What Happened to the V12 That Powered America’s Heaviest Trucks?

In the early 1960s, GMC built one of the most extreme gasoline truck engines America had ever seen: the 702-cubic-inch Twin-Six V12. Weighing roughly 1,485 pounds and delivering 630 lb-ft of torque at only 1,600 RPM, it was designed to move enormous loads across America’s expanding Interstate system with remarkable smoothness and durability. The Twin-Six wasn’t simply two V6 engines bolted together. It used a dedicated V12 block, a massive forged crankshaft, four cylinder heads, two carburetors, and two distributors—yet many of its service parts were shared with GMC’s commercial V6 family. It proved itself in heavy trucks, fire apparatus, logging operations, and even military applications. Yet only five years after production began, GMC stopped building it. The engine itself had not suddenly become weak or unreliable. Something much bigger was changing in American trucking. This is the story of the GMC 702 Twin-Six V12—a gasoline giant built to conquer heavy freight, and why even that much iron could not stop the industry from moving on. #GMCTwinSix #V12Engine #TruckEngineHistory