Flashback Friday: GMC’s Monster 702ci V12 “Twin-Six” engine

While the engine may look like two GMC V6 engine blocks welded together, the block is actually one single piece. The bore centers were increased to increase cooling, and the “Twin-Six” name stems from the fact that there are a lot of major components that are interchangable between this engine and the GMC V6 engine of the time.

When you hear “V12”, you probably think of some exotic performance car with a decidedly European name, RPM through the moon, and an exhaust note reminiscent of a Formula One car. (Hopefully it’s not just us.) However, there exists a domestic V12 engine, produced by GMC in the mid-1960s, that is the exact opposite of all that.

While the name implies it might be two 351-cube V6 engines mated to one another, that not the case at all. It’s a one-piece cast block, with increased bore spacing – not for a bigger bore and more displacement, but rather for additional cooling around each cylinder. Where the “Twin-Six” name comes from is the fact that it utilized almost 60 major components from the GMC V6 engines at the time.

With a massive 4.560-inch bore and a 3.580-inch stroke, high-RPM performance wasn’t the Twin-Six’s intended playground. Rather it was designed to be used in commercial vehicles and industrial and agricultural applications. Tipping the scales around 1,500 pounds (about the weight of an early Honda Civic CRX), and with features like one-and-a-quarter-inch wrist pins, four piston rings per piston, two distributors, three thermostats, and 56 head holts, the name of this engine’s game was reliability.

Surprisingly, the Twin-Six engine didn’t make much power by today’s standards, instead offering up a diesel-like 275 horsepower at 2,500 rpm with peak torque of 630 lb-ft coming in at only 1,600 rpm. However, its powerband has actually earned the engine distinction by being able to operate so low in the RPM range, on gasoline.

For the full scoop on this impressive engine of yesteryear, along with some cool videos of it being used in a couple of hot rods, check out former-editor Mike Magda’s article HERE.

About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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