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This 1993 GMC 3500HD is a trailer towing King with a 7.2L Caterpillar engine swap

Posted in Features on January 2, 2016
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“It all started with an engine,” says David Hackett of Renfrew, Pennsylvania. “I was always a little disappointed that a Caterpillar engine was never offered in any pickup trucks, so I decided to build my own, around a ’00 7.2L CAT 3126 engine.” What started as a simple idea evolved into a nine-year project that couldn’t have been completed without the help of a countless number of friends.


In the space where an anemic 6.5L diesel once resided now sits a mammoth 7.2L 3126 Caterpillar engine. The 1,290-pound monster makes 300 hp and 800 lb-ft of torque with factory programming. The CAT is governed at 2,700 rpm and makes peak torque as low as 1,400 rpm!

The firewall of the GMC had to be modified in order to fit the large I-6 engine. The doghouse engine cover is shallow enough to keep the stock dash intact.


“The CAT engine is really heavy (1,290 pounds), so I knew I needed a truck that was up to the task of supporting it.” After months of searching for both Fords and Chevrolets, David came across a truck that fit the bill: a GMC 3500HD that had a bucket and a boom. This hardware confirmed it was also equipped with a heavy-duty front axle. “There wasn't really anything wrong with the truck, they were just getting rid of it because it was old,” David adds.

A custom radiator and shroud were made for the engine swap. Although the radiator is still in the stock location, it features 5/8-inch tubes and 11 fins per inch, which cools the big diesel with ease. Behind the radiator is an intercooler that was originally designed for ’94-to-’98 Dodge Rams, which David managed to install behind the stock grille.

Since David’s GMC is hooked to a trailer almost full time, cooling is a major concern. In addition to the custom radiator, David added an aggressive fan clutch from Horton Airflow Solutions, which keeps airflow at optimum levels.


While the project started in 2002 at David’s shop, Diesel Services Unlimited, it was quickly put on hold when David was sidelined with health issues. After the truck sat on the back burner for a few years, David's head mechanic, Tom Coholich, and his shop foreman, Tom Koffler, said, “We need to build this thing!” So, with his health improving, David and the Toms went back to earnestly working on the truck in 2006, lowering the engine and Allison MD3060R close-ratio six-speed automatic transmission into the frame. “We started by mounting the engine using the factory crank centerline as an index and putting it as close to the radiator as possible. Then we fabricated our own engine and transmission mounts,” David says. From there, the team made the body fit over the engine by building a notched firewall with a doghouse-style cover. The rest of the truck was also restored or repaired as necessary from top to bottom to ensure cross-country trips would be no problem.

David incorporated the factory GM parts wherever he could. An aluminum airbox draws air from the factory location and sends it into the CAT's turbocharger.

Since industrial engines are designed for low-rpm operation, the relatively small turbo (for the size of the engine) produces a whopping 27 psi of boost at full throttle and gives David's rig good acceleration.


“We ran into a lot of unexpected issues with the swap,” David notes. Despite having complete wiring schematics for the truck itself, the engine, and the transmission (which also has an ECU), integrating it all was no small task. “Completing the wiring on the truck took me about a month solid,” David adds. There were also a few tasks that required outside help to accomplish, such as getting the speedometer to function properly. Kincer’s Service of Mt. Vernon, Kentucky, is the company that was able to make it happen.

At the back of the truck exits a custom 4-inch, stainless steel, turbo-back exhaust built specifically for the application.

“Since it was a boom truck, the front axle is designed to hold 5,000 pounds of weight,” David says. Still, with the weight of the big engine, an extra leaf spring per side was added to keep the front end level. The front suspension is also one of the best places to view the Chili Pepper Red paint that was applied to the whole undercarriage.


Ensuring the transmission operates normally was also a challenge, as the MD3060R transmission does not have a Park gear. To keep the truck from rolling away on hills, David and his team fabricated a braking system for the transmission and installed a line lock to keep the front brakes locked on steep hills. “If you forget to set anything, we set it up so the horn will blow as soon as you open the door. You can't ignore that!” David says.

