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Sep 01, 2023

Turning a TJ Wrangler Into an LJ Wrangler for the Next 4x4 Garage Build!

As Joe Friday used to say, "Just the facts"—we don't want to give away all the

As Joe Friday used to say, "Just the facts"—we don't want to give away all the cool secrets we're cooking up for the Cummins TJ and Cummins Dodge Ram Truck episodes of 4x4 Garage, presented by Duralast, but we'll cover the broad strokes. For starters, nothing is as synonymous with the phrase "quality engine" as the name Cummins. As we've done with our previous Bronco build, all episodes of the RealTruck Crawlerado build, and, of course, the Cherokee Chief build, we're going to tease you with a bit of a recap here before you dive into the video.

For starters, most off-roaders know of the Cummin R2.8 Turbodiesel Crate Engine program. It's a realistically priced, all-inclusive engine-repower package that includes a 161 hp, 310 lb-ft four-cylinder Cummins Turbodiesel engine, wiring harness, ECU, all engine accessories, and all the other stuff you need—you could literally fire it up inside the box it comes in. Basically, you bolt this sucker into your vehicle, drop a fuel pickup line into your tank, make a few easy wiring connections, and, vroom!

Related: Find every episode of 4x4 Garage HERE!

An emissions-legal alternative engine (for most states 1999-older vehicles) that's economical and torquey. And the vehicle Cummins Repower used to develop and perfect this system is this long-abused, rust-riddled TJ Wrangler that after years of dutiful service had started to literally fall apart around the Cummins engine. Sure, they could purchase a brand-new Wrangler to replace it—but where's the fun in that?

The Cummins Repower Jeep TJ test mule started life as a Pennsylvania vehicle, so road salt was a steady staple of this vehicle's diet, and after Cummins purchased it and relocated it to Indiana, its salt intake didn't diminish one bit. The resultant damage left the factory frame so rotten that despite many patch jobs you could still reach up and grab fistfulls of orange metal off of it with your bare hands. The body tub itself was in surprisingly good shape, however, and the Cummins engine that had powered this little test mule since 2016 or so was still raring to go.

Our plan was to obtain a new TJ frame and treat it to a GenRight Offroad TJ-to-LJ stretch kit. The first thing we did was bring the Jeep into the shop and remove the drivetrain so we could set it aside for some planned adapter, transmission, and T-case updates, although the Cummins R2.8 engine itself will remain completely untouched. With the drivetrain removed from the TJ, we pushed the Jeep back outside and brought in our junkyard TJ Wrangler frame to begin prepping it for the TJ to LJ stretch kit and Cummins engine.

Cummins Leader of North American Communications Stephen Sanders was instrumental in the development of the crate engine program, and he's a really handy builder as well, so he dove straight in with an angle grinder, cutting off the factory Jeep 4.0-liter engine mounts so we could weld in a pair of new Quick Draw Brand conversion mounts to accept the R2.8. Additionally, because we were converting to a long-arm suspension, we had to ditch all the factory TJ short-arm suspension mounts.

After lots of grinding discs, some plasma-cutting, and a whole lot of elbow grease, we had the framerails mostly bare and were ready to put the frame up on the chassis table where it could be cinched down tightly to prevent warping and twisting and keep things in square as we modified it.

We got the frame locked down, and Christian Hazel cut the rear of the donor TJ frame off while Ian Johnson assembled and welded up the pieces of the GenRight stretch kit. Then we brought the whole thing together and assembled it on the chassis table, welding it in place. You can actually do this with the tub still on the Jeep in your garage or driveway, but, hey, we had the tools and a bare frame, so sometimes life gives you lemonade instead of plain lemons.

To support bigger, 37-inch Nexen Roadian MTX tires that will give this Jeep plenty of off-road grip and performance, we determined a pair of JK Dana 44 axles would provide the requisite strength we needed while boosting the Jeep's track width. We buzzed off the JK brackets and ground the tubes smooth. The front was out of a Rubicon, so beyond the suspension and steering mods we don't be doing much to that apart from freshening any worn components with new Duralast replacement parts. In the rear, we'll be upgrading the differential to an Eaton ELocker and replacing the 3.73s with 4.10s to match the front and keep the little diesel in its happy place. To be sure nothing loosens up or gives us issues, we'll be using Stage 8 locking fasteners inside the diff, as well as in various other places throughout the Jeep that we'll be highlighting as we go.

