Reinforcing Our 1991 Suburban’s Frame For Post-Apocalyptic Durability

Doomsday Diesel Part 2 Modified Front Engine Crossmember

Modern truck frames feature fully boxed construction, hydroformed framerails, and robotic welding. Doomsday Diesel’s C-channel frame (’91 ¾-ton Suburban) has none of those technologies. It’s blacksmith simple and comprised of two side rails riveted together with six stamped-steel crossmembers. It’s not particularly rigid—but that should mean it’ll never crack or fail.

Modern truck frames feature fully boxed construction, hydroformed framerails, and robotic

No other engine in the light-truck world has a more proven reputation for long-lasting use than the 5.9L Cummins. Like many of you, we’ve admired this motor, yet we’ve always wished it came in more than just Dodge pickups. So when we scored a used ’95 Cummins for $1,200, we began looking for a new home for this Indiana-built bruiser.

After toying with the idea of constructing a Cummins-powered snowplow truck or a diesel-powered muscle car, we settled on swapping the 6BTA into a ¾-ton Suburban. We chose the Suburban because it was the right size for the engine, it offered four-wheel drive, and it would make an excellent platform for getting out of Los Angeles in a hurry—should the need ever arise. The path we chose for our buildup is part bug-out vehicle and part family camper, wrapped up in a package that should last as long as the engine. We call this project Doomsday Diesel.

Doomsday Diesel Part 2 Crossmember Clearance For Engine

If you read the Jan. ’12 issue, you saw how we had to cut the GMC’s front crossmember to provide clearance for our swapped-in 5.9L Cummins’ harmonic balancer. We made sure to remove as little metal as possible and cut on a curve to distribute the stresses over a large area. Cutting a square notch in this piece would’ve been easier—but would have likely led to the steel cracking.

If you read the Jan. ’12 issue, you saw how we had to cut the GMC’s front crossmember to p

Frame Fortification
To get the rest of our ’91 GMC Suburban 2500 up to par with the Cummins, we’ve brought it to Mercenary Offroad, in Camarillo, California, to fit the engine, reinforce the frame, and upgrade the drivetrain. This month, we’re focusing on our truck’s primitive ladder-style frame. While it’ll never be as rigid and robust as a modern truck frame—it should also never fail.

The first step to improving our frame was stripping the factory undercoating off. In hindsight, we should have taken it to be steam cleaned. But after a full day of cleaning the rails with scrapers, Simple Green, and wiping it down with acetone and Scotch-Brite pads, our GMC’s chassis looks almost new. After it was clean, Mercenary Offroad upgraded the frame for extreme-duty use with a few select parts from Offroad Design.

When it comes to frame modifications, smoother is always better. Mercenary Offroad ground the front crossmember down and welded a 1-inch-wide strip of 3⁄16-inch-thick steel to it to reduce its tendency to bend front to back.
When it comes to frame modifications, smoother is always better. Mercenary Offroad ground
Adding this steel won’t prevent the crossmember from trying to bend up and down, but it should help shore-up the section of frame the steering box is mounted to.
Adding this steel won’t prevent the crossmember from trying to bend up and down, but it sh
It turns out there is one area the ’67 to ’91 solid-front-axle GM truck frames are notorious for cracking: around the four steering box bolts. We think the frame cracks because the steering box bolts are subjected to extreme shear loads. These loads cause the frame to flex underneath the box and allow the bolts to loosen. If the steering box is allowed to move around, it fatigues the steel until the frame rips apart. The steering box then begins to move around and tear up the frame. Our ’91 3/4-ton Suburban showed no sign of damage, but none of these trucks are immune to this problem.
It turns out there is one area the ’67 to ’91 solid-front-axle GM truck frames are notorio

Offroad Design offers this 1⁄8-inch-thick steel reinforcement plate to add more material to the crack-prone steering box mount. Mercenary Offroad’s Todd Farrand bolted the plate in place using four 7⁄16-inch bolts and this large clamp.
Offroad Design offers this 1⁄8-inch-thick steel reinforcement plate to add more material t
Using a Miller 185 MIG-welder, Farrand stitch-welded the reinforcement plate onto the frame per the Offroad Design instructions. He took care not to put too much heat into any one area.
Using a Miller 185 MIG-welder, Farrand stitch-welded the reinforcement plate onto the fram
The finished job looked like this. By not fully welding the plate, Farrand limited the amount of heat he put into the frame. This way, the reinforcement adds structure to the high-stress area—without weakening the base metal.
The finished job looked like this. By not fully welding the plate, Farrand limited the amo

Whenever you weld near a factory rivet, it’s recommended you replace the rivet with a fine-thread Grade 8 bolt. In this case, Farrand welded within an inch of these two factory rivets, so we’ll drill them out and insert 7⁄16-inch bolts.
Whenever you weld near a factory rivet, it’s recommended you replace the rivet with a fine
We upgraded the front suspension with Offroad Design’s Front Upper Shackle Hanger (FUSH) kit. The FUSH kit replaces the factory cast-iron pieces that are riveted into the frame with these welded-steel units that allow for a 11/2-inch bushing (instead of the stock 11⁄8-inch bushings). These larger bushings support a 9⁄16-inch bolt that rides in a chromoly steel sleeve for utmost durability.
We upgraded the front suspension with Offroad Design’s Front Upper Shackle Hanger (FUSH) k
To mount the FUSH brackets, we drilled out the eight rivets (four per side) that held the stock hangers to the frame. Farrand then used a hole saw with a piece of tube mounted inside the cutter to act as a pilot bearing in the factory hole in the frame.
To mount the FUSH brackets, we drilled out the eight rivets (four per side) that held the

We then bolted the FUSH brackets into place using 7⁄16-inch Grade 8 bolts, washers, and lock nuts. Next, we slid the greaseable Offroad Design bushings in place and mounted a pair of Rancho leaf springs using Offroad Design’s extended shackles—more on that in a future issue.
We then bolted the FUSH brackets into place using 7⁄16-inch Grade 8 bolts, washers, and lo
With Doomsday Diesel’s frame fortified, it’s time for us to prep the firewall and engine compartment for our 5.9L to be reinstalled. Our plan calls for painting the frame and firewall with a gloss black finish to really make the engine stand out.
With Doomsday Diesel’s frame fortified, it’s time for us to prep the firewall and engine c
Speaking of the engine…our ’95 5.9L was wrapped up and sent to Industrial Injection for a million-mile rebuild that you’ll read about next month. Go online to dieselpowermag.com to see how we built this cheap engine-shipping crate.
Speaking of the engine…our ’95 5.9L was wrapped up and sent to Industrial Injection for a

SOURCES
Industrial Injection 1201 South 700 West Salt Lake City UT 84104 800-955-0476 www.industrialinjection.com	Screamin Seeman Off Road 18476 Eiler Ave Faribault MN 55021 507-330-3567 www.screaminseeman.com
Mercenary Off Road Sun Valley CA 91352 www.mercenaryoffroad.com	Off Road Design 970-945-7777 offroaddesign.com/