Learned something about GMC from eBay

[JamesR]

They actually made a large displacement V6 engine in the mid 60's. I did not know that, though I admit I have little GMC knowledge. And no experience. At first I thought the V6 in this truck was after market, then I saw the emblem on the side. "Very cool!" I said.

 

If I'd seen the motor without knowing it was a V6, I might've thought it was a 348 or 409 at first glace. It kind of has that look. Was this V6 used in any other GM vehicles? How long was it used in the trucks?

 

1966 GMC C20 | eBay

Edited October 28, 2019 by JamesR (see edit history)

[jrbartlett]

You might also be surprised to learn that they built some V-12s that I think were two of these engines in line. I think for larger trucks. Someone else will weigh in.

[cheezestaak2000]

yes it was the basis for the V12 and was used quite a bit into the 1970's as well. if you want to change the spark plugs, make sure you clean any debris from around them first, if not it could easily fall into the hole after you remove the old plugs

 if memory serves me correctly, they were 301 and 351 cubic inch for the six

 

Edited October 28, 2019 by cheezestaak2000 (see edit history)

[Frank DuVal]

That would be the 702 cubic inch GMC engine based on two of the V-6 blocks connected together at birth!😲

 

https://www.hotrod.com/articles/ccrp-1212-702-ci-thunder-v12-gmc-engines-horsepower/

 

I have seen one in person, stuffed into an Oliver tractor at the Field Days of the Past in Oilville, VA. What a sight!👍

 

AFAIK, the V-6 was only found in GMC trucks, pick ups and larger. The V-12 was originally for over the road tractors just before diesel got big.

 

 

[8E45E]

There was also the Toroflow diesel engine versions of that V6 and V12.

 

Those gasoline-engine 702's used up to 3 gallons of fuel a mile under heavy load.

 

Craig

[zepher]

I was not aware of this engine from GMC, either.

Was it produced to compete with the 300 ci Ford inline 6?

[Rusty_OToole]

GMC did make a V6 and a V12, designed at the same time and part of one family of engines. They must have been about the last all new  gasoline engines designed for heavy trucks before diesels took over. Their other claim to fame was the worst gas mileage of any pickup truck of equal horsepower.

[WQ59B]

The 1st gen GMC V6 was 305 CI, not 301.
A number of other variants were built, too : https://en.wikipedia.org/wiki/GMC_V6_engine

 

RE the V-12 : "It was mistaken as two V6 engines welded together, but it is its own separate engine design based on a single casting."

Edited October 29, 2019 by WQ59B (see edit history)

[Rusty_OToole]

1 hour ago, zepher said:

I was not aware of this engine from GMC, either.

Was it produced to compete with the 300 ci Ford inline 6?

It was introduced in 1959 and continued in production till 1974. The idea was to make a super powerful V12 for hiway transport trucks and a V6 using the same parts, for smaller trucks. It was not designed as a light truck engine but the smallest V6s were used in GMC pickups and Suburbans. Really it was designed for heavy duty truck use and was overkill for a light truck.

[Pfeil]

3 hours ago, JamesR said:

They actually made a large displacement V6 engine in the mid 60's. I did not know that, though I admit I have little GMC knowledge. And no experience. At first I thought the V6 in this truck was after market, then I saw the emblem on the side. "Very cool!" I said.

 

If I'd seen the motor without knowing it was a V6, I might've thought it was a 348 or 409 at first glace. It kind of has that look. Was this V6 used in any other GM vehicles? How long was it used in the trucks?

 

1966 GMC C20 | eBay

 

 

Check it out, 1959-1974

V6 engines were produced in 305, 351, 401 and 478 cubic-inch (5.0, 5.8, 6.6, and 7.8 respectively liter) displacements, with considerable parts commonality. During the latter years of production, 379-and-432-cubic-inch (6.2 and 7.1 L) versions with enlarged crankshaft journals were manufactured as well.