For both the front and rear axles, we replaced the brake rotors, pads, and calipers with brand-new Duralast Gold parts. The Duralast Gold is super nice-quality stuff and always performs for us when the going gets rough off-road. Duralast parts are OE-spec or better, and we find the Duralast Gold brake components are an awesome and affordable alternative to fancy race stuff that often wears out much faster and often delivers little, if any, performance advantage.

The Nexen Roadian MTX checks all the boxes of what we generally look for in an off-road tire. The sidewalls are of a heavy-duty construction, there's a nice, thick bead bundle to help prevent the tires from slipping at low trail pressure, the tread blocks are siped and chunky, and the MTX offers features like evacuation rips between the outer lugs to help break mud suction, as well as extra material down the sidewall for added protection and a dual sidewall design that delivers two different styling options—you can mount the tires so that either the more traditional tread blocks that extend down into the tire's sidewall or the more stylized claw-tooth design are visible. We initially ordered a set of 35x12.50R17, but after spacing our suspension we decided to reorder some larger 37x12.50R17s.

With the drivetrain out of the vehicle, we decided to swap the noisy, worn NV3550 that came in the Jeep for an AX15, simply because we prefer the AX15, even though it has a slightly taller 3.83:1 first gear compared to the NV3550's 4.01:1 first. The other ratios are so close as to make virtually no perceptible difference, but we find the AX15 nicer to drive in daily traffic, is quieter, and is just as durable as the NV3550, if not more so. While we were waiting on the newly rebuilt AX15 to arrive, Christian dove into the factory NP231 and installed an Advance Adapters slip yoke-eliminator kit. Not only does this bump the rear output shaft up to a much beefier 32-spline design, the shorter fixed yoke allows for a long rear driveshaft and the ability to run the Jeep without a rear driveshaft at all in the event that you damage it on the trail and need to remove it and drive home on the front shaft.

With the new rear framerails in place, we dropped the chassis down on some jackstands and temporarily positioned the axles under the chassis and dropped the drivetrain back in place to check potential interference when building our longer four-link control arms. We also wanted to get an idea of shock lengths for when we ordered our new RadFlo coil-over shocks. RadFlo offers all sorts of super high-performance shocks for virtually any application. On this particular build it's looking like we'll be going with some 14-inch-travel, 2.5-inch remote-reservoir coil-overs with beefy ⅞-inch shafts. The RadFlo shocks will allow us to dial-in our valving and adjust our final ride height exactly where we want it. We can't wait for them to land so we can show you what gorgeous pieces of art they are!

In addition to simplifying our antiquated adapter package with some updated parts from Quick Draw Brand that we'll show you in the next build installment, we're taking the opportunity to add a whole mess of clamping power to our clutch system by upgrading to a SPEC clutch package. SPEC offers all sorts of clutches for virtually any application. With our new adapter package we'll be able to utilize a factory Jeep 4.0-liter disc and pressure plate. We've selected SPEC's Stage 3+ carbon semi-metallic disc for long life and smooth engagement, along with its Super Clamp pressure plate, which will give much more clamping force than a factory-spec pressure plate.

Without giving too much away from the second episode of the Cummins Repower TJ to LJ conversion build, we've already been making some killer progress. After prepping the new frame, we rolled the Jeep back into the shop so we could lift the body off the chassis. With it suspended from the lift, we chopped the back half off according to GenRight's instructions and then slid the newly lengthened replacement TJ chassis under it—well, actually, we should be calling it an LJ chassis now, shouldn't we? The rear of the body is starting to get fleshed back out and the light at the end of the tunnel is getting brighter.

