heavy duty chain is a 1/2-inch wider than the OEM version.
Run a thin bead of RTV sealant (supplied in the rebuild kit) around the inside edge of the case half. Then spread it around with your finger tip. It just needs to be a thin coat. You don’t want any sealant getting inside the case or it’ll damage the oil pump.
JB uses double-lip seals on the new rear case that are far better performing than the OEM versions on the stock slip-yoke case.
You should be meticulous on rebuilds like this, torqueing every bolt to factory specs. Here, the 15mm case half bolts are getting torqued to 30 ft-lb.
The slip-yoke eliminator kit includes a new speedometer gear for the larger diameter shaft. It’s held in place by a new snap ring that also part of the JB Conversion kit. Important: The gear sits on a snap ring in the upper of two lower grooves and the top snap ring goes into the upper of the two upper grooves.
It’s very important to keep the RTV clear of the oil passage in the rear output shaft cover by going behind the bolt head as shown. Run the bead and use fingertip to flatten the bead out to a thin film. Use blue Loctite and torque the bolts to 30 lbs.-ft.
Reinstall the shift detent spring with the supplied O-ring and plug. It should be lubricated with ATF as well.
Apply a little black RTV to the rubber star washer seal that goes on the rear output shaft before the new 1310 yoke and nut go on. The nut (supplied along with the yoke) is then torqued to 180 ft-lb. The front yoke is installed the same way but only torqued to 90 ft-lbs.
The last step in the rebuild is to shift the T-case back and forth between 2Hi and 4Hi to verify the shift rod moves in and out as shifting occurs and is moving freely. If all is okay, install the threaded plug with the O-ring in place (where the old vacuum disconnect switch resided) and lightly tighten. A little RTV on the threads ensures it won’t leak. If the rod is the correct length, the bolt will not interfere with the shift rod’s in/out movement.
Our rebuilt NP231J is ready to rock. We also installed a JB Custom Fabrication cable shifter to make our rebuilt T-case an even sweeter package no matter where it’s being used.


JB Conversions
Sulphur, LA 70664
JB Custom Fabrication

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