There was no way David was going to let the interior of such a unique truck remain stock. Bob Kirik lent a hand in the design and color scheme and did all the work on the upholstery, as well as the blue vents in the dash

Perhaps the nicest feature of the interior is the suede-covered seat, which gives David and his wife, Eleanor, a comfortable spot to sit during their travels.


With the engine and transmission installed, the wiring done, the rearend rebuilt by John Kunzak, and everything more or less buttoned up, it was time for the finishing touches—the bodywork, paint, and interior. “It took about six months for Bacchus Brothers Auto Body to paint it, and it turned out excellent,” David says. The brothers used a combination of silver, Cobalt Blue, and Chili Pepper Red to make the truck really stand out, while Bob Kirik tackled the custom interior. “In 2011, it was finally ready to go.”

Oil temperature, tachometer, and exhaust gas temperature gauges are mounted on the A-pillar, right next to the CAT data system. The trick computer monitors throttle position, fuel consumption, and a variety of other engine data points.

The truck’s original Dana 80 rear axle has been rebuilt, painted, and upgraded with a Detroit Truetrac locker. The rear wheels are polished GM units fitted with Goodyear 8R19.5 G633 RSD tires.


Later that year, David and his wife, Eleanor, set out on a cross-country journey, towing their 27-foot Airstream trailer. “We really didn't know what we had built at that point,” David says. “It wasn't until we saw other people’s reactions to the truck that we knew it was something special. There were folks chasing us through parking lots, or asking if they could take pictures from underneath. I guess we built something pretty cool.”

The front tires are 225/70R19.5 Goodyear G647 RSS.

In addition to the exhaust, David had fabricator Tom Coholich build the 16-gauge stainless steel running boards and 1/8-inch stainless bumper that set the rig apart from virtually everything else out there.


With more than 45,000 miles on the unique setup since it was completed, David's definitely not afraid to drive his creation. “It gets about 12.5 to 12.6 mpg on the dot towing the Airstream, and I really couldn't ask for any better than that,” he says. “In fact, the whole truck turned out great.” With a First Place in the show ’n’ shine competition at the 2015 Scheid Diesel Extravaganza, it looks like other enthusiasts agree. “I plan to drive the wheels off it. Look for it out on the road, somewhere between the two oceans.”

The fuel tank is also, yep, you guessed it...stainless steel! A front tank that holds 19 gallons works in conjunction with the 46-gallon rear tank, which gives the GMC quite a mileage range.


Fast Facts:
Year/Make/Model: ’93 GMC 3500HD
Owner: David Hackett
Hometown: Renfrew, Pennsylvania
Odometer: 45,000 miles (since rebuild)
Engine: 7.2L Caterpillar I-6
Transmission: Allison MD3060R six-speed automatic
Horsepower: 300 hp
Torque: 800 lb-ft
Tires: 225/70R19.5 Goodyear G647 RSS (front)/8R19.5 Goodyear G633 RSD (rear)
Wheels: Factory, polished and powdercoated
Suspension: Extra leaf spring per side (front), factory rear suspension
Axle: Dana 80 with 3.55:1 gears and a Detroit Truetrac locker
Interior: Custom blue and gray suede interior by Bob Kirik; A-pillar pyrometer, tachometer, and EGT gauges; CAT data system; and Vintage Air air conditioning
Body: Stainless steel bumpers, fuel tank, and running boards; Cobalt Blue, silver, and Chili Pepper Red paint by Bacchus Brothers Auto Body; Jeraco camper shell
Fun Fact: In addition to the fabrication work, David had a lot of additional assistance. He told us he couldn't have done it without the help of Kenny Noland (body and fabrication), Joseph “Peppi” Yerace (ECM programming), Jimmy Miros (tech), and Ed Waterson (tech).

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