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Improper lubrication or towing a vehicle in four-wheel drive can lead to catastrophic failure of the transfer case. A worn-out chain can result in metal particles floating around in the oil which wears out other components, and the whole unit can grind itself to death or explode. Even without catastrophic failure, when it’s time to rebuild your NP231, you should look into the different upgrades available.
Common wear points on these T-cases are the plastic wear pads on the aluminum shift forks. They last relatively long, but when they disintegrate the fork itself takes a beating. Replacement steel forks for some styles are available from Tri-County Gear and are a worthwhile upgrade.
This notch in the case half is the result of a worn-out and loose drivechain eating away at the aluminum as the chain flops around. If not fixed, the housing will wear through and lose fluid, and then completely destruct. Sometimes pieces of metal get trapped between the sprockets and the chain, which forces the case apart sideways.
Tri-County Gear offers a chain and sprocket upgrade kit that increases the strength of the NP231 transfer case assembly by as much as 25 percent. The wider chain can handle more torque, and the matching gears are stronger as well. The 1/4-inch-width difference of the heavy-duty chain means it won’t stretch as easily for prolonged life expectancy.
For maximum strength, Bunch uses a six-gear planetary instead of the original three-gear unit. The matching input gear is also available in different lengths and input spline counts, should you decide to swap in a different type of transmission. The tooth pitch of the NP231 transfer cases varies in some model years, so swapping parts should be left to experienced professionals.
Variations of the shift forks and shafts have also been used throughout the years, and some may need to be modified with the new shaft and gears. The shaft on the left needs to be ground down for case clearance. This is especially needed if a heavy-duty, short-output shaft will be installed. However, some units have a switch or an engagement mechanism that the shaft operates, so don’t randomly hack off pieces.
The secret weapon in the Tri-County Gear upgrade is the shift pawl that allows the case to be in a true Neutral position for towing, rather than both the front and rear driveshafts being hooked together. The difference between the two is negligible from the outside but once installed, a distinct change in shift-lever pattern is noted.
Converting to the short, heavy-duty shaft so a longer driveshaft can be used is an important part of the buildup. Three different short-shaft conversions are offered by Tri-County: (left to right) the resplined M.I.T. shaft, the one-piece heavy-duty shaft, and the Currie shortened shaft. The next long shaft (shown) is a stocker from a YJ Wrangler, and the far right shift shaft is from a stock TJ Wrangler.
For Gary Downs of Tri-County, assembly is pretty straightforward since he does most of the custom transmission and transfer case work for the company. Downs uses all new bearing and seal kits for a rebuild. These kits and pieces can be ordered separately or fully assembled from Tri-County Gear.
The many variations and styles of the NP231 T-case mean experience is necessary before you swap in nonstock parts, and this steel shift fork, instead of the aluminum one, is but one example. Some cases and input shafts feature different bearing widths that match certain gears, so care must be taken on assembly.
With the new heavy-duty planetary chain, sprockets, and shaft in place, the shift-shaft spring is installed and the case halves are bolted together. The heavy-duty shaft doesn’t neck down at the output splines like the stock or resplined ones do. This also means a different speedometer drive gear is needed, a speciality item not to be overlooked.
The oil pump is critical for proper lubrication, yet is often overlooked during a rebuild. Six small screws hold the pump together, and the simple gerotor design usually needs only a thorough cleaning and reassembly. Make sure the pump gears and surfaces are coated with assembly lube before installation to provide ample pressure on initial start-up.
The heavy-duty shaft kit also needs a new four-bolt output cap and adapter, which is shorter than the stock three-bolt style. The heavy-duty double-ball bearing in the new output cap lasts much longer than the single-row stock unit, and a quality seal tops off the adapter to eliminate leaks.
The final step before you install and fill the unit with oil is tighten down the output yoke. Several output yoke options are available for different series and sizes of U-joints, should you wish to upgrade from the stock style. And Tri-County Gear stocks virtually any part available.
Exploding your truck's transfer case can really be a drag, whether you're on the trail or just flat-towing your 4x4 down the road. The NP231 transfer case found in many late-model Jeep, Dodge, and GM vehicles has a fairly good reputation for a chaindriven, aluminum-cased unit, but improper towing procedures can cause it to explode, and some of the internals are iffy for big tires and lots of torque. These facts make the NP231 a good candidate for upgrades and modifications to make it a truly durable unit.
One of the first upgrades for Jeep owners should be the short-shaft kit, which converts the rear output yoke to a fixed style rather than a slip yoke, to allow removal of the driveshaft without fluid loss and fitment of a longer driveshaft for better driveline angles. The story "Instant Superhero" in our Dec. '96 issue showed a heavy-duty JB Conversions shaft installed in a '97 Jeep TJ, which is much stronger than stock and shortens the overall transfer case length. In the same story, we featured a 4-to-1 reduction unit that greatly increased the low-range ratio for outstanding off-road gearing without the highway drawback of lower axle gears.
But even these trick parts and pieces won't overcome a serious drawback of most NP231 cases--towing the vehicle without a true Neutral position. We talked to Jason Bunch at Tri-County Gear, who said that the standard NP231 such as that found in a Jeep YJ Wrangler connects the front and rear driveshaft together when it's shifted into Neutral, and drivetrain windup is relieved by the front-axle disconnect being disengaged. Unfortunately, when the engine is started and the case is put in Neutral, the vacuum-operated disconnect engages and stays engaged even after the engine is turned off. After a few hundred miles of towing, the resulting drivetrain windup can literally cause the transfer case to explode, just as though it were driven in four-wheel drive on the pavement.
The simple solution is not to start the engine before shifting the transfer case into Neutral for towing or to unhook the vacuum hoses to the front axle. But Bunch has some special parts he installs to provide a true Neutral inside the T-case for hassle-free towing, as well as some other beef-up parts for strength and longevity. Although some people believe that converting to an older-style cast-iron case is the only solution to the problem, the NP231 has proven to be a perfectly adequate unit. Follow along as Bunch shows how he modifies and beefs up the NP231 into a durable, towable transfer case that wont leave your rig with a bang.