1. home
  2. project vehicles
  3. Jeep Flatfender - Garage Project GPW: Part 7

Jeep Flatfender - Garage Project GPW: Part 7

Engine, transmission, and cooling for our flatfender Jeep

John CappaPhotographer, Writer

We considered several different engines for the Garage Project GPW flatfender. Having spent quite a bit of time behind the wheel and under the hood of early Jeeps with a variety of engines, we’ve developed some strong opinions and came to a few conclusions about what works best where. The stock 60hp inline-four would be far too gutless for what we want to do with this Jeep. A GM small-block V-8 makes great torque down low for crawling. It would be a blast in the sand dunes and on the street, but the added weight of the big engine and the necessary heavy-duty drivetrain components detract from the natural light and nimble capabilities of a flatfender in tight technical terrain. The GM V-8 would also have to be shoehorned in and could be a challenge to keep cool in triple-digit summer weather. We even considered a Mazda rotary engine, which still intrigues us if used with the right gearing.

Way To Go
Ultimately, the GM 4.3L V-6 made the most sense for our project. It’s relatively lightweight and produces admirable torque and horsepower but not enough to turn our light-duty drivetrain into metal-flake gear-oil mist. A V-6 fits well in the engine bay of a flattie, leaving plenty of room for a properly sized radiator, cooling fan, and rear driveshaft. The GM 4.3L was produced in several different variations from 1985 through 2014. Each version has its pros and cons. We accidentally bumped into a brand-new ’04 GM 4.3L V-6 still in the crate for sale online, so we jumped on it. This engine produces about 200 hp and 260 lb-ft of torque, which should easily propel our little 4x4 over rocks and through dunes. It shouldn’t have a problem buzzing down the highway at 60-70 mph in front of our SM420 manual transmission and Spicer 18 transfer case backed up with an Advance Adapters Saturn overdrive. The 7.05:1 First gear in the SM420, combined with the 2.46:1 low range in the Spicer 18 transfer case and 5.38:1 ratio axle gears will provide an impressive crawl ratio of 93:1.

Time And Space
It’s no secret that space is at a premium on an early flatfender Jeep. We often joke about how something like the steering box location will dictate where the rear axle needs to be mounted. Not directly of course, but the steering box dictates where the radiator and engine are located, which affects the transmission and transfer case location, which in turn alters the driveshaft angles and how the rear axle needs to be situated. Once you work on a flatfender for any length of time, this ideology becomes your mantra. Moving something even 1/4-inch can make all the difference in the world. Take your time when installing an engine in a flatfender. It’s a good idea to mock up as much as you can, constantly checking for clearance all around the engine, before throwing down any permanent weld beads.

Our ’04 4.3L was originally destined for a van with an automatic transmission. So we made a call to Centerforce to get our hands on an externally balanced 168-tooth steel 35-pound flywheel (PN 700160). We tapped an Advance Adapters Gen III pilot bushing (PN 716155) into the back of the crank to keep the SM420 input inline.

For extra clutch-holding capacity and to maintain a light pedal feel, we backed the new flywheel up with a Centerforce II clutch disc (PN 383735) and clutch cover (PN CFT165552). We cleaned the clutch surfaces with brake cleaner prior to cinching down the cover and used thread-locking compound on the bolts before torqueing them to 35 ft-lb.

The factory GM cast-iron and aluminum bellhousings can become fatigued and crack, so we opted for a Quick Time thick stamped-steel race bellhousing (PN RM-6022). It provides heavy-duty 360-degree protection for the spinning flywheel and clutch and mates our GM engine and transmission. The clutch fork is from Lakewood (PN 15500), and the release bearing is from Centerforce (PN N1716).

Our Advance Adapters conversion motor mounts (PN 713001) should be fail-safe. Massive 5/8-inch bolts keep the engine in place and large rubber bushings absorb vibrations and frame flex. They can be welded or bolted to the frame.

We slung the assembled engine, transmission, and transfer case into the Jeep and began the process of finding the optimum location. Call us OCD, but we actually got down to moving the engine around 1/8-inch at a time. Our motor is pushed back to the firewall, sits level side to side, and is tilted forward a few degrees to improve transfer case ground clearance.

