Presenting one of the largest gasoline engines ever produced by General Motors: the 702 cubic-inch Twin Six used in GMC trucks of the 1960s. Here’s more on this fascinating dinosaur.
Not a common sight even in the GMC trucks it was intended for, the General Motors Twin Six of 1960 through 1966 was largely forgotten by gearheads until just a few years ago, when it suddenly captured the imaginations of extreme custom car builders, most notably Randy Grubb of Blastolene Special fame. The fabulous Blastolene B702 proudly sports a GMC V12, and why not? After all, bigger is usually better in the hot rod world, and there’s no replacement for displacement.
Now, when we think of V12 powerplants for highway use, we usually recall the 12-cylinder luxury cars of the classic era, such as Cadillac, Packard, and Lincoln, or exotic sports cars like Ferrari or Lamborghini. But in fact, there’s a solid historical precedent for V12 engines in large trucks. Seagrave and American LaFrance, to name two, offered V12 gas engines in fire trucks for decades. GMC’s V12 came along toward the end of the gasoline truck era, when diesels were quickly taking over the industry but there was still a demand for large-displacement gasoline engines. Here’s a look inside GMC’s Twin Six.
Introduced in 1960, the Twin Six was based on GMC’s 60-degree V6 engine series, also unveiled that year. Offered in 305, 351, 401, and 478 cubic-inch versions, the heavy-duty V6 was used across the GMC truck line in specific applications through 1974. The 702 CID Twin Six version was essentially two 351 CID V6 engines on a single cylinder-block casting (see above). The one-piece crankshaft forging weighed nearly 200 lbs., while the cylinder heads, intake and exhaust manifolds, and so on, were taken from the 351 V6. There were 54 shared components in all, reportedly.
While the GMC V6 family also included a diesel variant, the not terribly successful ToroFlow series, there was no diesel version of the Twin Six, only a gasoline model.
This view of the right side of the engine clearly shows the paired cylinder heads, manifolds, and carburetors as borrowed from the 351 V6. The ignition (not visible) employed a pair of distributors on a common drive. Note the belt-driven air compressor and the triple-belt drive for the DC generator and mammoth water pump, rated at nearly 120 gallons per minute. Cooling fan diameter was 25 inches.
Compression ratio was 7.5:1. Bore and stroke were 4.56 inches by 3.58 inches, respectively, yielding a displacement of 702.4 cubic inches. The gross brake horsepower rating was 275 horsepower at a lazy 2400 rpm. And here’s the number we were all waiting for: 630 lb-ft of torque at 1600 to 1900 rpm.
This left-side view shows an alternate accessory drive configuration with a crank-driven fan and twin-belt water pump drive. A block-mounted oil filter housing allows easy access, while oil sump capacity was a generous four gallons (16 quarts). The Twin Six was offered in 7000 and 9000 series chassis in a number of straight truck and cabover configurations, which explains the variations in exterior engine dress. Ready to run, the Twin Six weighed in at 1485 lbs.
In 1967 the big V12 was replaced by a V8 gas engine of 637 CID that offered more reasonable cost and packaging. While the GMC Twin Six was successfully used in thousands of big trucks in the 1960s, the engine was also a popular choice for irrigation pumps, especially in the American southwest. There, the V12 was prized for its ability to run 15,000 hours or more with only minor maintenance. We have it on good information that a few are still out there running today.
I figured it was just a matter of time before this motor was featured here. It, by all rights, should get any motorhead aroused. Being an old trucker, I’ve known about this motor for quite some time, although, never actually drove a truck with one. As stated, diesels were coming of age in the early 60’s, but there were still a lot of truckers who weren’t familiar with them, and preferred to stay with gas motors. This motor, while impressive, had a bunch of problems, one being fuel mileage ( some claim as little as 3 mpg) And while they claimed 275 hp, a 180 or 200 hp diesel would leave them in the dust, and get double the mileage, and last 5 times as long. Still, it shows what GM was willing to go through to keep the gas motor alive. Ford wasn’t much better, and I believe was the last truck maker to adopt diesels. Diesel’s have been the motor of choice in trucks for the last 50 years, and for good reason. If you’ve ever driven a “gas job” truck, you’d know why. Still, you can’t argue what an impressive unit this is. Thanks, MCG.
