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Super-Duty Jeep Transfer Case Conversion

Posted in How To: Transmission Drivetrain on July 16, 2018
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Much of the project plan for this 2006 Jeep Wrangler TJ has been to maintain an old-school vibe and keep it somewhere in the middle between dirt road cruiser and rockcrawler. We wanted a simple and reliable build that we can have a lot of fun with. As of this report, it sports 4 inches of Currie lift, Currie Antirocks front and rear, Rancho RS9000XL shocks, and 15x8 Mickey Thompson wheels. At the moment, a set of 33x10.50R15 BFGoodrich KM2 Mud Terrain tires are wrapped around the aluminum wheels. It can take 35s if we want. All that change created a chain reaction, as do most modifications to any vehicle. The lift demanded new driveshafts, and a slip-yoke eliminator kit was needed for the TJ’s New Process 231 transfer case to allow a longer-than-stock rear shaft for better pinion and U-joint angles to help eliminate vibrations.

After browsing for a few hours we settled on the Tom Wood’s Custom Drive Shafts Super-Duty 231 slip -yoke eliminator kit for the NP231 transfer case. Why not a low-range gear ratio conversion? The NP231 offers a versatile 2.72 low range and direct drive in high range. With the 4.0L straight-six engine, six-speed manual transmission first-gear ratio of 4.46:1, and a ring-and-pinion gear swap that will come with the new locking and limited-slip differentials (that’s a whole ’nother story to come soon), we can run 33- or 35-inch tires, and the 2006 TJ will have all the giddyup it needs to go exploring.

What’s the difference between heavy duty and super duty? Well, the Tom Wood’s Super-Duty 231 conversion kit comes with the manufacturer’s unique rear output flange that allows the installation of a 1210, 1310, 1330, and 1350 series driveshaft with a conventional two-joint CV (double-Cardan) setup, or a 1410 series conventional two-joint driveshaft, all without changing the flange. As is very common, a Jeep goes through many evolutions throughout its lifetime. Who knows what further modifications may be made down the trail. With this setup, we’re not limited to one type or size of driveshaft, and the flange also offers a convenient spot for a park brake rotor if that’s ever in the plan.

