GM Built a One-of-a-Kind V12 in the ’60s That Weighed More Than a Classic Mini Cooper: Facts About a Retro Engine

In the 1960s, General Motors released an unusual V12 known as the Twin Six. Weighing nearly 1,500 pounds—more than a classic Mini—this massive engine powered GMC’s heavy-duty trucks. Why did it remain a rarity, and what made it so remarkable? Here’s the story.

In the mid-20th century, American auto giant General Motors embarked on a bold experiment: its engineers developed a gasoline V12 engine that outweighed some passenger cars. The engine, named the Twin Six, was created specifically for GMC trucks and became a true technical sensation of the 1960s.

At a time when U.S. roads were dominated by large trucks, dump trucks, and buses, GMC was searching for ways to boost performance. Standard V6 engines could no longer meet rising demands, and diesel power was only beginning to take hold. In response, engineers built a unique 702-cubic-inch (11.5-liter) V12 that produced 275 horsepower and 465 lb-ft of torque. Most remarkable of all: it weighed almost 3,000 pounds—more than a classic Mini Cooper.

Despite its size, the Twin Six wasn’t simply two V6s bolted together. It featured a compact, integrated block and a single crankshaft nearly four feet long that weighed 190 pounds. Cooling this giant required 120 gallons of water per minute, and the oil pan held 4 gallons of oil. Its fuel consumption was exactly what you’d expect from an engine this big—but for commercial trucks, that wasn’t a deal-breaker.

The engine’s biggest advantage was its durability. GMC claimed the Twin Six could run up to 200,000 miles without major repairs, with minor maintenance needed only every 15,000 miles. For truck owners, that kind of longevity meant money earned rather than lost to downtime. Not surprisingly, the engine became popular with fire departments and farmers—fire crews used it to power high-capacity pumps, while farmers relied on it for irrigation systems.

But the era of the Twin Six was short-lived. As diesel engines gained popularity, the massive gasoline V12 lost its edge. By 1965, production had already been scaled back, eventually giving way to a V8 with similarly sized cylinders. Only about 5,000 Twin Six engines were ever built, and roughly 200 are believed to survive today. Some still work on farms or in fire stations, while others have found a second life in the hands of enthusiasts building hot rods.