GMC 702 V12: 5 Twin-Six Myths You Still Believe
It’s just two V6 engines welded together.
It was a commercial failure that nobody wanted.
GMC built it because they did not know how to make a real V12.
If you’ve spent any time researching the GMC702 twin 6512, you have heard these claims repeated over and over again.
The problem, almost all of it is wrong.
Today we are destroying the myths and giving you the actual facts about one of the most misunderstood engines in American automotive history.
The GMC702 cubic in V12, the Twin 6.
What it really was, what it was not, and why the truth is far more interesting than the legends.
If you want the real story backed by actual engineering facts, hit that subscribe button because we are about to set the record straight.
Myth one.
Many people say the twin 6 is just two V6 engines welded together.
That is the most persistent myth about the 7002 and it is completely wrong.
Let me be crystal clear.
The GMC Twin 6 is not two 351 cubic in V6 engines bolted or welded together.
Here is the reality.
The Twin 6 has its own unique one-piece cast iron block.
It is a dedicated design with oversized bore spacing.
And that extra spacing was not for larger cylinders to increase displacement.
It was for improved cooling around each cylinder.
This engine was engineered from the ground up to run at maximum capacity for extended periods.
The crankshaft is one solid piece of forged steel running in seven main bearings with fourbolt main bearing caps.
The block featured strengthened saddles and supportive webbing cast directly into it.
The cam shaft sat relatively high in the block to allow for shorter push rods and greater valvatrain stability.
So where does the myth come from?
Simple.
The twin 6 shared approximately 54 to 60 major components with its V6 siblings, particularly the 351 cubic inch version.
It used the same cylinder heads, just four of them instead of two.
Same valve covers, same exhaust manifolds, four instead of two.
It used two carburetors, two intake manifolds, and two distributor caps driven by a single distributor drive.
So yes, the twin 6 operated functionally like two V6 engines working in harmony, sharing a common crank case, but it was absolutely not two separate engines joined together.
This was a purpose-designed V12 that simply leveraged existing components for cost efficiency and parts availability.
Myth two, it was a high performance engine.
Let’s talk numbers.
The Twin 6 produced 275 gross horsepower at 2,400 revolutions per minute.
Some sources say 250 horsepower.
Either way, that is pathetic by today’s standards.
An 11.5 L V12 making less power than a modern Honda Civic Type R.
But here is what everyone misses.
This engine produced 630 lb feet of torque at just 1,600 revolutions per minute.
Peak torque at barely above idle speed.
For comparison, a 2024 6.7 L Powerstroke diesel 58 makes 475 lb feet of torque.
The legendary 7.3 Power Stroke made about 505 lb feet.
The twin 6 running on regular 87 octane gasoline in 1963 was making more torque than modern turbo diesels and doing it at an engine speed so low it was practically loafing.
The twin 6 holds a unique distinction.
It makes the most power between 2,000 and 3,000 revolutions per minute of any automotivebased naturally aspirated 87 octane gasoline engine ever produced.
This was not a performance engine.
This was a diesel killer that ran on gas.
Myth three, it was a commercial failure.
Between 1961 and 1965, only about 5,000 twin 6 engines were produced.
Production was limited, and by 1967, it was replaced by a 637 in V8.
But was it a failure?
Not exactly.
The twin 6 was expensive to manufacture.
It was heavy and hard to package.
You could only get it in specific GMC 7000 and 9000 series trucks, certain B series conventionals, and L series cabovers.
And yes, it drank fuel like your college roommate drank cheap beer.
We are talking 3 m per gallon.
But GMC never intended to sell millions of these.
The Twin 6 found its niche.
Fire departments loved it because GMC advertised that it could pump 1,500 gall of water per minute at the lowest cost per gallon of any competing engine.
Farmers bought them for irrigation pumps where they would run wide open for weeks at a time without complaint.
Many were sold as stationary industrial engines, so it was not a failure.
It was a specialized tool for a specific job, and it did that job remarkably well until diesel technology finally caught up.
Myth four, it had terrible reliability.
This one persists because people confuse fuel consumption with mechanical reliability.
Yes, the Twin 6 drank fuel, approximately 3 m per gallon under load, but mechanically this engine was built like a tank.
GMC claimed the Twin 6 only needed minor servicing at 15,000 mi and would not require a major overhaul until 200,000 mi.