 

 

 

 

 

GMC produced a 637-cubic-inch (10.4 L) 60° V-8 with a single camshaft using the same general layout (bore and stroke) as the 478 V6. The 637 V8 was the largest displacement production gasoline V8 ever made for highway trucks.

 

 

 

 

 

 

 

The largest engine derived from the series was a 702-cubic-inch (11.5 L) "Twin Six" V-12, which had a unique block and crankshaft, but shared many exterior parts with the 351.

 

 

 The crew;

[Locomobile]

3 hours ago, Frank DuVal said:

AFAIK, the V-6 was only found in GMC trucks, pick ups and larger.

 

I recall the V-6 351 and what I remember it was used in heavy trucks, referred to as the "dump truck" engine. That engine had huge bores in it, like 4-1/2". It was a very low torque bull power plant, not fast at all. A lot of people back then didn't believe GM had a 351, soon as someone said "351" they immediately assumed it was a Ford. Which always prompted the follow up question, "Are you sure GM made that?"

 

-Ron

[PONTIAC1953]

3 hours ago, cheezestaak2000 said:

yes it was the basis for the V12 and was used quite a bit into the 1970's as well. if you want to change the spark plugs, make sure you clean any debris from around them first, if not it could easily fall into the hole after you remove the old plugs

 if memory serves me correctly, they were 301 and 351 cubic inch for the six

 

they were 305 and 351

 gmc V6's

Edited October 29, 2019 by pontiac1953
correction (see edit history)

[Pfeil]

14 minutes ago, pontiac1953 said:

they were 305 and 351

 gmc V6's

What I already said.

[PONTIAC1953]

1 minute ago, Pfeil said:

What I already said.

you were correct, cheesestaak2000 made an error

[PONTIAC1953]

even as much crazy was the stacking of two R type dual range hydra-matic transmissions known as the Twin Hydra-Matic, a 7 speed automatic transmission with the upper trans turned upside down and connected to the lower trans, both on the input and on the output side.

[PONTIAC1953]

[trini]

Cool engine , baby.

[Pfeil]

50 minutes ago, pontiac1953 said:

even as much crazy was the stacking of two R type dual range hydra-matic transmissions known as the Twin Hydra-Matic, a 7 speed automatic transmission with the upper trans turned upside down and connected to the lower trans, both on the input and on the output side.

 

 

Now I like that Charles! Could you post / scan the cutaway?

Remember the "Twin" "Tempest Torque" called the "Power Shift" 4 speed semi Hydra-Matic?

Edited October 29, 2019 by Pfeil (see edit history)

[8E45E]

41 minutes ago, Pfeil said:

 

 

Now I like that Charles! Could you post / scan the cutaway?

Seen in 2010 at the ATHS show.

 

Craig

Edited October 29, 2019 by 8E45E (see edit history)

[PONTIAC1953]

58 minutes ago, Pfeil said:

 

 

Now I like that Charles! Could you post / scan the cutaway?

Remember the "Twin" "Tempest Torque" called the "Power Shift" 4 speed semi Hydra-Matic?

 

[PONTIAC1953]

[PONTIAC1953]

[zepher]

And just think, the 702 ci V12 is still considerably smaller than the 825 ci inline 6 Pierce Arrow made in the late teens.

I guess the big V6's from GMC were great stump pullers but since they didn't rev real high they would be of no interest to performance enthusiasts.

Must be why not many, including myself, have never heard of them.

[JamesR]

Thanks for the replies and education, everyone. Fascinating to learn about an entirely new unique engine from a mainstream vehicle. To me, just the giant engine badge on the side of the hood is worth the price of admission.

[joe_padavano]

You might find this website interesting:

 

http://6066gmcguy.com/gmcv6a.html

 

Also, I suspect that ebay didn't tell you about the plaid valve covers...

