Step By Step
The easiest way to drain the case is to pull the bottom two cover bolts and let the fluid pour out. After the shift linkage and driveshafts were unhooked and secured out of the way, the adapter-to-transfer-case bolts were removed and the case was slid backward a little so the transmission fluid could drain from the adapter housing. Make sure the unit is well supported on a transmission jack when you do this, since the NP203 weighs around 300 pounds. While our model had no mounts on the case itself, some rigs have struts or brackets on the side that must also be removed.
Once the beast was out, the input splines were checked for wear. The shaft coming out of the transmission should also be checked and replaced if worn. On the Ford divorced case with a yoke on the front, the splines cant wear unless the yoke becomes loose. For manual-transmission Chevy and GMC applications, a coupler sleeve slides over the transmission output shaft and the transfer case input shaft, which are male style with external splines rather than the internally splined female style as shown on this Dodge. GM coupler sleeves commonly wear out and are a major source of drivetrain slop and noise.
After the rear yoke and output cap were removed, the output shaft and differential assembly were pulled out. The mating gear needs to be held inside the case with your thumb to prevent the internal gear from coming out as well. The needle bearings between these two gears will fall out but must not be allowed to drop into the main case unless a total rebuild is being performed.
The spider gears in the differential function just like an open-axle assembly and can wear the same way. Check the teeth and the thrust washers behind the gears for wear. If a standard part-time conversion kit is being installed, this step is not as important because the gears will be locked together and wont rotate.
These gear teeth on the inside of the carrier (which are chipped and worn) index into the sliding cogs on the shifter. Excessive wear in this area accounted for the majority of noise in this 203, and new parts would be relatively expensive to fix just this one problem. A shaft-style part-time conversion kit could be used to replace the differential side gears but the sliding cogs would remain worn, so noise would still be evident. It was cheaper and better for us to install another NP203 from Boyce.
The rebuilt 203 came from Boyce Equipment, which specializes in military axles and cases. At $150 and painted olive drab, the NP203 cant be beat. The case was popped apart to remove the tailshaft and differential shown here. Even though these components were in perfect shape, the part-time kit conversion will keep them from wearing out again in less than 10,000 miles.
The one-piece-shaft part-time conversion from MileMarker is a truly heavy-duty unit that completely replaces the differential gears and carrier; a lighter-duty kit simply locks the spiders together. On a few Ford conversions, a new flange yoke is required to match the spline count on the new shaft, and early Chevys with cast-iron one-piece tailhousings need extra gaskets to shim the housing for clearance.
Mopar shafts also differ from the GM style in that instead of a snap ring near the main bearing, a thrust bearing and shims are used to control endplay. The Torrington bearing and thrust washers were coated with white lithium grease and inserted into the back of the shaft before installation. The loose needle bearings described earlier were also eliminated with the addition of the shaft kit, but the O-ring on the indexing shaft must be in good shape or else replaced.
The small plastic gear off the old shaft was greased and slid onto the new shaft. This helical gear looks like a speedometer drive gear but is actually an oil pump gear. This gear installs flush with the top of the case and pumps oil to critical parts when the gear is turning.
Once the oil pump gear was in place, the speedometer drive gear was slid down the shaft next. Rather than changing this gear and the driven gear to compensate for different axle-gear ratios of larger tires, an external reducer on the speedometer cable output gear was used.
To set endplay, the original shims were lubed and put in place as a starting point. If no endplay is evident, remove shims until some endplay is felt. If excessive endplay is observed, add shims to tighten the assembly. The existing shims are usually adequate, but more are available in a variety of sizes.
After the end housing was bolted on, the shaft was moved in and out to check for endplay. Notice that this was done before the speedometer driven gear was installed, because it could cause a false indication. The instructions call for some endplay but dont give a specific amount. Generally, the shaft must move in and out a perceptible amount, but not much more than that.
There was no endplay even after all the stock shims were removed. Situations like this are common and simply require the addition of one extra gasket under the differential housing to provide endplay. Use silicone sealer sparingly on the gaskets as shown to prevent excessive goo from circulating into the oil and clogging up the passages.
