Editor Hazel is a total control freak. We are not talking about enforcing employee dress codes or tracking how on-time stories are turned in. Nah, we mean in terms of how he likes his vehicles set up. Some UA vehicles have really gone over-the-top in terms of complexity. But for Hazel’s rig he insisted on a healthy dose of analog with a heaping side of manual control. Outside of the small, simple ECU that controls the Cummins R2.8 Turbo Diesel engine and turns on the Flex-a-lite electric fan, everything is driver-operated. And that includes deciding in which gear the UACJ-6D moves.
But his insanity goes beyond simply eschewing an automatic transmission for a manual. Nope, he’s gotta have a gear for each and every occasion—no exaggeration. The drivetrain he chose to mount behind the 161hp, 267–lb-ft turbodiesel was, um, varied, to say the least. For starters, in order to mate the Cummins-specific bellhousing to a 4x4 transmission, Axis Industries got the nod. The company has come up with a super-nice billet adapter system that mates the Cummins bellhousing to your choice of AMC AW4 auto, AX15, NSG370, or NV3550 as well as Chevy 4L60E, 4L80E, and 6L80E transmissions. The adapter system retains the Cummins flywheel in addition to the flywheel or flexplate of the donor transmission. The starter engages the Cummins flywheel, and the clutch or flexplate handles the engine-to-transmission coupling. By the time you read this, Axis should have its Chrysler 8HP70 adapter kits ready as well. But when we were working on the UACJ-6D, Axis only had the AMC components ready.
While Tech Editor Verne Simons was busy fabricating on the frame, freelancer Trent McGee assembled the many, many gearboxes, starting with prepping the junkyard SM420 to go between the Advance Adapters Dana 300 adapter and Ranger Torque Splitter.
We sent our factory Cummins flywheel to Axis to be lightened from its roughly 30 pounds to 15 pounds and grabbed a stock-spec, neutral-balanced 1995 Jeep 4.0L flywheel from a local auto parts store. A super high-quality South Bend Cluth pressure plate and disc ride the Jeep flywheel. We nabbed a new AMC bellhousing from Advance Adapters and then ordered a Ranger Torque Splitter with a 27 percent overdrive built with AMC front and Chevy rear transmission patterns. Advance can build the Ranger with several different bellhousing patterns from AMC/Ford, Toyota, and large- or small-bearing retainer GM. Hazel has used the Ranger in other projects and knew from experience how well the units work.
Behind the Ranger is a dead-stock SM420 that Hazel purchased for $100 without even cracking the top for inspection. It was then fitted with an Advance Adapters Dana 300 1x23 spline conversion stub shaft and adapter before getting mated to the Offroad Design Magnum Box and a Ford NP205 T-case, which we covered in depth in the story “NP205 Gearing Options From Offroad Design” (goo.gl/yx8eBP).
The resulting flexibility is staggering. Need to go super-crazy, slinky-slow? Drop the SM420 into its 7:05:1 First, engage the Offroad Design Magnum Box’s 2.72:1 Low, and put the Ford NP205 in 1.96:1 Low for (factoring in the 4.88 axle gears) a combined crawl ratio of 183:1. That’s slow enough to watch paint dry while you drive. But if it’s ever just too slow, Hazel can engage the Advance Adapters Ranger Torque Splitter overdrive to kick that down 27 percent, giving him a 134:1 Low. Or, use the Ranger to split the SM420 gears on the street, effectively turning the four-speed SM420 into an eight-speed multi-ratio transmission with a 0:73:1 Overdrive. Yes, you can engage any of these gearboxes in any combination in either two- or four-wheel drive. Enjoy the five-shifter drivetrain assembly, and next time we’ll show you how we mounted and plumbed the Cummins R2.8 Turbo Diesel.
The Offroad Design Magnum Box shares the same six-bolt round pattern as the super-common Dana 300 and NP231/241 T-cases, and ORD can install just about any input shaft available. We spec’d a Jeep/Chrysler 1x23 input. The Ford NP205 features 32-spline front and rear output shafts and, of course, the gargantuan 32-spline chromoly Magnum Box “fat shaft,” which doesn’t neck down like a factory NP205 intermediate shaft. It’s the strongest four-speed T-cases system available.
To mate our 2WD SM420 to the Dana 300–pattern Magnum Box, we used Advance Adapters PN 50-9702, which uses a set collar and conversion stub shaft to go from the SM420’s coarse 10-spline output to a 1x23. In lieu of using the set collar with this kit you can remove the SM420 output shaft and machine it for a snap ring, but we have had good success with the collar in the past and, honestly, in the interest of time, wanted to avoid disassembling the transmission.
