The best hardcore transfer cases are gear driven with heavy-duty steel or billet aluminum cases. Having said that, there are many chain-driven cast aluminum transfer cases out on the road and trail today, and to ignore them would be silly. These T-cases are lighter and quieter than gear-driven units, making them popular with auto manufacturers and drivers looking for a quiet 4x4. If you are like us you don’t care about noise too much and a little extra weight in exchange for strength is not a big deal. Still, despite these transfer cases not being quite as strong as some others, they are more than adequate for many off-roaders and there are tons of them in 4x4s and junkyards around the country.
The New Process (NP) 231 transfer case is one of the most common manually operated (lever shifted) T-cases found in all domestic 4x4s except Fords. In fact, the NP231 can be found in many Jeeps, S-10 trucks, SUVs, and light-duty Dodge trucks (Dakotas and 1/2-ton 4x4s) built since the mid-1980s. And while many parts will interchange between NP231s (and some NP233s found in Chevys), there are a few areas where upgrading an NP231 can be inexpensive and just about as easy as a trip to the junkyard.
We’ve modified several NP231s over the years, usually installing the nearly compulsory slip-yoke eliminator, but until now we haven’t delved too deeply into how to take that everyday NP231, rebuild it to extend service life, and add aftermarket and a few junkyard parts and pieces to beef it up to be as strong a transfer case as possible. Together with some new aftermarket parts from JB Conversions and a few runs to the junkyard for bits and the parts store for some new seals, we are building the ultimate NP231 for use in a V-6–powered GM 4x4.
So GM NP231 tech, you say? Yes, but most of the tech we are going to cover can be used in your Jeep or Dodge NP231 to add beef or simply renew that Chevy NP231. Did we mention that NP231s are easy to work on? With a little knowhow and some good tools, just about anyone should be able to rebuild an NP231.
There is some debate as to the naming conventions for the NP231s available over the years. It seems to us that New Process was filling orders that met various manufacturer specifications and thus most NP231s can have different internal components while looking almost identical on the outside. The main differences between all NP231s are the number of planetary gears and the chain width. Some GM NP231s or NP231Cs are marked “HD” (heavy-duty) and contain wide chains and six-gear planetaries, but we have heard of non-HD NP231Cs with wide chains and six-gear planetaries. Most Dodge NP231s or NP231Ds have a wide chain and may or may not have a six-gear planetary. Our NP231C from a 1997 S-10 truck has a wide chain (1 1/4 inches) and a four-gear planetary.
Most of our experience is with modifying Jeep NP231s or the NP231J. These cases generally have a three-gear planetary and a narrow chain (1 inch) from the factory (more on the chains later). First, let’s talk about planetaries. All NP231J planetaries we have come across are only machined for three planetary gears. Presumably these planetary housings could be machined for three more planetary gears, but that would be prohibitively expensive and difficult. It is much easier to order a new six-gear planetary from JB Conversions. You can also rob the six-gear planetary from an NP241 housing or scrounge the junkyards looking for a possibly mythical six-gear planetary from a Dodge or Chevy NP231. Alternately, if you have a four-gear planetary and a spare three-gear planetary sitting around you can roll the dice and do what we did.
As we said, the NP231C we are building has a four-gear planetary housing. This housing is machined for two more planetary gears, and with an old spare planetary from an NP231J (from a 1997 TJ in fact) we can rob two of the planetary gears and install them in this machined four-gear housing. There is a gear pitch change in 1995, so use planitaries and gears from, and in, either before or after 1995 cases. We used our shop press and a piece of steel to push the center out of two of the planetary gears. Be careful not to lose any of the needle bearings, spacers, and plastic thrust washers. To reinstall, use grease to hold two sets of 17 needle bearings (one on top of the other; grand total of 34), three metal spacer rings (one between the needle bearings and one on each end), and two thrust washers (one on each end) per planetary gear.
Then carefully press the center pin through the housing, past the thrust washers, rings, and needle bearings. To secure the center pin in place, tack-weld it or use a chisel to deform the end (as the factory did). Alternately, you can order a six-gear planetary from JB Conversions. Those thrust washers and spacer rings are easy to damage, but you will get a spare set from your NP231J planetary.
Now that we have our cheap but not labor-free six-gear planetary it is time to talk about wide chains. Most NP231Js we have worked on had a 1-inch chain, and any 231 can be upgraded with a wide chain and the corresponding wider sprockets (again available from JB conversions, or the wide sprockets can be scrounged from the junkyard from some NP231Cs or NP231Ds).
One of the wider sprockets drives the front output, but the other wider sprocket rides on the main or output shaft of the NP231. Here our NP231C sprocket and hub on the left is in contrast to a NP231J sprocket and hub on the right. At some point New Process also switched from roller bearings (right) to a metal-on-metal design (left). Both seem to work well, but shafts and hubs must match to avoid damage. Some, if not all, Dodge NP231s also have larger hubs and shift collars than the Jeep and GM T-cases we have seen.
