1987 Chevy C30 Square Body Duramax Swap

Duramax 6600 Diesel & Allison 1000 Conversion
Updated August 2024

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Text by Jim Bigley
Photos by Kean Clark

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Bringing new life to an older truck is more than just a way to save a ton of money. A surprising number of members keep the same vehicle for decades, and want to do all they can to keep it on the road. Converting the drivetrain to Duramax/Allison power has become a popular upgrade for the GM fans, which increases the vehicle's capability. Member Kean Clark is currently working his way through a 1987 Chevy C30 Square Body restoration and Duramax conversion project, and we wanted to share his progress with the GM community.

Kean acquired a 2005 K2500 extended cab that had been hit hard in the front. While the insurance company sold the truck as salvage, Kean bought it knowing that all of the essential components he needed were undamaged. A short list of the transplanted components included the engine, transmission, wiring harnesses, fuse boxes, electrical center, various electronic modules, instrument panel gauge cluster, steering column, complete fuel system, radiator, charge air cooler, and coolant surge tank. Aftermarket items included electric engine cooling fans and a Banks air intake system.

Before beginning this project, Kean acquired The Diesel Page's Duramax Diesel Conversion Guide, which provided the full-size drawings for the motor mounts as well as essential information regarding engine fitment and electrical setup. As of February 6th, Ken has the engine running and is working toward completion. This photo (taken from behind the behind the driver's side wheel/tire) shows the driver's side motor mount.

The front section of the aluminum crankcase need to be modified to allow clearance over the Chevy engine crossmember. The cast aluminum crankcase is easily welded using a TIG welder. No essential engine components are disturbed inside the crankcase; the engine oil pickup is farther back in the oil pan.

We had heard rumors that the frame rails used in the 80's GM trucks were a little too narrow to comfortably accept the Duramax. However, motor mounts made to The Diesel Page's specs position the engine so just a careful application of a cutoff wheel can be used to provide adequate clearance without weakening the frame.

The factory transmission crossmember was modified to accept the 2005 factory Allison mount and shift cable anchor. A 4WD transfer case currently resides behind the transmission, but that'll soon be replaced with a 2WD tailshaft.

Inside the cab, Kean installed the 2005 steering column and matching instrument panel. Incorporating the new steering column and instrument panel solved many of the interface problems that would have occurred otherwise. In addition, having the new parts allow you to take advantage of the newest features incorporated in the new drivetrain.

The 34 gallon fuel tank from the 2005 donor truck was transferred to the 1987 K30. Due to frame rail width, the tank is mounted outboard of the driver's side frame rail - just like the original 1987 tank had been. Using the new fuel tank allowed Kean to maintain the correct diesel sender package and fuel cooler - again, solving several problems that he would have had to deal with if he had converted one of the original fuel tanks.

Out back, the GM Corporate 10.5" rear axle was kept in-place, but disc brakes were added to help modernize the braking system. Kean has done a great job with his swap/conversion. TDP