 

[JamesR]

2 hours ago, joe_padavano said:

 

 

Thanks, Joe. Now that's a real milestone in the history of automotive power!

[Buffalowed Bill]

Two things that I remember about these V6, they really ran rough, and got lousy gas mileage. 

[GARY F]

In 63 my father bought a new 63 GMC p/u. It had   a 305 V/6 with plaid valve  covers. After he passed away I bought it from my mother. Great truck.

[Pfeil]

36 minutes ago, Buffalowed Bill said:

Two things that I remember about these V6, they really ran rough, and got lousy gas mileage. 

You mean like the 3.8 before Buick offset ground the crankpins.

[Bill Harmatuk]

Had to find this.

Thought you guys might like to see.

[Locomobile]

I don't think they ever used the engine being discussed (351 V6) in light trucks. That is a massive heavy V-6 engine. It's a commercial/Industrial engine. If someone has one in a pickup, I'm guessing it's been retrofitted to the vehicle and I'll bet its a real dog on the road.

 

-Ron

[AzBob]

One thing I recall about the GMC V6 of that era is that it had a distinctive sound. One could always tell when one was coming down the road. The later V6’s sound different. 

[Pfeil]

50 minutes ago, Locomobile said:

I don't think they ever used the engine being discussed (351 V6) in light trucks. That is a massive heavy V-6 engine. It's a commercial/Industrial engine. If someone has one in a pickup, I'm guessing it's been retrofitted to the vehicle and I'll bet its a real dog on the road.

 

-Ron

 

 

Collectible Automobile Magazine published in it's June 2017 issue all the engines listed for the GMC light duty trucks for 1967- 1972

Those engines are;

GMC

305 V-6

351 V-6 *

Chevrolet Gen 3 inline 6

250- I-6

292 I-6

Chevrolet Small block V-8

283 V-8

307 V-8

327 V-8

350 V-8

Chevrolet Big block V-8

396 V-8

402 V-8

 

* used in 1967 and 1968

 

Edited October 29, 2019 by Pfeil (see edit history)

[Pancho's ride]

My 67 GMC had a 351 V6. The truck was a vacation vehicle and had an Alaskan camper and pulled an airstream trailer.  I’m a carpenter and it’s now my daily driver. It no longer has the V6 in it. 7 miles per gallon and top speed of 70 downhill was all it took to change things up. 

[chistech]

That double stacked tranny looks like two of the 4 speed tranny’s I used to work on in the 51-54 GMC deuce and a half military trucks. They were basically clones of the 48 Oldsmobile auto with a 4 to 1 reduction on the back of them. I used to buy and recondition military trucks and rebuilt a ton of those tranny’s. The reason they failed was the rubber the lip seals were made out of was junk plus they used a main shaft centrifugal weight governor to shift which often caused premature shifting from a band to a clutch pack. Replacing the lip seals with modern replacements and putting in a manual shift valve body while removing the governor made them bomb proof trannys.

 

on another note: we had an old GMC box truck with one of those big block V-6 in it. I can still visualize the emblem on the side of the fender noses!

[58L-Y8]

Our school district had a fleet of GMC V6 school buses in the 1960's, the local Pontiac-GMC dealer placed the lowest bid.  They always had an odd sound running and to the exhaust.   Apparently they were quite 'thirsty' too, but tough, durable and long-lived, rust got the bodies before the engine wore out.  Some ended up as cut-down farm trucks after the district sold them off.  They made great hay haulers.

[8E45E]

7 minutes ago, 58L-Y8 said:

Some ended up as cut-down farm trucks after the district sold them off.  They made great hay haulers.

Many of the V12's ran irrigation pumps once they finished their life under the hood of a truck.

 

Here is a loose 702.

 

[junkyardjeff]

I remember seeing a GMC truck in a junkyard years ago with the V12 badges so I had to look since I did not know there were made,opened the hood and seen what I thought was a V6 but then looked back and seen another intake and set of heads.