With proper endplay set, the rear yoke was installed, but not before the rubber washer went in the end of the shaft. This special washer does a superior job of sealing the end of the shaft and should be in good shape or else replaced. In a pinch, silicone sealer can be used, but it doesnt always seal as well as the rubber washer does.
The front yoke on the replacement case came with a square front flange, so the yoke from the old case needed to be washed and swapped onto the new one. Early 203s use a 10-spline front output shaft, while later ones use either a 30- or a 32-spline. If youre switching cases or flanges and yokes, make sure of the spline count so you end up with the right combination of parts.
The replacement case had been sitting around in a humid environment, causing the gasket surfaces to rust. A good seal between the cases is essential to prevent leaks, so a cleaning wheel was used to buff the rust off the surfaces. The input shaft surface should also be polished if its rusty to prevent seal damage and leakage.
Fully assembled and ready to install, the NP203 assembly took two strong people to lift it. An old transmission shaft was slid into the input splines to provide a convenient handle to hold the awkward assembly. When installed, the case should be refilled with 30W motor oil and shifted into Hi-Loc every few weeks to adequately lubricate immobile parts.
The full-time four-wheel-drive NP203 transfer case is one of the most standard items found in 70s fullsize 4x4s. Big, heavy, and relatively strong despite its chaindrive, this box can be converted to part-time with a simple gear or shaft swap inside the case. But few people understand how the insides tick on this case, so we thought wed clue you in to some of the innards and theory without a complete rebuild or overhaul.
The full-time aspect of the 203 is all in the rear section of the unit. The case has an internal differential that selectively splits the power to the front and rear axles when the shifter is not in one of the locked positions. When shifted into Hi-Loc or Lo-Loc, the differential locks to provide equal power to both driveshafts. If you removed either driveshaft and tried to drive the truck with the case in the nonlocked position, the internal diff would send all the power to the yoke with the missing shaft. Its just like an open rear axle differential, where all the power goes to the tire with the least traction. However, if the case was shifted into a locked position, power would be equally distributed to the front and rear, so youd be able to drive the rig as long as one driveshaft was still in place.
Many of you with 203s are running around without a front driveshaft and with the shifter in Hi-Loc since thats the free method of overriding the full-time 4x4 feature. But after a while youll probably notice a clunk emanating from the case. Whether your rig is a Dodge, a Chevy, or a Ford, the NP203 will develop internal wear when subjected to this type of use. Minimal driving isnt a problem, but continual driving in the Hi-Loc position without the front shaft installed causes the internal coupler on the differential to develop a lot of slop, eventually causing a banging noise and then destruction.
The differential carrier is also an item that can shred with this type of abuse. Many 203 cases use a pressed-steel cage for the differential gears instead of the two-piece heavy-duty style, and Jason Bunch at Tri-County Gear says the pressed-steel unit wears out quickly if you do a quasi conversion this way. The best bet is to install a conversion kit in the 203 case and add locking front hubs to eliminate wear and drag on the front axle internal components. Even the 74-79 Dodge trucks and Ramchargers with the live-axle front end can be converted to use hubs with a MileMarker kit.
Such was the situation on our subject vehicle, a 74 Dodge Ramcharger with a couple thousand miles in Hi-Locand a terrible clunk. The noise from the T-case got so bad it made us think the whole shebang was going to be scattered across the freeway during our rush-hour commute. Fortunately, it made it to Tri-County Gear, where Bunch diagnosed and inspected the dead Dodge case. After a thorough evaluation, the good Dr. Bunch recommended installing the MileMarker part-time kit into a rebuilt NP203 from Boyce Equipmentsomething that had been rolling around in the back of the Ramcharger for a while, adding ballast and just waiting for this opportunity.
Once finished, the new parts had eliminated the drivetrain clunk and provided extra peace of mind. Because of the conversion, the Dodge now has a two-wheel-drive low-range position, although were going to use it sparingly to prevent overstressing the rear axle. The case we got from Boyce Equipment was also a near-direct replacement for the Dodge, but many of the internals will also fit Fords and Chevys.