Not to be left out, it goes without saying that a Cummins-powered Ram is one of the most popular, brutally capable work machines for towing, hauling, and pulling. Ram trucks and Cummins turbodiesels go together like peas and carrots, but it all started somewhere. And that somewhere was with the 400-lb-ft 12-valve 5.9-liter Cummins that was first offered to the public in the 1989 Dodge 250 and 350 D- and W-series pickups. And it just so happens that Cummins mule truck No. 6 is next in line for some 4x4 Garage love as soon as the TJ mule is wrapped up. What's in store for this historic Dodge Ram? Keep an eye out and we'll show you!

Christian and the gang have been busy working away on the TJ-to-LJ conversion for Episode 2 of the Cummins Repower TJ build project. In this episode, we did some head scratching and decided to pull the chassis out from under the body to make even more modifications, replacing the factory front framerails with a Motobilt TJ Front Half Frame Kit that replaces the factory framerails. The reason for this is the Motobilt rails provide a bit more width between the frame inside the engine compartment, as well as more clearance between the axle and bottom of the framerail, allowing a lower ride height.

With the Motobilt kit installed, we then turned our attention to prepping the Cummins R2.8 Turbo Diesel Crate Engine to mate to our rebuilt AX15 with some new, updated adapter components that Chad from Quick Draw Brand brought down. Quick Draw Brand were early adopters in not only Cummins R2.8 engine conversions, but several other diesel and gas engine configurations. In short, the new Quick Draw conversion bellhousing, flywheel, and additional Wrangler conversion components Chad brought down, decrease clutter and complexity while simultaneously improving durability, strength, and performance by omitting clunky adapter plates and two-part flywheel contrivances that were the only way to mate this engine to a Jeep transmission when the program was first launched. In short, there's a better mousetrap now, and Quick Draw Brand has it.

With the Cummins R2.8 loaded with the new Spec clutch, Christian slapped the Quick Draw Brand conversion bellhousing on the AX15 and mated it to the back of the Cummins engine with no drama. Then we added the NP231 and set the drivetrain aside while we once again slung the chassis back under the tub and moved forward with finalizing components of the stretch kit before getting ready to prep the Rubicon Dana 44 front and JK Dana 44 rear and build the supsenison. There's a lot going on, but don't worry because it'll all come together by the end of next episode before we dive into the super cool 1989 W-350 Dodge pickup that was one of Cummins' original 12-valve 5.9L proof-of-concept "Skunk Works" trucks. We think you'll like what we've got in store for good 'ol "D-06."

Stephen Sanders of Cummins checks some specs using the swanky new Quick Draw Brand Cummins R2.8-to-Jeep AX15/NV3550 transmission conversion bellhousing as a computer desk. Chad McKinney of Quick Draw Brand designed a killer conversion package to mate these engines to many Jeep transmission applications that saves both drivetrain length and weight compared with other older, out of date alternatives that are out there.

The "early days" method of adapting a Cummins R2.8 in front of a Jeep transmission can be seen at the top of this image. Compared with the Quick Draw conversion bellhousing (lower right), the old school method involved using a thick adapter between the Cummins block and a factory Jeep bellhousing. These conversion also utilized two flywheels. The Cummins flywheel remained on the back of the engine crank so the starter had something to index. However, a long, thick spacer was used to mate a factory 4.0L Jeep flywheel behind that to which the clutch was mounted. Now, you just replace the Cummins flywheel with the Quick Draw Brand conversion flywheel and bolt the Quick Draw conversion bellhousing on. Fewer parts bolted together and about half the weight!

Admittedly, there isn't much factory TJ left in this TJ. We thought for a hot second the frame was done, but then we decided to upgrade the front as well as the rear. We yanked the frame back out and put it back on the chassis table to install a Motobilt TJ Front Half Frame Kit. The Motobilt kit spreads the front rails a tad wider, but more critically, raises them up to allow more front axle uptravel with a lower ride height.

While the drivetrain was in easy reach we upgraded much of the hardware to State 8 Locking Fasteners where it made sense, including the oil pan and exhaust to ensure they never shake loose with the kind of extreme off-roading to which we'll now be subjecting this Jeep. At the same time, the Dana 44 rear got a regear to 4.10 to match the front Rubicon axle and an Eaton ELocker for instant traction at the flick of a button off-road without giving up the open-diff civility you want on-road. And some new Duralast Gold pads, rotors, and calipers rounded out the axle builds.