A full bearing-and-seal rebuild on the NP231 is a bench job you can do in an afternoon that fixes leaks, a stretched chain, and hard 4-Lo shifts — and it's the right time to add a slip yoke eliminator if your XJ is lifted.
The NP231 is the part-time chain-drive transfer case behind almost every 1987–2001 XJ, and it's built to be rebuilt. Two decades of ATF and trail use wear the same handful of parts: the chain stretches and slaps the case, the oil pump wears a groove through the rear aluminum and weeps onto the driveshaft, the mode fork pads erode until 4-Lo won't engage cleanly, and the front and rear output seals start dripping. None of that means a replacement case. A master rebuild kit covers every wear item for well under the cost of a reman unit, and the whole thing comes apart with hand tools once it's out of the truck.
You'd tackle this when you find ATF pooling under the transfer case, hear a rattle or slap on deceleration, feel the shifter fight you going into low range, or you're already pulling the case for a slip yoke eliminator on a lifted rig. Do it now rather than later — a worn pump that eats through the rear case leaves you stranded with no drive, and a stretched chain that jumps is a bad day on the trail. Plan to rebuild on a clean bench with the case drained and pressure-washed, and keep every snap ring and shim in the order it came out.
This guide is the rebuild overview — the sequence, the parts that matter, and the torque values. Pull a factory service manual or a detailed teardown video up alongside it for the exact snap-ring and shim positions on your year.
Steps
Drain and clean the case before anything comes apart. Pull the drain plug, dump the old ATF, then pressure-wash or wipe the exterior spotless. Grit that falls into an open case ends up in a new bearing. Set the case in a vise by a mounting ear or in a holding fixture.
Remove the rear output/extension housing. Take out the housing bolts and separate it from the case. On a slip-yoke case this is where the SYE goes if you're converting; on a stock case, note the speedometer components and the rear bearing and seal for replacement.
Separate the case halves. Remove the perimeter case-half bolts, then split the case with a soft drift in the pry slots — never pry on the sealing faces. Lift the front half straight off so the chain, sprockets, and shafts stay together as an assembly.
Lift out the chain and sprocket assembly as a unit. The drive sprocket, driven sprocket, and chain come out together. This is your primary inspection point: lay the chain on the bench and check for stretch and side-plate wear, and inspect both sprocket teeth. If the case has 150k+ miles, replace the chain regardless of how it looks — a new 1" HV-027 chain is cheap insurance against a jump on the trail.
Inspect and replace the oil pump. Slide the pump off the rear output shaft and check the rear of the case bore for the classic "pump rub" groove worn into the aluminum. A grooved case leaks ATF onto the driveshaft; fix it with a pump rub repair kit or an updated wide-body pump, or plan on a replacement/SYE housing if it's worn through. Always fit the new pump and pickup from the kit.
Pull the mode fork and shift components. Remove the shift rail, the mode fork, and the range fork. Inspect the nylon/bronze fork pads — worn pads are the reason 4-Lo won't engage or pops out. Replace all fork pads from the kit; they're the cheapest part in the job and the most common cause of shift complaints.
Disassemble the planetary and mainshaft as needed. Break down the input/planetary assembly, mode hub, and rear output shaft only as far as the kit requires. Keep thrust washers and shims stacked in order — endplay depends on them going back exactly as they came out.
Replace every bearing and seal in the kit. Drive out the old input bearing, front and rear output bearings, and the pilot bearing, and press in the new ones squarely with a driver or a socket that bears on the outer race only. Fit new input, front output, and rear output seals. A cocked bearing or a nicked seal lip means you're doing this twice.
Reassemble the shafts, forks, and chain onto the front case half. Reverse the teardown: install the output shaft and planetary, set the forks with new pads, then lower the chain-and-sprocket assembly on as a unit. Confirm the forks sit in their sprocket/hub grooves and the shift rail moves freely through its range before the case goes back together.
Seal and join the case halves. Clean both sealing faces down to bare aluminum, lay a continuous bead of the specified RTV/anaerobic sealant, and mate the halves. Torque the case-half bolts evenly in a crossing pattern to roughly 23–35 ft-lb (verify the exact value for your year in the FSM — Chrysler lists these on the higher end near 35 ft-lb / 47 N·m).
Install the front output bearing retainer and rear housing. Fit new gaskets/seals, then torque the front output retainer bolts to about 15–20 ft-lb and the rear output/extension housing bolts to about 23–30 ft-lb. If you're installing an SYE, this is where the new short housing, seal, and yoke go.
Set and torque the front yoke. Install the front yoke with a new seal riding on a clean, unscored sealing surface, and torque the yoke nut to roughly 90–130 ft-lb. Replace the yoke if the seal contact area is grooved — a new seal on an old worn yoke still leaks.
Confirm it shifts through every position by hand. With the case fully assembled, work the shift lever through 2H, 4H, N, and 4L and confirm each engages positively and the output turns as expected. Fixing a missed fork now beats pulling the case again after it's back in the truck.
Fill with 2.2 quarts of ATF and torque the plugs. The NP231 runs Dexron III / Mercon ATF, not gear oil — it's a chain case. Fill to the bottom of the fill plug hole (about 2.2 qt), then torque the fill and drain plugs to roughly 30–35 ft-lb. Reinstall in the truck, top off after a short drive, and check for leaks.
Why it works
A master kit rebuild restores a leaking, hard-shifting NP231 to like-new for well under the price of a reman case, and every wear item — chain, pump, bearings, seals, fork pads — gets addressed at once.
It's a bench job with hand tools: no special press is strictly required if you drive bearings carefully, and the case comes apart in a logical sequence you can photograph as you go.
Doing it yourself means you're the one who knows the chain and pump are fresh — critical peace of mind before a trail trip, and it pairs naturally with an SYE if your XJ is lifted 3" or more.
Trade-offs
Endplay and shim setup matter. Get thrust washers and shims back out of order and you can end up with binding or excess play — keep everything stacked in sequence and consult the FSM for your year.
If the pump has already worn through the rear case, a repair kit may not save it and you'll need an updated housing or SYE case, which adds cost.
Torque values vary by model year and source; the ranges here are a starting point. Confirm the exact case-half, retainer, and yoke-nut specs against a Chrysler service manual before final assembly.
If you need a 4:1 low range or true full-time AWD, rebuilding the 2.72:1 NP231 is money spent on a case you may replace later — weigh an Atlas or NP242 swap first.
Tools required
Bench vise or transfer-case holding fixture
Snap-ring pliers (internal and external)
Dead-blow hammer and soft brass drift
Bearing/seal driver set (or sockets sized to the outer races)
Torque wrench (ft-lb and in-lb)
Large slip-joint or channel-lock pliers for the pump
Gasket scraper and brake cleaner
Drain pan
Dial indicator (optional, for output-shaft endplay check)
RTV sealant (Mopar-approved anaerobic or gray)
Parts
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Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.