Thanks as always for your input, Howard. Do you have any recollections about the ToroFlow diesels or the 637 CID V8?
I have not had any experience with the ToroFlow or 637. By the time I started trucking, mid 70’s, everything was Detroit 2 cycles, which I never cared for. Not sure why GM even used the ToroFlow, when they already had access to the Detroit. I can only imagine the ToroFlow was pretty gutless, and I read, many replaced them with gas motors, so that right there tells you something.
Hi guys. Through my years I managed to get everything weird through our shop, including a couple of well-used 702s; lots of V-6s including a couple of diesel Toro-Flops. Quite a challenge, those 702s were, attempting to synchronize two carbs. Three mpg? That’s a little flattering. I’d say closer to breaking even, loaded, and 3-5 empty. But then, they were usually running stump-puller (7-17) gears. With a Spicer 4+4 transmission they didn’t offer much in the way of OD. Saw a couple of 702s on CNG pumping irrigation water. And like what was just said, they lasted a long time. The Toro-Flo engines were great in transit buses except that someone thought you should actually crawl in there with them to work on them; access in some was horrible. I found that out the hard way, when a hockey team bus showed up with a broken injection line. The Toro Flo used an American Bosch PSJ distributor injection pump which spun at crankshaft speed. It sometimes spun the bearings and deep-fried them and the pump camshaft. The Detroit Diesel 2-stroke was a much more versatile engine. Just keep them full of oil and slam your finger in the door every time you got in and you were ready to run. Don’t forget your ear plugs.
An engine I had never heard of! Not even the V6 version.
We never got them here in Oz, though we lost Chev trucks in the late 50s. And I have never seen the larger ones anyway. Just trucks up to about 8 ton capacity with petrol 6s.
Plenty of Detroits around though, used in trucks buses and even earthmoving equipment.
In smaller trucks we still had petrols up until about 80, a few C60 Chevs with 350s [reintroduced mid 70s], Bedfords with Bedford 6s and 308 Holdens. Louseyvilles with the petrol V8, D series Fords with the 300 six and Dodges with hemi 245s & 318s. Internationals with the 345. Mostly in single axle trucks up too 8 ton capacity though the Fords and Inters used them in tandem drive trucks as well as lighter semi trailer application.
In tandem tipper application the 345 Inters were a LOT quicker than diesels [ Loadstars and Accos] and a lot thirstier.
I had the pleasure of working on some 478s. Wonderfull running engines, you could idle them down to a point of putting your finger on the centre of the Water pump pulley and go around with it. The fit of the Pistons, rings, and all of it were very exacting, and would last for a long time, until over revved , then rod bolts and other bad things happened.
In a scrap yard in Regina a coule years ago the V12s were laying around everywhere.
The 478s I worked on we’re in American Flyer coaches, and were on rails for working on. Old memories!
The Fountain Valley Fire Dept bought A used 1948 Seagrave pumper that had belonged to the Signal Hill Fire Dept. Their fire station was half way up the hill. If a call came in that was up hill they could not get the engine out of first gear. The Pierce Arrow V-12 was replaced with a 702 cu in V-12 GMC. Then you could get into second going up the hill.
We restored the pumper and used it as a reserve unit. After restoration I took it out on the 405 freeway that was not yet in service to see what the truck felt like at speed. The speedometer only went to 80 and the needle was past that. Driving it on city street you never used high gear until over 55mph.
I used to do the annual pump tests on all out rigs. They were all 1250 gem pumps except for the 48 Seagrave, it was a 1000gpm pump. I forgot and pumped it as a 1250 pump and it passed the test better our 1964 American Lafrance pumper.
Checking the fuel consumption annually on all the equipment the GMC V-12 usually averaged a out 1 mile per gallon when counting pump time.
