NP231 Rebuild Kit

Vehicle transfer box is designed for the distribution of torque between several drive axles of all-wheel drive (multi-drive) vehicles, which have high support and coupling qualities, which is achieved by an optimal distribution of the mass on the drive wheels.

NP231 sye

Introduced in 1987, this part-time transfer case became the main one for the XJ, YJ and TJ models and continues to be marketed on these models to this day as the NV231 or NVG23.

A lot of upgrades are made for the NP231, which can make from a good box a real monster of off-road.

For a start, you can replace the three-satellite planetary gear with a 6-satellite from the NP241 box, because the 3-satellite can withstand 600 lb-ft (813 N-m or 82 kg-m) of torque, and the 6-satellite – 1,400 lb-ft (1898 N-m or 193 kg-m). In some cases, the input shaft will also have to be changed at the same time.

It is also possible to supply a more powerful chain. Model 231D uses a 1.25-inch wide chain instead of the 1-inch wide chain used on Jeep and GM car boxes. All you have to do is buy a chain and chain pinions.

SYE kit has a new output axis with superior design qualities like 18% larger spline diameter and 32 splines on the output end compared to the original 27. Also, the output axle of a larger diameter gets a 54% increase in torque capacity over the original. 231 master rebuild kit includes all the bearings, seals, gaskets, small parts, chain, and instruction guide.

NP231 parts

The transfer gearbox is installed between the gearbox and the shafts leading to the axis. Despite a great variety of designs, some parts of the transfer gearbox are available in any model: driveshaft, locking mechanism and inter-axis differential as well as a gear or chain reduction gear and actuator. It also includes drive axles of the front and rear axis and a synchronizer.

NP231 disassembly

1. For loosening the nuts of shafts, first, fix their flanges with a double lever gripper with a movable – sliding link two bolts with nuts. The output axle flange of the rear axle drive is pressed together with a two-arm lift.
   1. Loosen the screws of the rear cover of the input, intermediate and output shafts of the rear axle drive with a pneumatic wrench.
   2. Take off the cover of the intermediate and output shafts of the rear axle drive, the cover and the circlip of the piston in the gearshift mechanism.
   3. Crushing the retaining ring, remove the toothed crown of the tachometer.
2. Unscrewing the bolts to secure the rear cover of the output shaft drive rear axis, also lightly tapping on its body copper (plastic) hammer, take off the cover together with the gland seal.
   1. Remove the adjustment rings on the rear bearings.
   2. Then pull out the cylinder of the switching mechanism.
   3. Then unscrew the round nut of the intermediate axis.
3. Also, unscrew the round nut of the input axle using a similar tool.
4. Screw the standard nut onto the output shaft shank of the rear axle drive. Unscrew the screws that secure the dispenser box cover to the housing.
   1. Turn the stand box to the working vertical position and put on two bolts, the intermediate flange of the rear cover of the input axis.
   2. Similarly, remove the nut and flange of the input shaft.
   3. Unscrew the bolts that secure the cover of the input and intermediate axis.
   4. Unscrew the nuts of the front axle drive cover attachment.
   5. Pull out the intermediate shaft and front axis covers.
   6. Remove the gearshift cylinder housing.
   7. Pull out the gearshift adjustment device.
   8. Then remove the gear shift piston, loosen the bolts of the gearbox cover attachment.
   9. Remove the front cover of the front axis driveshaft complete with the axis using the mounting levers.
   10. After removing the cover bolts to the crankcase, screw the special eyebolt into the central threaded hole. Lift the front cover off.
5. Unscrew the two locking bolts of the gear shift fork, note the position of the fork (e.g. part number), pull out the stem, and then remove the fork. Install a hook-away attachment instead of the nut.
   1. Pull out the input axle.
   2. Screw an extension cord with nut into the output shaft, remove the complete output shaft of the rear axle drive.
6. Pull out the gear shift piston, loosen the bolts of the gearbox cover attachment.
   1. Also, remove the front cover of the front driveshaft complete with the axle using the mounting levers.
   2. After removing the cover bolts to the crankcase, screw the special eyebolt into the central threaded hole. Lift the front cover off.
   3. Unscrew the two locking bolts of the gear shift fork, note the position of the fork (e.g. part number), pull out the stem, and then remove the fork.
   4. Install a hook-away attachment instead of the nut.
   5. Pull out the input axle.
   6. Screw an extension cord with nut into the output shaft, remove the complete output shaft of the rear drive.
7. After that pull out the oil inlet differential.
   1. The output axle of the rear drive is screwed back on, the nut of the flange attachment.
   2. Remove the intermediate shaft.
   3. Take off the screen of the intermediate axle.
   4. Finally, press cylindrical and ball bearings out with a mandrel. Remove the gear shift piston, loosen the bolts of the gearbox cover attachment.
   5. Take off the front cover of the front axle drive shaft assembly from the crankcase by removing the cover bolts, screw the special eyebolt into the central threaded hole.
8. Unscrew the two locking bolts of the gear shift fork, note the position of the fork (e.g. part number), remove the stem, and then the fork.
   1. Install a hook-away attachment instead of the nut.
   2. Remove the input axle.
   3. Screw an extension cord with nut into the output shaft, pull out the complete output shaft of the rear axle drive.
   4. While the rear axle drive axle is in the hinged position, pull out the oil inlet differential.
   5. On the output shaft of the rear axle, the drive is screwed back on, the nut flange mount.
   6. Dismantle the intermediate shaft. Remove the screen of the intermediate shaft.
   7. Finally, press cylindrical and ball bearings with a mandrel.

NP231 small parts

Also, it is possible to rebuild only small parts of NP231. To do that, you may use a small parts kit made specifically for changing the small elements of NP.