Square Body Cummins

An ’85 GMC Body, A Second-Gen Chassis, and A Compound Turbo’d 4BT

How many of you spotted your dream truck sitting alongside the highway? When it comes to old iron—especially the classics we grew up with—this method of shopping for a vehicle is more common than you think. Some time ago, Austin Magruder spotted this ’85 GMC, then in two-wheel drive form, while passing through Rolla, Missouri. He promptly inquired about the truck, purchased it, and then spent years roaming all over the Show-Me state in it. Then a friend put his 4BT Cummins-powered step van up for sale and the wheels started turning…

Perfect Timing

Luckily for Austin, around the same time he acquired his delivery van, the folks at nearby MCS Midwest Performance Diesel were parting out a second-gen. Not only did Austin buy the ¾-ton chassis, suspension, NV5600, and NP241 transfer case, but he also enlisted their help in making his 4BT square body dream a reality. With a trip up to Brown’s Auto Body, the GMC’s body was flawlessly swapped over to the Dodge chassis and the project was returned to MCS, where months of work would be poured into Austin’s classic creation.

Performance Overhaul

Not willing to take a chance on the 400,000-mile take-out engine, Austin had MCS tear the 4BT down for an overhaul and a few select upgrades. While the forged-steel crank and connecting rods were retained, the rod beams were micro-polished. The overbore pistons would be OEM replacements, but their skirts were treated to dry film coating to reduce friction, and the tops received a ceramic thermal barrier. The stock camshaft was swapped in favor a 188/220 unit from Hamilton Cams to drive the turbo(s) harder, with Hamilton’s bolt-on cam retainer, tappets and heavy-duty pushrods making the cut as well. Upon discovering that the original cylinder head was cracked, an OEM replacement was ordered, and then fitted with Hamilton valve springs prior to being machined to accept O-rings at LinCo Diesel Performance. The head is anchored to the block thanks to ARP 425 head studs.

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DW-2106-4BTGM-02 — Plucked from an Interstate Battery step van, the 4BT Cummins in Austin Magruder’s ’85 GMC was rebuilt by Keith Summers of MCS Midwest Performance Diesel. With the 4BT’s stock hardware more than capable of handling in excess of 400-hp, nothing crazy was required to ensure the 3.9L remained reliable. The factory crankshaft, original forged-steel (yet micro-polished) connecting rods, overbore OE pistons with dry film-coated skirts and ceramic-coated tops, and a 188/220 camshaft from Hamilton sums up the extent of the short-block upgrades.

DW-2106-4BTGM-03 — Due to a crack being found in the original cylinder head, MCS sourced Austin a brand-new one and equipped it with stiffer valve springs from Hamilton Cams. In order to maintain a solid combustion seal under big boost, the cylinder head was also O-ringed, by LinCo Diesel Performance. An OEM composite head gasket is employed and the head is anchored to the block via ARP 425 head studs.

DW-2106-4BTGM-05 — Just as it is with 6BT engines, relocating the oil filter makes oil changes much easier to tackle once compound turbos are in the mix. Pacbrake’s remote oil filter relocation kit makes greater accessibility possible on Austin’s square body, and LinCo mounted the oil filter on the passenger side of the engine bay, right next to the air filter.

The Spooling Solution

To get around the 4BT Cummins’ inherent spooling issues, Austin enlisted the help of LinCo Diesel Performance to build a compound turbo arrangement. The guys at LDP responded by spec’ing out a combination that placed the factory Holset HX25 over a BorgWarner S300-based charger from Stainless Diesel, then fabricated the piping to make it work. The Stainless Diesel S300 makes use of a 5-blade compressor wheel with a 63mm inducer, and a 68mm turbine wheel inside a non-wastegated, T4 divided .83 A/R exhaust housing. The HX25 bolts to a T3 Steed Speed exhaust manifold and the combination sends 70-psi of boost through a factory second-gen intercooler.

