The cast-iron madness of GM’s 11.5-liter V-12

gmc v-12 v12 engine gm twin six

Much as the V-8 engine format has been closely associated with American automakers, V-12s are generally regarded as the province of European luxury manufacturers. Consequently, the latter type of engine is widely thought of as sophisticated, at least next to the more plebian bent-eight.

Even when General Motors considered making a V-12 for Cadillac in the 1960s, the 12-cylinder “V Future” engine eschewed conventional Detroit engineering like cast-iron blocks and pushrods for aluminum heads, aluminum blocks, and overhead cams. At the time, however, the General was already making a V-12—a 1485-pound, 702-cubic-inch behemoth called the Twin Six. It came from GM’s GMC truck division.

This leviathan was made for just five years, from 1961 to 1965. It came at a time when gasoline was so cheap that the Twin Six’s three-mile-per-gallon consumption was deemed acceptable by trucking companies. It remains the largest-displacement gasoline production engine produced by a major car company since World War II.

gmc v-12 v12 engine gm twin six

In 1960, GMC unveiled a new family of smooth-running, 60-degree V-6s. These engines, designed for use in heavy-duty trucks, featured a stout six-throw crankshaft, pushrods, and a single cam. They were available in a range of displacements from 305 to 478 cubic inches.

One year later, GMC announced the Twin Six, derived from that line, essentially two 351 V-6s stuck end to end. Total displacement was 702.4 cubes or 11.5 liters. The compression ratio was 7.5:1. The 4.56-inch bore was paired with a 3.58-inch stroke.

gmc v-12 v12 engine gm twin six

Though some think GMC’s V-12 was made by welding or bolting two of those V-6 blocks together, the Twin Six had a dedicated, single-piece, cast-iron crankcase. The forged crank was one piece, four feet long, and 190 pounds. It spun in seven main bearings, with four-bolt main bearing caps, strengthened saddles, and supportive webbing cast into the block.

The Twin Six’s camshaft was also four feet long. It sat relatively high in the block, to allow for shorter pushrods and greater valvetrain stability at (relatively) high rpm. Public confusion as to the block’s construction may be due to the fact that the V-12 used the same cylinder heads as its V-6 siblings, just four instead of two.

By the way, that meant a total of 56 head bolts. Intake and exhaust manifolds were also shared with the six—in all, the two engine families had some 54 parts in common.

The small four-cylinder engine in my car holds about four quarts of oil. The Twin Six’s sump held four gallons. To keep things cool, it had a water pump capable of delivering a remarkable 118 gallons per minute. To keep things lubricated, the engine’s high-output oil pump flowed 17 gallons per minute. Two large downdraft carburetors parked in the vee provided fuel and air.

gmc v-12 v12 engine gm twin six

GMC rated the engine for 275 hp at 2400 rpm (SAE) and 630 lb-ft of torque. The latter figure arrived just off idle, at only 1600 rpm. The Twin Six was marketed to fleet operators based on ease of maintenance—the engine, GMC boasted, could remain in service for at least 15,000 miles with only minor maintenance. On top of that, it was claimed to be good for 200,000 miles without a major overhaul.

Predictably, when this substantial piece of work was discontinued, its successor wore fewer cylinders. When the Twin Six was removed from GMC’s lineup for 1967, the replacement was a 60-degree, 637-cubic-inch V-8 with the same bore and stroke. That engine was cheaper to produce, cheaper to sell, and easier to package.

Thunder V12

The big V-12 was also marketed to fire departments, installed in GMC 7000-series fire trucks, and popular with American fire-truck manufacturers. The engine layout was not uncommon in that field. When Lycoming decided to concentrate on aircraft engines, American LaFrance (ALF) bought the rights and the tooling for that company’s overhead-cam V-12 and kept it in production for decades. Seagrave used a modified Pierce-Arrow V-12 with twin distributors and two spark plugs per cylinder.

Not all Twin Sixes ended up in over-the-road trucks—some farmers pressed them into service driving irrigation pumps, impressed with the V-12’s ability to reliably endure harsh conditions. In some stationary applications, the Twin Six has been known to last the equivalent of nearly 23 years of continuous running time. It’s rumored that some are still in use in the American Southwest.

As for today, some custom-car folks see the Twin Six as a less-common alternative to the GM LS or Ford Coyote V-8s. Notable Twin Six projects include the Blastolene B702 built by Michael Leed and Randy Grubb, Pat McNeal’s mid-engine 1942 GMC COE rat rod, and Robert Wunderlin’s stretched 1964 GMC pickup.

Here are some other Twin Six–powered customs.

For those who want something yet even more special, engine builder Bob Walker makes a high-performance iteration of the Twin Six that he calls the Thunder V12. The Evansville, Indiana, engine builder claims his version of the engine “makes the most power between 2000 and 3000 rpm of any automotive-based, naturally aspirated, 87-octane-fueled crate engine you can buy for under $35,000.” Walker’s firm sells parts for the engine as well as bellhousings to mate it to popular automotive transmissions. He is not currently taking rebuild orders, but when the company does have engines available, a turn-key stock rebuild costs $18,590. The “Thunder” version is $24,900.