Our NP231C has the 1 1/4-inch chain from the factory but also has an odd front output shaft for an undesirable GM front driveshaft (right). Our S-10 Blazer that this T-case is going in will require a collapsible shaft without a double-cardan joint. That means we had to source a front output with a wide sprocket (for the wide chain) and a traditional splined shaft with a nut (middle). The best source for this was a Dodge NP231, or NP231D. In the factory application the NP231D uses a large flange, but a YJ front yoke will fit the Dodge output shaft and accept a normal 1310 jointed driveshaft, while a yoke from a TJ or XJ will fit and accept a 1310 double-cardan joint driveshaft. The front output on the left is from a narrow-chain NP231J that was in a YJ.
A slip-yoke eliminator is a must upgrade for anyone running an NP231, NP241, or NP242 on a vehicle that’s lifted or that gets wheeled. This modification gets your 4x4 away from the undesirable rear slip yoke that these transfer cases came from the factory with. Factory slip yokes have a few undesirable side effects that make them a liability on any lifted 4x4 used off-road. For one, the slip yoke limits the overall length of the rear driveshaft. Shorter driveshaft means steeper angles and more wear on the U-joints. A SYE allows the use of a longer rear driveshaft. Also, lifting a vehicle with a factory slip yoke means that the driveshaft now has more leverage on the sliding portion of the yoke. Hitting bumps now imparts more damaging jacking force to the slip yoke as the axle moves up and closer to the transfer case. This leads to bearing and seal wear with light hits (which can kill your transfer case with time), or spectacular failure when the slip yoke and shaft try to bend.
Lastly, not all NP231 slip yokes are built the same. Here we have the output shaft from our NP231C (center) and NP231J (right) and, for comparison, the JB Conversions Heavy-Duty Slip Yoke Eliminator output shaft (left). You can see that the NP231C uses a much larger shaft. That means it is stronger and less likely to fail or wear out. Still, the SYE shaft from JB conversions is the strongest because of the fixed rear output (and is the same part as an NP241, a transfer case with much higher load ratings). The SYE also allows for the use of a longer traditional collapsible rear driveshaft. This upgrade just makes sense for any NP transfer case that will get used off-road.
Not all NP231 slip-yoke eliminators are built the same. We have installed just about all the SYEs out there, and the fit and finish on some is just better than others. This is reflected in the price. And while any SYE is better than the factory slip yoke, we have seen cheaper SYEs develop leaks a few hundred miles after installation. Like with many things, when it comes to an SYE you get what you pay for. Also, all of the parts of an SYE for a NP231J won’t necessarily work on an NP231C or NP231D. The mainshaft may plug into the transfer case, but the speedometer drive mechanisms and housings (shown) are different between Chevys (right) and Jeep/Dodge (left) NP231s. With JB Conversions you get quality and knowhow and parts for all three manufacturers’ NP231s and the variations within manufacturers (especially Jeep).
All NP231s are lubricated with ATF, which is fed to the bearings using a filtered pickup and a simple pump. We like to clean the pickup of any and all debris and check the seal around the pump. If this seal is cracked the pump won’t move fluid properly, and that can lead to excessive heat and t-case damage. The pumps are easy to replace, but we have had good luck reusing them. Just check the seal and be sure the pick-up tube O-ring is good and the pick-up tube is properly inserted in the pump when the t-case is reassembled. Also, we’ve seen people disassemble the pump and inspect it, but we’d just replace the pump or the pump seal if either showed signs of wear.
Input gears are specific to the transmission the transfer case will be bolted up to in both spline count and stick-out length. Different length and spline count input gears are available from a number of sources, and are the first part in during a rebuild and the last part out when an NP231 is torn apart (so replacing one basically requires a full teardown of the t-case). Transfer case clocking is also somewhat variable, but generally if a transfer case was behind the same transmission as the one you are using, both should work for you. Clocking rings are available but add length to the assembly. Also, to add confusion to the S-10, NP231s use a five-hole mounting pattern rather than a six-hole mounting pattern common to Jeep, Dodge, and GM fullsize 4x4 trucks.
If you are in search of superlow gearing for that rockcrawler, Tera Flex also offers a 4:1 NP231 kit that includes a new front half of the transfer case and a new five-pinion planetary gear (that is somewhat different than stock NP231 planetary gears). Also available from Tera is a modified shift plate that allows your NP231 to be shifted to 2WD Lo range. We contemplated the 4:1 Tera Lo gears for our project, but the differing mounting bolt pattern would have complicated our use of this product (but could have been resolved by swapping input gears and adapters).
As said, we’ve rebuilt more than a few NP231s, and as far as we can tell there isn’t much that can go wrong during assembly. If you can feel the difference between a good and bad bearing, use snap-ring pliers, and replace a seal or two, you should be good to go. Most of the parts only fit together one way, so if something isn’t lining up you are doing it wrong. A good shake and a wiggle should allow everything to fall into place. The only part we think you can get installed upside-down is this large shift ring (silver ring in the larger shift fork) that engages the front drive. It needs to be oriented with the beveled teeth facing the back of the transfer case, or with the L-shaped shoulder facing the back of the case (rather than the rounded shoulder).
The only other mistakes we have made or seen are forgetting the spring that goes on the shift fork rod (pictured), seals that go bad, and the overuse of silicone in NP231s. Seals go bad and are relatively easy to replace, if not just a pain. As far as silicon use goes, it doesn’t take a whole lot of silicon to seal the aluminum surfaces of an NP231, but silicon can and will clog the oil pump pickup.