The department still has the pumper and uses it in parades .
MY dad was a fleet mechanic for Coke years ago when they were running a lot of GMC’s. He spoke of these engines and had nothing but bad things to say about them. There were a few of these in their route trucks and they had chronic bottom end issues with them. These engines as well as their smaller v6 family members are probably best kept in the history books.
Your dad obviously had a high expectancy on Everything. My 305 V6 in my 64 is a Great motor that is more Reliable than anything I own. Sucker pulls loads with ease and doesn’t have any problems doing it. Oh yeah, not to mention I know Several guys with V6’s who have over 400,000 on them…maybe your dad should hear that. When used how they were meant to be and taken care of how any motor should be these engines lasted forever. But then there was guys like your dad probably who would beat the living day lights out of them and then expect them to be fine and blame everything on the engine.
my family owned a GMC truck dealership in Cullman, Ala. when these engines were in trucks. Very durable, long-lived engines. Not flashy like the passenger-car powered chevs, but could out pull and out live them 10-to-1. only black sheep of the family was the toro-flow diesel conversions-these had bad crankshafts. the hardening on the toro-flow was a process known as “tuffriding” a chemical wash on hardener, which only penetrated 0.006″ into the surface of the shaft. could not be turned.
The Air Force used a transporter erector TE tractor that used this engine. Did oil changes and and general maintenance on them while in the service. They were neat trucks, but no one enjoyed working on them.
I remember these in some log trucks, and I was much younger then. The 478’s did a respectable job- for what they were. My father did the work on this small fleet of three with those 478’s. He got bold and put a v-12 in one truck…..but somehow my memory keeps bringing back that it was the diesel V-12 terrible flow engine. As I said earlier, I was much younger then, and could likely have my facts mixed up. I do remember having to go out with dad once to rescue it from a cold weather gell-up. Pulled it 12 miles back home with a 72 F-100 4×2 pickup with a 390/C-6. I would have been around 12 or 13 years old at the time.
The GMC V-6 series were the best gasoline engines GM ever built. Pretty much indestructible and plenty of low speed grunt. I’d take one over a V8 any day of the week!
Man you couldn’t have said it better!
The GMC V6’s series and V12 were the best they ever built. Solid, Durable, Made to last, and Torque monsters, and way overbuilt and under tuned. 630 foot pounds at 1600 RPM? And idiots call that lame? Boy, some people are Stupid. There’s guys with V6’s that have 400,000 on them! How many stinkin 350’s have made it that far? I have a 305E in my 64 3/4 ton. Has a 4 barrel and dual exhaust, Runs like a Champ and pulls a load with Ease. NEVER gives me issues. WHY? I take CARE of it. I don’t scream it at 5000 RPM like all these idiot haters do/did. I use it how it was meant to be used. Peak torque at 1600 and peak hp at 3800. I shift at 3300 with a load, gets up to 50-55 no problem! And hell! What more could you expect for 305 cubic inches and a wide a** ratio 4 speed!?! I tell ya…if people should quit hating and whining and start seeing the good in these engines.
Hi Tom. I’d have to say, for argument’s sake, that the V-6/V-12 rather outgrew themselves. It seems that the new generations of drivers lost their patience to some extent. From a dealership level, I’d see the GMC V-6s on one side and the Chevy V-8s on the other. Take a couple of neighboring farmers running grain trucks circa 1964; one has a GMC 970 with a 351 and the other has a Chevy C-70 with a 327. Both have 5×2 drivelines with 7.17/9.25 diffs. The GMC has a lot more power out of the hole but you don’t want to push it more than 55 down the road. The Chevy might have more difficulty getting the load moving but 65 on the road is attainable. Then there’s the cost of repairs; the V-6 is more costly to rebuild than the Chevy. Those of us in the service departments got it from both sides. I might add that most of my customers didn’t care about how long they lasted (you’re right; the V-6 went a lot further), and when you hit them with that, they were the proverbial ‘Deer in the Headlights.’