Around 5000 Twin Sixes are believed to have been built. Thunder V12’s website claims only about 200 still exist, and that most are now “basket cases.” Walker says he spent a year and a half tracking down about 100 of those 200, tearing them down and inspecting and grading their blocks. Only a few make the cut for his treatment.

Thunder V12 rates the performance version of its engines at 425 hp at 4200 rpm and 630 lb-ft of torque at 1800 rpm. A video on the site shows an engine briefly hitting its hard redline of 5000 rpm and staying in one piece. (Walker doesn’t recommend going higher than 4000 normally.) That horsepower rating can be doubled, Thunder V12 says, with predictable performance mods—porting and polishing the head, a roller cam, and better pistons.

As for torque? Thunder V12 says more is available through paths like nitrous oxide or supercharging: The engine is apparently stout enough to take the strain. “An excess of 1200 lb-ft of torque at 2500 rpm would surely satisfy even the most jaded power junkie,” the site says.


Because these engines often have a history of severe-duty use, because remaining examples are often in poor condition, and because parts are often difficult to source, Bob Walker cautions against trying to rebuild one yourself. If you are so inclined, however, at press time, eBay holds a Twin Six in “for parts, not working” condition. It is priced at $3500.

Not surprisingly, it’s local pickup only. And your pickup better be at least a three-quarter-ton.




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    I love this stuff – but I think I’ve caught a typo: a 702 cid 12 made into an 8 keeping the same bore and stroke would be far less than 636 inches.

    Saint George Fire department In southern Utah Has one of these in a pumper that is still passing pump testing yearly

    I guess one would say that Continental was not a “major car company”. But Continental produced, after WWII, large numbers of 1790 cu in, V-12, air cooled, gasoline fueled engines for several different military vehicles including M47, M48 and M60 tanks.

    But they definitely did build cars, in 1932 and 1933, and I think maybe we have had a look at them here.

    I remember a local fuel distributor who had the engine in a truck they used to pick up butane at the refinery and haul to their storage facility. At that time, propane was almost free at the refinery in the summer when they were filling their storage facility, so it was super inexpensive to operate.

    C’mon Mr. Hagerty, buy that eBay motor, give it to Davin and Jeff and they’ll have it running in a half day.

    Beautiful and nostalgic article, since I was working in the GMC Truck and Coach (later GMC Truck and Bus) engine lab in the mid-60s. My responsibility was testing of the 478 cu. in. V6 gasoline engine.

    Later, Toro-Flo V6 and V8 4-stroke Diesel engines were developed based on the gasoline engines. Displacements were 351 and 478 for the V6s and 637 for the V8. The V8 and the larger V6 were available naturally aspirated as well as twin turbocharged. Both intake and exhaust ports were located on the outboard sides of the cylinder heads, the valley being occupied by the mechanical fuel injection pump. These engines are not to be confused with the 2-stroke heavy truck engines of the Detroit Diesel Division of General Motors.

    Many years ago, I was about 13. While waiting for the school bus I’d see the Alpha Beta food market GMC cab overs coming up the street. You could hear them approaching by the sound. While no expert at 13, I knew what they were running. What a sound

    We have one in a short-nose GMC D-7000. it is a lot of engine. Would like to get it apart and freshened up one of these days

    I still work a 478 V6. All I can say is that those GMC big blocks were the best damn HD gasoline truck engines ever built.

    This brings back memories. Growing up in Oshawa, Ontario, a dyed-in-the-wool GM town, there was always something lurking around the corner. We lived down the street from a local inn, The Cadillac Hotel, believe it or not, and it was where the broker truckers stayed overnight in between loads. Their trucks were beautiful, perfectly turned out, red, with blue and white pin-striping and everything. Showed pride of ownership. All GMs of course, as bringing other makes onto the GM grounds was frowned upon. One of my dad’s co-workers at the local Hydro decided to go trucking, and his vehicle of choice was a GMC with the Twin-Six, a COE sleeper I believe. These were wonderful years, growing up in Oshawa. I used to walk up and down the street admiring the trucks parked there. The trucking company was ICL, International Carriers Ltd., I think. Great memories.

    ICL was based in Windsor I believe but being dedicated to GM they had terminals at many GM towns. St Catharines had an ICL terminal that is now Wolverine. Direct Winters Transport had a fleet of highway tractors powered by the V12. Their truck shop in St Catharines had more stock than any local dealer.

    Very intresting we had a guy who tryed one in a pulling tractor he gave up up on soon and went to an allison. It was intersting to me because I also ran an allison I have intest in all v-12’s _ Rolls Royce ! The unlimited hy dro plane got my attention also!

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