Jeep has been outfitting its Wranglers, Commanches, and Cherokees with the New Process/New Venture Gear 231 transfer case since 1988. It has been a great case for the majority of the owners, but some of us just want more. The 231s were not the first chaindriven transfer cases used in Jeeps, but they are the most common, and their 2.72:1 low range was the lowest low range available in a Jeep before the 4:1 Rubicon transfer cases became available. We recently started messing about with an old Jeep YJ Wrangler and, in doing so, found that the 231 transfer case had seen better days. The rear tailhousing was cracked, and we could tell the case had been taken apart in a previous life, so we decided some upgrades were needed.
The first upgrade we wanted was lower gears, so we opted for the Low 231 HD Kit from Teraflex. This will give us a 4:1 low range, perfect for big tires and the small 2.5L four-cylinder engine the YJ is running. While we are opening up the case we opted for the two-wheel-drive low-range option, which allows us to creep around in low-range rear-wheel-drive only. This is useful when turning off road if you have a constant locking front differential like a Detroit Locker.
We upgraded our cracked tailhousing to the Extreme short-shaft kit, which offers both a fixed rear output yoke and more space for a longer rear driveshaft or some sort of future crawler box if need be.
Why go with all these upgrades instead of merely swapping in an aftermarket transfer case like an Atlas? The Atlas is a great case and the pricing has gotten almost as cheap as all these upgrades to the 231, but the 231 has some things going for it. The chaindrive 231 cases are quieter on the road, lightweight, and already set up with front driveline and shifter, and we had the case, making the upgrades just a little more affordable. The next step in upgrading the 231 would have been to add a wider chain, which is available from JB Conversions, but by then we would have surpassed the price of the Atlas. We decided to settle for this current build. It will give us a strong entry-level case with plenty of grunt and factory-style reliability.
 1. Our NVG231 was in an old...  1. Our NVG231 was in an old ranch Jeep, so we started the rebuild with a thorough cleaning. Because the transfer case will come completely apart, it is important to take some time to get all mud, crud, and pasture pies off the case for ease of dis- and reassembly. |  2. We quickly jumped into...  2. We quickly jumped into the disassembly by following the step-by-step instructions that came with our Teraflex parts. The rear tailhousing came off first (ours was cracked and leaking) followed by the speedometer gear housing. We're not going to show you all the steps, but we will recommend a quality pair of snap-ring pliers. |  3. When removing the speedometer...  3. When removing the speedometer gear housing, notice how the oil pump is installed in the back of the transfer case. This must be correctly reinstalled during assembly or your transfer case will not work and you'll shear off the four tabs inside the transfer case. |
4. The Tera Low 231 HD Kit comes with a new front half of the transfer case that packages the heavy-duty, five-gear, low-range planetary, giving it the deeper 4:1 low range. The case bolts back onto the transmission the same as the stock case and doesn't change the front driveshaft length at all. We needed the 21-spline version for our four-cylinder YJ; there are also various 23-spline versions for the different YJs, TJs, and Jeep trucks and Cherokees.
 5. Inside the front half of...  5. Inside the front half of the case we installed the two-wheel-drive low-range kit, making the Jeep easier to turn during tight wheeling without losing the low-range gearing. |  6. The Extreme short-shaft...  6. The Extreme short-shaft kit replaces the rear output shaft of the transfer case and the tailhousing for additional driveshaft length. The kit allows for a 7-inch-longer rear driveshaft, helping to reduce rear driveshaft angles and replacing the rear slip yoke with a fixed yoke. This kit does require a new rear driveshaft to be made. When you compare it to the stock output shaft, you can see the length decrease and the shaft diameter increase we gained. |  7. The super-short rear tailhousing...  7. The super-short rear tailhousing uses a tone ring and an electronic sensor for speedometer readings, so any early mechanical-speedometer Jeeps will require an electronic speedometer. |
8. Our new Tera Low 231 was out of the Jeep, cleaned up, rebuilt, and back in the Jeep in less than a day. Reassembly of the transfer case is simple enough that an amateur mechanic can build this assembly with a few specialty tools. We do need to order a new rear driveshaft, but we'll be upgrading the rear axle soon so that wasn't a concern. The lower, slower gearing will help the little four-cylinder turn bigger tires when crawling down our favorite rocky trails.