We removed and installed the engine several times while checking and creating the needed clearance for the exhaust, radiator, steering, accessory group, front axle, and so on. Once we were happy, we tack-welded the motor and transmission mounts into place, pulled everything back out, and then permanently welded the motor mounts to the frame.

With pretty much all the fabrication done, we rolled the body and chassis out into the yard where it received a coat of Gillespie Coatings Red Oxide III primer and 33070 WWII lusterless paint we bought from Army Jeep Parts. The paint mixes, sprays, and cleans up easily. We needed about 3 gallons to hose down the entire frame, suspension axles, rollcage, and all the body panels.

Traditional push-style clutch slave cylinders generally take up valuable space where an in-frame exhaust wants to be on a flatfender. To give us more exhaust room, we opted for this Wilwood pull-style clutch slave cylinder (PN 260-1333). We fabricated a mount that bolts to the side of the SM420 transmission where a PTO would go. On the opposite side, we went with a compact Powermaster starter from Summit Racing (PN PWM-9112). This combination of parts gives us the ability to use the stock cast-iron exhaust manifolds.

We used a front serpentine accessory group from a ’02 Chevy Silverado 1500 pickup. We added a Powermaster 215-amp alternator (PN PWM-58237) we got from Summit Racing. We also needed a smaller March crank pulley kit (PN MCH-4460-08) to clear the axle with the suspension fully compressed.

We wanted to avoid overly complex driveshafts. After some careful measurements, we found we could still get the 10-11 inches of wheel travel we wanted while using off-the-shelf driveshaft components. Tom Wood’s Custom Drive Shafts built us a pair of polished and clearcoated driveshafts with 1310-sized U-joints and a CV for the rear.

Our Tom Wood’s front driveshaft was a no-brainer. It features traditional 1310 U-joints on each end and an extra-long slip-joint. Our suspension doesn’t require much plunge, but the heavy-duty extended-life slip-joint is more robust and will last longer than a shorter slip-joint.

At 14 1/8 inches long, the rear Tom Wood’s CV driveshaft in our GPW is a miracle. It wasn’t even long enough to fit a sticker. At ride height, the CV runs at an acceptable 10 degrees. At full suspension droop, it cycles to about 25 degrees with no binding.

To fuel our 4.3L, we made another call to Summit Racing and ordered an Edelbrock EFI fuel pump (PN EDL-3594). We plumbed it with a standard plastic filter before the pump and a 10-micron high-pressure filter after the pump. This protects the pump and injectors from any garbage that accumulates in the fuel tank. A good eye will spot a couple Ford F-script GPW bolts repurposed for pump mounting.

Thanks to smaller headlights, the nine-slot MB/GPW grille from Omix-ADA (PN 12021.99) provides more space for a larger radiator than the CJ-2A and CJ-3A grilles. For cooling, we used a Griffin Combo Unit (PN CU-55201-XS). The crossflow aluminum radiator features dual-row 1 1/4-inch MegaCool tubes and measures 24 inches wide by 15 1/2 inches tall. It’s rated for more than 400 hp, so it should keep our V-6 running cool. Many other sizes are available.

Our Griffin Combo Unit radiator features a built-in aluminum shroud and an electric fan that pulls 1,038 cfm. This particular radiator has GM-style outlets, but many different inlet and outlet variations can be ordered. We built a sandwich-style mount that clamps the radiator between the factory front frame crossmember and a fabricated bracket at the top of the grille, which will also support our headlights.

We cut out the floor and raised the SM420 transmission and Spicer 18 transfer case for more ground clearance. We then built a removable cover for the transfer case but left the transmission open. We could have built a cover for it, but this can result in a cramped footwell for the clutch, brake, and throttle pedals in a flattie, depending on how high you raise the drivetrain. We’ll suffer the heat coming up through the gaps around the transmission for the improved foot and leg ergonomics.

Underneath, we fabricated this reinforced 1/4-inch-thick skidplate to protect the underbelly of our Jeep. It doubles as a transmission/transfer case crossmember and mounts to the frame with threaded weld-in inserts. It’s similar in design to what you find under YJ or TJ Wranglers.