Although this entire operation can be done with the NP231 still bolted up to the transmission, you will need an assortment of hand tools (that include some specialty items), as well as the extra kick of pneumatics (or a huge breaker bar) to uncork the gland nuts on the front output shaft and axle pinions.
With driveshafts out of the way and yokes on both ends removed, it was time to unscrew the drain plug from the NP231 transfer case. We opened up the inspection plug too, allowing the fluid to drain quickly. Although we’ll soon be seeing the innards of the transfer case, inspecting the newly drained fluid for metal shavings can help identify early signs of wear and tear. The transfer case should be placed in 4 Low before any more work is done.
There are some slight differences between the ’96-and-newer NP231 and the ’95-and-older NP231. For ’96-and-newer transfer cases the balancer/dampener hub and slinger/shield is discarded for this upgrade, as is the factory trail housing. Our transfer case was sitting in a 2006 Jeep Wrangler TJ and had the balancer/dampener; a three-jaw gear puller was the ideal tool to remove it. The slinger/shield seal was gently popped with a mallet and chisel, and then the snap ring inside was removed using snap-ring pliers. The innermost snap ring holding the bearing in the tail housing does not need to be removed.
Next was removal of the wiring harness from the transfer case, including the speedometer drive signal converter. Be careful with the speedometer drive during removal; it will be reused. If the drive’s seal or gear is damaged during removal it will leak or not function properly when reinstalled.
The five 10mm hex bolts holding the tail housing to the case were removed, and after carefully and slowly prying at the edges of the housing, it easily came loose and was removed from the case.
To separate the case halves and get to the innards, all eight bolts holding them together must be removed. One of the eight bolts was a 12-point 10mm head, but all the others were 15mm heads. We kept track of where each bolt came from to be sure each went back where it belonged later. There are two pry tabs that can be gently separated with a flat screwdriver, helping loosen the two case halves from each other.
Support the oil pump when removing the rear case half, as it’s connected to the pick-up tube located inside the rear case half. If the pump and/or tube do become disconnected, they can be set aside until it is time to reassemble the case.
Jp Pro Tip: There is a spring on the shift rail that can easily fall out when separating the case halves or pulling out the chain and output shafts. Keep an eye on it, and make sure this spring is there before the case is reassembled. This is a good time to also inspect the spring. If the case is very old or any wear and tear appears on the spring, it should be replaced.
Now that the case was separated, the chain and output shafts were carefully pulled rearward simultaneously from the front half of the case while keeping the chain on the sprockets. There is one shift fork attached to the shift rail, and it’s best to leave it attached. Keeping an eye on all the parts and their proper location and orientation during this process is important.
Using snap-ring pliers, remove the large retaining ring and then slide the mode hub and chaindrive sprocket off the main output shaft. Keep the hub and sprocket in the proper order for installation on the new Tom Wood’s NP231 Super Duty slip-yoke eliminator kit mainshaft.
Jp Pro Tip: Here’s another essential part that can fall out (and break) during disassembly of the transfer case halves. This ring-shaped magnet sits in a special slot at the very bottom of the case and attracts metal shavings to help keep the case’s fluid clean. Save it, clean it, and place it back in its slot before the case halves are reassembled.
This photograph of the old (top) and the new (bottom) main output shafts shows the difference in length and thickness between the two. You can also see the blue nylon speedo drive gear on the new Tom Wood’s mainshaft. At this stage we did some pre-assembly of the new tail housing, factory speedo signal converter, and new mainshaft (without mode hub and chaindrive sprocket) to align the gears of the signal converter and drive gear correctly, and then marked the converter’s clocking position on the case so it could be correctly realigned upon reassembly.
The mode hub and chaindrive socket were slid into place on the new Tom Wood’s slip-yoke eliminator main output shaft, and the large snap ring was installed. Our ’06 mode hub and chaindrive sprocket fit right on, but ’94-and-older factory mainshafts used caged bearings in the chaindrive socket. These caged bearings must be removed before the chaindrive socket will fit on the newer and thicker Tom Wood’s mainshaft. A small punch and hammer can be carefully used to remove the cages.
If your NP231 has a lot of hard miles on it (ours carried just 65,000 miles), it’s worth taking a look at the condition of the shift forks, bearings, and bearing surfaces for any concerning wear patterns. We did it anyway. It’s better to replace now that you have the case open rather than months down the road. Jp Pro Tip: Make a photographic record of how all the bits and pieces were in place before and during removal so you know exactly how they all go back together correctly.
The factory chain, factory front output shaft, and new Tom Wood’s main output shaft (with factory mode hub and chaindrive socket) were assembled. Then after making sure the shift forks were in their proper positions, we carefully guided the entire shaft and chain assembly back into its home in the front case half of the NP231.
First, we made sure the pick-up line and oil pump were connected and in place properly, double-checked that the spring was back on the shift rail, and degreased and then applied a smear of silicone to the mating surfaces on both case halves. Next, the rear case half was slid back into place, taking care to not loosen the oil pump as it went over the main output shaft. Force is not needed here: If it doesn’t go back together easily, then try rotating the mainshaft while sliding the rear half case on. It should take no more than a couple taps with a rubber mallet, otherwise something’s not right and you need to take a look inside.
To assemble the speedo drive gear to the new mainshaft, an inner snap ring, the blue nylon speedo drive gear, and an outer snap ring must be placed on the mainshaft, in that order. The snap rings don’t need to be tight, they just need to be in the snap-ring grooves on the shaft.
Installing the new tail housing was easy enough. We made sure a thin film of silicone was applied to the housing mating surface, and then slid the housing over the mainshaft, taking care to line it up with the pump tabs, bolt holes, and shift rail pocket. We made sure the housing seated flush against the case, and then pulled on the mainshaft to be sure it rested on the bearings in the tail housing. The five tail housing bolts were dabbed with thread locker and torqued down to 15 lb-ft (specs are 15 to 20 and we’ve had no leaks).
Now came the icing on the cake—the rear output flange. This is not just any rear output flange. Aside from accommodating a much sturdier and shorter main (rear) output shaft from Tom Wood’s now inside the NP231 in our ’06 Jeep Wrangler TJ, this new output flange is what makes the Super-Duty 231 kit special. The manufacturer’s unique flange allows the installation of a 1210, 1310, 1330, or 1350 series driveshaft with a conventional two-joint CV (double-Cardan) setup, or a 1410 series conventional two joint driveshaft, all without changing the flange. After degreasing the splines on the rear output shaft and inside the output flange, the splines on both received a small amount of silicone to help prevent fluid leaks. The output flange was slid onto the rear output shaft, and the gland nut was torqued to 140 lb-ft. Specs are 140 to 150 lb-ft.
The final reinstallation of the factory speedo signal converter and its alignment with the blue nylon drive gear on the new main output shaft was easy because we had done a pre-fit to mark the converter’s proper clocking on the case to correctly align its gear splines with those of the drive gear. The 4WD electrical connector was also plugged back in at this time.
Final button-up on the upgraded NP231 under our 2006 Jeep Wrangler TJ was to reinstall the drain plug, fill the case with ATF to the point it bled out of the inspection hole, and then reinstall the inspection hole plug.
Part of the Tom Wood’s Super-Duty 231 tailshaft conversion kit (sometimes referred to as a slip-yoke eliminator) package can be a custom rear driveshaft, and in order to take full advantage of the stout nature of the new tailshaft we opted for the recommended Tom Wood’s 3-inch tube, 32-spline, flanged double-Cardan 1350 CV-style shaft.
This new Tom Wood’s rear driveshaft allowed us to upgrade the yoke at the rear axle too. We went from the factory 1310 to a much beefier 1350 yoke for the factory optioned Dana 44 rear end underneath our 2006 Jeep Wrangler TJ. It’s plain to see the size (and strength) difference between stock 1310 and the 1350 upgrade in this photo.
Here’s what the new rear driveshaft from Tom Wood’s looked like completely assembled and bolted up to the flange on the new Tom Wood’s rear output shaft and the 2006 Jeep Wrangler TJ’s Dana 44 rear axle. In order to get a reference measurement for our new rear driveshaft, we measured (per instructions from the manufacturer) from the forward edge of the rubber boot on the stock transfer case’s rear output shaft to the centerline of the U-joint at the rear differential. This measurement should be made with the suspension lift already installed, the stock transfer case still in the vehicle, and with the vehicle sitting at static ride height. The tape measure should follow the approximate slope of the stock rear driveshaft, and go from point to point (no bends in the tape measure). Consult with Tom Wood’s when making the measurement, as the NP231 was available in a number of models of Jeeps—there are different models of the NP231, and each requires a different method of measurement to arrive at the reference number you need to supply Tom Wood’s Custom Drive Shafts with for your custom rear driveshaft.


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Tom Wood's Custom Drive Shafts

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