In an era where downtime meant lost profits, that reliability was worth its weight in gold, and this engine weighed a lot.
The block featured strengthened saddles and supportive webbing cast directly into it.
The crankshaft ran in seven main bearings with fourbolt main bearing caps.
The wrist pins measured 1.24 in in diameter.
The cooling system used three thermostats and a water pump that could move 118 gall per minute to keep everything at optimal temperature.
Fire departments loved it because it could pump 1,500 gallons of water per minute continuously without overheating or breaking down.
Farmers pulled these engines and mounted them to irrigation pumps where they would run at governed throttle for weeks on end, pumping thousands of gallons of water.
Across fields.
There are even reports of these engines being used in Boeing GMC Minuteman missile transporter erectors, vehicles that moved intercontinental ballistic missiles in and out of their silos.
These units reportedly served until the early 1990s.
You do not use unreliable equipment for nuclear weapons operations.
Myth five, you can’t do anything with them today.
Here is where modern reality intersects with vintage iron.
Today, the Twin 6 has become a cult object for custom builders who want something truly unique.
The most famous Twin 6 build is probably the Blastoalene B72 by Randy Grub and Michael Le.
Other notable builds include Pat McNeel’s mid-enine 1942 GMC cab over engine and Robert Wonderland’s stretched 1964 GMC pickup.
If you are enjoying this mythbusting deep dive, don’t forget to hit that subscribe button.
Now that we’ve destroyed the myths, let’s talk about what this engine actually was from an engineering standpoint.
The GMC 702 twin 6 was introduced in 1961 as part of GMC’s V series engine family.
It shared approximately 54 to 60 major components with the V6 engines in the lineup, particularly the 351 cubic in version, but it was not simply two V6s joined together.
The engine displaced 702.4 C in or 11.5 L.
It had a bore of 4.875 in and a stroke of 3.937 in.
The compression ratio was a modest 7.5 to1 designed to run on regular 87 octane gasoline available at any pump.
Power output was 275 gross horsepower at 2,400 RPM, though some sources site 250 horsepower.
But the real story was torque, 630 lb feet at just 1,600 RPM.
The twin 6 was primarily installed in GMC 7000 and 9000 series trucks, heavyduty conventionals and cabovers meant for serious commercial work.
We’re talking dump trucks, cement mixers, fire engines, and heavy haulers.
Fire departments were a major customer.
GMC marketed the Twin 6 as the most cost-effective fire engine power plant available, capable of pumping 1,500 gallons per minute at the lowest cost per gallon of any competing engine.
Only about 5,000 twin 6 engines were produced between 1961 and 1965, with some sources extending that to 1960 to 1966.
By 1967, the engine was replaced by a 637 in V8 that was lighter, more fuel efficient, and easier to package.
The twin 6 represented the last gasp of gasoline in heavyduty commercial applications.
By the mid 1960s, diesel technology had advanced to the point where it was simply superior in every metric that mattered.
Fuel economy, torque, durability, and operating costs.
The Twin 6 was expensive to manufacture, heavy, hard to package, and drank fuel at an alarming rate.
As diesel engines improved and became more reliable, especially in cold weather starting, the advantages of gasoline evaporated.
GMC did not discontinue the Twin 6 because it failed.
They discontinued it because diesel had won.
If this video gave you the real story behind the Twin 6 myths, please subscribe and ring the bell.
We’re committed to bringing you accurate automotive history, not just repeating internet myths.
The GMC Twin 672 was never meant to be everything to everyone.
It was meant to be a gasoline alternative to diesel that could deliver massive low-end torque with the cold starting reliability and simplicity that fleet operators wanted in the early 1960s.
And for about 5 years, it did exactly that at 702.4 4 cub in producing 630 lb feet of torque at 1,600 RPM, weighing 1,485 lb and gulping fuel at 3 m per gallon, the Twin 6 was excessive, expensive, and exactly what some operators needed.
It holds the distinction of being the largest displacement gasoline production engine from a major automaker since World War II, a record that still stands today.
The real story of the twin 6 is more interesting than the myths.
It is not a tale of two engines welded together or a commercial disaster.
It is the story of GMC trying to hold back the inevitable tide of diesel with pure displacement and American engineering and almost succeeding.
Fewer than 200 of these engines survive today.
Every time you see one running, you are witnessing a piece of automotive history that most people completely misunderstand.
