The heart of a four-wheel-drive drivetrain is the transfer case. In two-wheel-drive, power moves straight back from the transmission to the rear axle, driving your vehicle. That’s perfect for everyday driving needs but not adequate for off-road purposes. When extra traction is needed, the transfer case is what splits the power and sends it to the front and rear axles.
Your typical factory transfer case is a two-speed transfer case, having a high range and a low range. For the average off-road conditions, two ranges are perfectly adequate. However, when things get really hardcore and you’re running really gnarly trails, much more is needed.
This cut-away of an Advance Adapters Atlas 4 transfer case shows off the unit’s sturdiness and quality of workmanship. The complex design and engineering that went into the Atlas 4 is easy to see when you can gaze at its guts.
We knew we needed something that would take on that frequently messy off-road world, and give us the control to pick our way through sheet-metal munching rocks. The Atlas transfer case from Advance Adapters was the solution. Better yet, its four-speed (three low-range options) Atlas 4 (PN Atlas-4sp-jk) was the perfect choice for this JK destined to be a rock hound. New driveshafts would be necessary too, as the Atlas is difference in length than the stock transfer case, so we went to JE Reel for a set of custom ’shafts with 1350 U-joints.
With close to 100,000 hard miles on the factory clutch of our subject JK, it was time to replace it before it failed on the trail. Because we were going to already be underneath the JK and elbow deep in a transfer case swap, it was a good time to replace the clutch and flywheel too. Centerforce Performance Clutch offered a dual-friction clutch kit (PN DF098391), throw-out bearing (PN N1764), and a new billet steel flywheel (PN 700474) for our JK. While the new dual-friction clutch was a huge improvement over the factory clutch, the flywheel made even more of a difference in drivability.
If you’re going to be doing a lot of this sort of work, we recommend investing in a Universal Drivetrain Jack from Advance Adapters. It’s simple yet immensely useful for any kind of work where you may have to raise, lower, or support a transmission or transfer case. This one is adjustable from to side-to-side and can be hung from J-bolts that hook into existing holes in the vehicle frame, and a 3/4-inch wrench or socket is all it takes to raise and lower the padded jack-foot.
The new Centerforce billet steel flywheel was 18 pounds heavier than the factory flywheel. At low speeds the added rotating mass of the new flywheel helped keep the engine turning several hundred rpm lower than before, and when rockcrawling, we found we didn’t have to worry about feathering the clutch until about 500 rpm. While it doesn’t sound like much, it makes a big difference when slowly navigating a stone-studded trail.
Thanks to the experts at Advance Adapters, the installation went without a hitch, and its website had an excellent video detailing how to properly adjust the shifter cables. While not exceptionally difficult, it does need to be done correctly to ensure smooth operation and many happy miles of use. The clutch and flywheel install went smooth as butter too. We haven’t had any issues with the new transfer case or clutch/flywheel assembly, and our only regret is that with a demanding 9-5 job, we haven’t been able to go Jeeping as much as we really want to. Oh well, first-world problems. Follow along as we show you some highlights and things we learned during the installation.
Sitting side by side, the Advance Adapters Atlas 4 transfer case looks like a piece of automotive art compared to the factory NP241OR. The factory T-case has a much thinner (and weaker) cast housing, whereas the Atlas 4 is machined from a block of aluminum. This combination of material and design allows Advance Adapters to build an immensely strong T-case while keeping the weight reasonable.
The clocking ring is what allows for different installation angles, as well the as fine-tuning of ground clearance and fit within your particular vehicle. The bolts were threaded in hand-tight and then the transfer case was test fit to the vehicle. Once we were happy with driveline angles and clearances, we removed the transfer case and tightened the bolts into the clocking ring. It took some effort and is best to remember that they are interference fit threads. While they may feel like you cross-threaded the bolts, it’s just how they work.
There were a lot of little brackets, bolts, and adjustments that went with installing the Atlas 4 transfer case. All of this is best done outside the vehicle where you have room to work. We installed the cables with the Atlas 4 outside of the vehicle. That way they could be removed later if necessary for installation. Proper cable and bracket adjustment was critical to the smooth operation of the transfer case.
The new oil level sight tube on the Atlas transfer cases is a great addition. No more sticking your finger into the fill hole and hoping that you guessed right about the level. When you initially fill the transfer case with the proper amount of oil, a zip-tie around the tube can be used to mark the oil level. Checking the oil after that is as simple as crawling underneath the vehicle and taking a quick look. The top fitting of the sight tube is also one of the easier places to add oil to the transfer case once it’s installed in your vehicle.
Advance Adapters includes this cover with Atlas transfer case JK kits, to plug the hole for the speedometer gear. Previous models of the Jeep Wrangler used gear- and cable-driven speedometers, but the JK uses wheel-speed-sensors on each wheel. They electronically monitor wheel speed, sending a signal to the vehicles computer, which then displays the vehicle speed.
Due to the differences in length between the factory and the Atlas transfer cases, new driveshafts were necessary. J.E. Reel built custom ’shafts that were the perfect length and perfectly balanced. The new ’shafts bolted right in. The new ’shafts were sturdier than the OE units and came with beefy 1350 U-joints.
The parts kit from Centerforce Performance Clutch was complete. The only thing we needed to do was use solvent to clean the cosmoline residue off of the flywheel before installation.
In a side-by-side comparison it’s easy to see where Centerforce added 18 pounds of weight. The recessed center has more metal around it, fewer holes were drilled into the material, and overall the flywheel is thicker. The greatest advantage to the heavier flywheel is that it helps the engine to keep turning at lower rpm. That extra rotating mass has more inertia for low-speed crawling, and you can lug the engine down much more without stalling than you can with the factory setup.
The Atlas 4 really is a thing of beauty. It looked even better mounted under our JK and getting closer to being drivable. It also took up considerably more space. While we got it in there without losing any ground clearance, room between the tub and the transfer case was much tighter. It was not tight enough to cause any problems, but it did make for a very snug fit for the shifter cables, and it is something to be aware of if you have wiring or any kind of plumbing running up there.
The Advance Adapters shifter bracket was designed to use the existing bolts and holes. We removed the center console and shifter, then bolted the new bracket in its place and dropped the center console back over the top. The hardest part of all was routing the shifter cables. They are high-quality shifter cables with very strong sheaths. This meant they don’t like to bend to a very small radius. Patience—and a helper—were key to still having hair once we were done here.
Mounting the shifter for the planetary gear of the Atlas 4 is where some might balk a little, but it is well worth the trouble. It required cutting a hole in the side of the transmission hump. A step drill and a die-grinder were all we needed to complete it, using the mounting plate as a template.