With the Advance Adapter adapter (PN 50-9702) mated to the rear of the SM420 and the conversion stub shaft in place, the Magnum Box could be mocked into position. The Magnum Box features a huge number of threaded stud holes to clock the unit in a wide variety of angles. Once McGee determined the best angle for the UACJ-6D’s front output, he installed the studs into the Magnum Box and bolted it to the adapter with some high-quality RTV Silicone.
The beauty of the JK Unlimited frame we used is that the framerails have no front-to-rear taper almost entirely between the control arm mounts. That meant we could modify the stock JK crossmember for mockup and simply slide the assembly front-to-rear for positioning as needed. To get the crossmember to squeeze between the adapter and Magnum Box mounts, Simons lopped off the portion that stuck out and welded in a plate to box it in (arrow).
Next, Simons figured out he could use Energy Suspension body bushings from a Jeep CJ under the Offroad Design Magnum Box mounting foot and a custom mount built off the bottom of the Advance Adapters adapter mount to keep the drivetrain at the correct angle with respect to the crossmember.
A bottom plate was designed that would sandwich the Energy Suspension bushings between it and the mounts on the T-case and adapter. With the template drawn up, Simons had Rob Bonney Fabrication cut the plate on its plasma table.
In this shot you see the whole T-case mounting setup, including the mounts on the underside of the T-case and adapter, the bushings, and the sandwich plate. The system actually uses eight Energy bushings: two on each of the bolts that sandwich the mounts for the T-case and adapter. This helps keep the drivetrain from binding on the crossmember mounts in the case of extreme chassis.
The input bearing retainer of the SM420 was shortened in accordance with the Ranger Torque Splitter direction and with the factory crossmember bolted up, the tranny and T-case were slung under the frame and then the engine, bellhousing, and Ranger Torque Splitter were prepped so the whole drivetrain could be bolted together and mocked into place.
Since we could not properly mount the drivetrain without knowing where the engine would sit in the chassis, we happily uncrated the Cummins R2.8 Turbo Diesel and slung it on a stand to be prepped.
The first step was to remove the 30-pound factory cast flywheel and send it to Axis Industries to be machined. Since the Axis adapter setup retains both the Cummins and donor transmission flywheel, the resulting mass would be substantial.
Here you can see the Axis Industries bellhousing adapter and CNC-lightened and -relieved Cummins flywheel. If we were only using a single-speed T-case with no low-range gearing or a transmission with a high First gear, we would have run the Cummins flywheel uncut for additional inertia, but given our super-low crawl ratio, we opted to have Axis cut 15 pounds out of the Cummins flywheel. The billet adapter indexes inside the Cummins flywheel and bolts between it and the donor transmission flywheel. In our case, we used a 1995 neutral-balanced unit for a 4.0L Jeep Cherokee.
With the Jeep 4.0L bolted up, we used a high-quality South Bend Clutch disc and pressure plate for a 1980 Jeep 258 inline-six application. Having earned its stripes in the diesel market holding upwards of 2,000 lb-ft, we have no worries that our South Bend Clutch will withstand the rigors of off-roading with our four-cylinder diesel even with large tires and extreme underdrive gearing.
The Advance Adapters AMC bellhousing has a 4.84-inch bellhousing retainer index and is a very nice unit that’s thicker than factory offerings. In addition to the ability to split gears at any speed on-or off-road and with the T-case in any configuration, a huge advantage of the Ranger Torque Splitter is its relatively light weight, which makes inserting the input shaft into the clutch and aligning the mounting bolts a piece of cake.
With the Ranger assembled to the bellhousing, the SM420 was easily mounted and the whole drivetrain could be mocked into place. We tried to get the engine as high as possible under the hood and took full advantage of the floor’s removal to raise the drivetrain up out of harm’s way.
After careful measuring to make sure the drivetrain was square in the chassis side-to-side and double checking our front-to-back engine clearance under the hood, the JK T-case crossmember mounts were tack-welded into place.
The engine, bellhousing, and Ranger Torque splitter were yanked back out so that work could continue inside the engine bay. We will show you the slick engine mounts with really nice Axis Industries isolators next time.
The resulting silhouette is pretty close to flat. Ideally we would have had time to procure a heavy-duty aftermarket T-case crossmember, but for now the factory one will do.