Easy NP231 Upgrade

Every MJ, XJ, YJ and TJ that came from the factory with an NP231 T-case was saddled with an assembly-line-friendly slip yoke. Slip yokes are fine for street use, but for off-roading they pose several distinct disadvantages. First, they take up lots of driveshaft length, which is a real bummer in a short-wheelbase rig like a TJ or YJ. Once a lift of over 4 inches is added to these vehicles, it's nearly impossible to obtain correct driveshaft angles. Second, if you break your rear driveshaft, you can't remove it and drive home on the front axle because all the T-case oil will flow out the gaping hole where the driveshaft enters the T-case. Finally, there really isn't any easy way to upgrade the strength of the output shaft for high horsepower and big tires.

Last month we installed a Superlift 31/2-inch suspension on a YJ and promised to give you a step-by-step install of the company's heavy-duty fixed-yoke kit (PN5080) that we installed in the vehicle's NP231 T-case. Most folks are a little nervous about cracking into their T-case themselves, but it isn't brain surgery. The Superlift fixed-yoke kit comes with everything you'll need minus the driveshaft and has pretty detailed instructions. The installation isn't anything to be nervous about. You can do it with the T-case in the vehicle without too much fuss and with common hand tools. Experienced techs can do the job in about 30-45 minutes, but the weekend warrior should be able to convert a T-case in a few leisurely hours.

The Superlift kit not only increases the rear driveshaft length by about 41/4 inches on an '88-'95 YJ and about 61/2 inches on a '97-'03 TJ for better driveshaft angles, but it also upgrades the strength by about 50 percent due to its 32-spline shaft, which is about 18 percent bigger than the stock piece. The kit comes with a 1310-series CV yoke, but you could run any 32-spline flange or yoke you need to. Here's how:

The Superlift heavy-duty fixed-yoke kit (PN 5080) fits MJ, XJ, YJ and TJ vehicles equipped with an NP231 T-case and comes with everything you need to convert your slip yoke over to a fixed yoke minus the driveshaft. We got doinked at our local driveline shop to the tune of $175 to shorten and balance a TJ front 1310 CV shaft to the length we needed. Next time we'll just buy a new shaft.	Start by chocking the wheels, removing the front and rear driveshafts and front-driveshaft yoke and draining the T-case oil. Remove the three bolts on the tailhousing extension and give it a whack with a dead blow or rubber hammer to break the seal. With the extension housing removed, use a good pair of snap-ring pliers to remove the output-bearing snap ring.
Next, remove the five bolts on the rear bearing retainer housing and use a prybar to gently break the seal. If your T-case has been apart before and has been reassembled with RTV, this step is going to suck. Just take your time and only pry gently where indicated (arrows) to avoid breaking the T-case housing.	With the rear-bearing retainer housing removed, trial fit the new Superlift housing to check if the shift rail bottoms out in the housing when fully extended. If it does, you need to shift the T-case so the shift rail is as far in as it goes, then cut it off flush with the housing. You can use a cutoff wheel or even a hacksaw to cut the rail if needed. Most newer Wranglers, such as our '95, already have the shorter shift rail.
Remove the eight bolts holding the two case halves together and gently pry with a large flathead screwdriver to separate the pieces. Our case had lots of black RTV holding it together, so we needed lots of patience, elbow grease and dirty words to get it apart. There's two or three small recesses where you should pry. Bashing the case willy-nilly around the bolt holes can result in a cracked housing, so be careful.	With the case halves separated, remove the oil pump and rear of the case as an assembly. Take the mode spring off the shift rail, but leave the rest alone.
Grab the front and rear output shafts and slide the whole assembly out of the case. The chain should stay on the drive sprockets and the synchro hub should stay on the rear output shaft. The shift rail, shift fork and synchro sleeve should stay in the case. Transfer the parts over to the workbench.	Here you can see the front and rear output shafts, chain and synchro hub assembly that came out together. You need to transfer the synchro hub assembly and drive sprocket from the old output shaft to the new shaft.
Remove the snap ring that holds the drive sprocket in place, then remove it and the synchro ring from the shaft. If your drive sprocket doesn't have two caged needle bearings installed in it, then you'll need to install the ones that came with the Superlift kit.	The caged bearings are a light-press fit, so you'll need to tap them in gently with a brass drift or dead-blow hammer. The two bearings go in one from each side, with the front bearing seated flush with the inside lip and the rear bearing recessed 11/64 inch.
With the new caged bearings installed in the drive sprocket, reassemble the components the same way they came off the old output shaft. The only difference is that you must install the supplied flat-thrust plate onto the shaft before installing the supplied snap ring. With the shaft reassembled, line up the drive sprockets in the chain and reassemble the shafts into the T-case the same way they came out.	Before you install the rear T-case half, clean all gasket surfaces and make sure the oil pump hasn't dislodged itself from the pickup tube. It's critical the tube stays in the pump, or you will burn up your T-case. Run a bead of RTV around the case half and bolt holes, install the pump and rear half as an assembly, then torque the bolts to 30 lb-ft.
Install the new snap ring in front of the oil pump, then install the speedo geardrive with the teeth as shown; otherwise your speedo will run backward. Then install the Superlift rear retainer, and torque the three bolts to 18 lb-ft.	Apply a thin coat of grease to the front and rear CV yokes and gun down the nuts to about 110 lb-ft. Don't forget to use red Loctite on the yoke nut and install the supplied rubber yoke seals to prevent leaks. If your T-case doesn't have a vacuum or 4WD indicator switch in the rear bearing retainer at the shift rail, install the supplied 5/8-inch Allen head bolt. For vehicles with electronic speedometers, you will also want to make sure the speedo sensor is installed in the same orientation as it was removed; otherwise the "Check Engine" light will come on and the Speedo won't work.