If someone with no prior knowledge of General Motors were dropped into their Warren Tech Center somewhere between 1960 and 1964, in the manner of Hertz dropping a guy into a moving Impala in their old commercials, that someone would most likely think of GM as a trailblazing, fast-moving corporation with innovation as its cornerstone. It’s been well-documented here and elsewhere. The Corvair. The Tempest’s “rope-drive” with rear-mounted transaxle. The Buick aluminum V8. Turbocharging. The Buick V6. Left out of that conversation to some extent is the GMC 60-degree engine family, introduced for the 1960 model year. Like most of GM’s other innovations of this heady period, it took the primrose path to obsolescence, but what an interesting path it was.
The V6 was perhaps GMC’s final opportunity, at least in the light truck line, to show the world that it was not just some tarted-up Chevrolet, and early advertisements beamed with pride. Colorful illustrated ads and comprehensive text extolled the benefits of this truck-only engine, which also included a V12 “Twin-Six” for the heavier models. There are about a dozen ads in my collection that feature this engine family in some way.
I’ve been studying the 60-degree GMC for weeks now, watching rebuild videos on YouTube and reading articles, and I’m fascinated by just how overbuilt it is. The lightest V6 weighs over 700 pounds, the Twin-Six nearly 1500. The connecting rods and pistons are monolithic. The block is stout; even the front cover is cast iron and weighs a ton. The camshaft and lifters are oiled by their own separate trough high up in the block – the camshaft is basically swimming in oil. The valves feature positive rotation. The bearings are huge. By most accounts, these things were built to pull and outlast a house; unfortunately, a house might have gotten better fuel mileage if it had had a motivating force. By many accounts, poor fuel mileage was the engine’s main shortcoming.
The crown jewel of this engine family, as I mentioned above, was the “Twin-Six,” which displaced a staggering 702 cubic inches. A late effort to out-Diesel a Diesel, the Twin-Six was about the only thing to make the V6 look efficient.
The Twin-Six earned its own V12 badge (where can I find one of those?) and has found some moderate popularity as an off-the-wall hot rod engine. Wearing four cylinder heads and two carburetors, the Twin-Six produced a mere 275 horsepower at a low 2400 RPM, but a staggering 630 lbs./ft. of torque at only 1600-1900 RPM. Like the V6, the Twin-Six was not built for revving; according to 6066gmcguy.com, the maximum recommended engine speed was 2400 RPM – the power peak. Even the V6 was all done well before 4000 RPM.
The ads touted greater fuel economy. Compared to what? Anecdotes from the internet seem to agree that mileage was dismal. Regardless, the Twin-Six is a fascinating engine from a fascinating period.
Most advertising, however, focused on the V6, which was offered in 305, 351, 401, and even 478 cubic inch variations; later “Magnum” versions offered a few more displacement options. The 478 had a monstrous 5.125 inch bore, so the bore spacing was unreal, as you can clearly see in the picture below. GMC eventually converted the V6 to a 637 cubic inch V8 with a balance shaft (remember that a 60 degree design is not optimal for a V8) to replace the Twin-Six.
Another selling point of this engine family was the well-designed cooling system; this ad may not actually prove anything, because I’ve never seen another engine set up to resemble a fire hose, but GMC claimed that there was no more than four-degrees temperature variation anywhere in the cooling system. Minimum cooling capacity for any truck with a 60 degree engine was 32 quarts. Also notice that the narrow block necessitated a narrow valley cover, especially compared to the GMC’s interdivisional brethren such as the Pontiac V8.
This 1963 advertisement foreshadowed the move to the blander use of photography as the 1960s elapsed. The last great year for automotive advertising was, in my opinion, 1964 (Pontiac ads illustrated by Fitzpatrick and Kaufman notwithstanding).
The V6 (“ONLY GMC HAS IT!”) was available in the entire GMC line of light, medium, and heavy-duty trucks, with the little-loved Toroflow Diesel variation being offered in some models. I’ve read almost nothing good about those engines, but have no experience with them myself.
GMC still had a few colorful ads up its corporate sleeve in 1963 and 1964. This one illustrates nothing new, just repeating the same claims about superior cooling and oiling, although it does allude to the engine’s burgeoning reputation for durability.
This ad is one of my favorites in my GMC collection. Once again, little new is said, but one of the illustrations highlights the beefy crankshaft, which was cast instead of forged (except for, apparently, the Twin Six). The caption on the bottom right is illuminating: GMC offered “a new pickup at a low, budget price. Its thrifty inline engine has a simple design that assures economical operation.” This well-hidden nugget of information would become more overt the following year.
By 1965, things began to change in advertising and at GMC. First, the advertisements transitioned to photography almost exclusively, with bland backgrounds and copious text. Second, GMC began to pictorially advertise a standard Chevy straight-six. Apparently, dealers wanted a lower-cost alternative to the V6 that they could use as a “price-leader” to make them more competitive with the low-priced three.
Here’s another ad from 1965 discussing the merits of the inline-six.
Most ads, however, still highlighted the good qualities of the V6, including its durability. Also advertised was GMC’s use of a printed circuit dashboard, apparently the only one in its field.
I’d love one of these trucks simply for the emblem on the hood. There are still a couple of V6 diehards out on internet forums; some claim gas mileage up to 16 or 17 MPG, while others claim 4. Like anything else on the internet, your mileage may vary.
If nothing else, this ghost-like image of the GMC’s major mechanical components is interesting.
My personal advertising collection contains no GMC V6 ads for 1966 and 1967, and only one for 1968. This ad shows the V6 in the picture, but does not mention it at all in the text. Although GMC didn’t wind down V6 production until 1974, the writing was on the wall when Diesels and Chevy V8s became totally reasonable alternatives to GMC’s engines. The Chevy V8s (both big block and small) are without a doubt great engines, some of the best V8s in American history, but their proliferation ended a truly unique period for General Motors. The “Chevification” of GMC was but one step in the General’s eventual homogenization.
Although the 60 degree GMC is a somewhat forgotten and still controversial engine family, its quirkiness combined with the handsome styling of ’60s GM trucks has to put this truck high on my list of “someday” vehicles. I might have a hard time finding engine parts, but I’m sure that unique exhaust note will put a smile on my face, at least as long as I can afford the gas.