DW-2106-4BTGM-04 — The 4BT is force-fed 70 psi of boost courtesy of a compound turbo arrangement. Spec’d and fabricated at LinCo Diesel Performance, the combination includes a stock-based Holset HX25 over an S363 atmosphere charger from Stainless Diesel. The non-wastegated S300 sports a 5-blade, 63mm billet compressor wheel, a 68mm turbine wheel, and a .83 A/R T4 divided exhaust housing. A factory second-gen intercooler helps limit EGT to a very manageable 1,100 degrees F.

DW-2106-4BTGM-06 — Northeast Diesel Service finessed a significant amount of additional fuel out of the engine’s Bosch P7100. During its overhaul, the P-pump was fitted with fresh 12mm plungers and barrels, higher rpm governor springs, and essentially had its fueling maxed out using stock parts. Incredibly, the little P7100 is capable of flowing 425cc’s.

Rebuilt And Benched P7100

It was up to Northeast Diesel Service to unleash the full potential of the 12mm Bosch P7100, and the renowned pump shop produced a street-friendly version that provides solid power and near-smoke-free operation. Using genuine factory parts, including brand new plungers and barrels, the P7100 is capable of flowing 425cc’s worth of fuel but timing is set conservatively in order to aid spool up and cold-weather starting. Fine-tuning is handled in the cab, courtesy of an AFC Live controller from Power Driven Diesel, while a single filter Platinum series FASS system keeps ample fuel supply on tap for the P-pump at all times.

DW-2106-4BTGM-07 — Low-pressure fuel supply is sent to the P-pump via a single filter Platinum series FASS system mounted along the driver side frame rail. The lift pump pulls fuel from a FASS sump installed in the factory second-gen tank, the tank having also been part of the deal when the GMC body was swapped on top of the ’00 Dodge chassis.

Second-Gen Drivetrain

The ’00 Ram 2500 donor at MCS Midwest Performance Diesel proved invaluable in transforming the square body into a true heavy-duty truck. The transmission, a six-speed NV5600, was matched with a single disc clutch from South Bend along with a new flywheel. The solid front Dodge Dana 60 remained with the second-gen chassis, with MCS adding a free-spin hub kit to the equation. The tough-as-nails Dana 80 was retained, too—complete with all the factory leaf springs above it. Both axles were re-geared with 4.10’s (from 3.55’s) while in the care of Northeast Diesel Service.

DW-2106-4BTGM-13 — While in the care of Northeast Diesel Service, the front Dana 60 and rear Dana 80 were treated to a gear change. In switching from 3.55’s to 4.10’s, the 4BT is kept perfectly within its happy place in the power curve. A Rancho RS5000 steering stabilizer helps limit bump steer and vibration.

DW-2106-4BTGM-08 — Along with the frame, axles, and most of the suspension coming from MCS Midwest Performance Diesel’s second-gen donor, Austin received the truck’s NV5600 manual and NP241 transfer case as well. The combination of a single disc clutch from South Bend and the six-speed transmission has zero issues coping with the square body’s estimated 400 to 450 hp.

Work & Play

Even though it wasn’t necessarily built for work, with quick spool up, low EGT, a solid foundation, and a manual transmission, Austin’s GMC will have zero issues when it’s attached to the occasional bumper-tow trailer. For the time being, however, Austin plans to enjoy the truck, making the rounds at all the local car shows with it. If he gets the urge, he may even hook the square body to the sled a time or two this summer. Wherever it goes, we expect Austin’s old-school, tractor engine’d creation to spark numerous conversations—as well as inspire other swap projects.

DW-2106-4BTGM-14 — Up front, a pair of F-O-A 2.5-inch coil-over shocks with remote reservoirs handle suspension duties, while the factory second-gen leaf packs sit above the Dana 80 in the rear. With a ¾-ton frame, axles, and suspension, the truck will be more than ready for its occasional towing duties.

DW-2106-4BTGM-10 — Though the original interior had been well-preserved, MCS did have to source a new dash from LMC, install a new headliner, and paint the factory door panels. Other in-cab upgrades include a 130-mph second-generation Camaro speedometer (along with a SpeedHut SpeedBox), a 14:1 ratio steering gearbox from AGR Steering Pros, and a Dakota Digital universal electronic cruise control system.