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Old 10-08-2014, 04:32 PM   #1
OrrieG
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GMC 702 Twin Six V-12

I did not even know these existed until a few years ago. I found them on a site for irrigation engines (I was researching Hemis). Here is a link to some info. Googling GMC Twin Six turns up a bunch of information. The look cool but unfortunately are a mule, max rpm is about 2500 (275 HP) but they have 630 ft # torgue at 1600, just right for running the large irrigation pumps found around here and in CA. Also good for heavy trucks of the era. They are expensive to buy in any shape and expensive to rebuild (like 8-10K).

http://www.curbsideclassic.com/blog/...s-at-1600-rpm/

A couple of months ago I went to a wrecking yard in another town I had not been to in a while. They had one sitting in the parking lot. Here are some pics. It has one block casting, 4 heads, 2 intakes, 4 exhaust manfolds and unique distributor. The intakes and valve covers are different from the V-6 engines of the era.
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Old 10-08-2014, 04:53 PM   #2
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Re: GMC 702 Twin Six V-12

Thanks, Orrie!

I love the GMC V6-60 family from the '60s. Engine sizes range from "tiny" at 305 ci to 478 cubes. It had gobs of torque and burned fuel like it was limitless. There was also a 60 deg V8 at 637 cubes, a "Toro flow" diesel in V6 and V8 form, and the above pictured 702 ci V12.

http://en.wikipedia.org/wiki/GMC_V6_engine
http://www.6066gmcguy.com/EngineData.html

I have more thoughts and a neat story I'll post later when I have more time.
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Old 10-08-2014, 07:41 PM   #3
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Re: GMC 702 Twin Six V-12

That is a very cool engine.
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Old 10-08-2014, 08:32 PM   #4
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Re: GMC 702 Twin Six V-12

Quote:
Originally Posted by OrrieG View Post
but they have 630 ft # torgue at 1600,
and THAT makes them cool

that 59 gmc 450 needs one of those and 2 super single tires on it
read my post huh?
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Old 10-08-2014, 09:02 PM   #5
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Re: GMC 702 Twin Six V-12

Thanks for the cool photos!
I learned something today.
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Old 10-08-2014, 09:32 PM   #6
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Re: GMC 702 Twin Six V-12

There have been a few rods and even a 60-66 tru k built with one in it. They extended the nose a foot and used two hoods to cover the motor.
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Old 10-09-2014, 12:46 AM   #7
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Re: GMC 702 Twin Six V-12

So the story is this: Once, back when I was in my 20s, I was traveling out of Yellowstone through Cody, WY and decided I'd explore the local roads a bit. So I turned and headed up a hill on a two lane and drove until I passed a house with a pile of old Chevy and GMC trucks out front. Now, seeing a bunch of old trucks in front of a house out that way isn't so uncommon but something about this yard stood out. I told my girlfriend I'd love to stop and surprisingly, she said "Go ahead." I spun around and proceeded up the fairly long driveway passing outfits from every decade prior to 1990 before stopping at the house.

The house wasn't much to look at and no one was around and I was beginning to rethink my move when an older curmudgeon came out to ask what I wanted. Well, I told him I liked the trucks and wondered if I could just wander around and look at them as they sat in various stages of disrepair. He let me, so I spent a few minutes looking over various trucks. On the way back to the car I said thanks and talked a bit more intending to head out. During the conversation I mentioned that I'd had a friend who'd built a pulling truck of a '68 GMC using an old, stock 305 and that it seemed like a better engine than most folks expected. Well, that just seemed to light the guy up. He tells me to follow him out to the barn and lo and behold, he's building a '67 (or '68) GMC 4X4 with a GMC V6. Only he wasn't using a stock engine.

Turns out this guy had been a serious drag racer in CA in the '60s campaigning Nailhead Buick powered cars and he'd developed liking for going fast with unexpected parts like the V6 Jimmy. He'd bolted heads from a larger displacement engine onto a smaller displacement shortblock (larger valves, less shrouding), used oversize 455 Buick pistons to raise compression, had a cam made, and had a very nice custom made 4bbl intake set on the top. Well holeee mackerel... Then I told him about my 455 Buick powered 79 Monte and after that, he just opened right up. Showed me several different V6 engines, a V8 version, a V12 assembled and sitting on a pallet (from a fire engine) and another in pieces in a shed along with piles of cylinder heads and cranks. I wish I'd had a way to record everything he was telling me because he had more ideas, tips, and tricks than I could begin to remember today. He went so far as to tell me Jim Bell still owed him money for a lost drag race years ago, and told me if I ever called Kenne Bell to order parts I should ask for Jim directly and mention this guy was still waiting on his check! What a character! I've had a lot of great random experiences by stopping and knocking on doors where old cars or trucks are parked but that one was one of the best.

An interesting thread for anyone considering these engines:
http://www.jalopyjournal.com/forum/t...series.153130/
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Old 10-09-2014, 10:08 AM   #8
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Re: GMC 702 Twin Six V-12

^^^^ Awesome
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Old 10-09-2014, 11:08 AM   #9
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Re: GMC 702 Twin Six V-12

Thanks, project!
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Old 10-09-2014, 11:25 AM   #10
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Re: GMC 702 Twin Six V-12

now that is not an everyday engine nor in locating any part(s) for one either ....

talk about neat ...
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Old 10-09-2014, 01:38 PM   #11
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Re: GMC 702 Twin Six V-12

Great story!
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Old 10-09-2014, 06:10 PM   #12
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Re: GMC 702 Twin Six V-12

Back in the day the V6 was Ok for what it was designed for. They like a Y block ford where dogs when it came to high performance. Slow turning poor design and lots of internal friction. Ive owned several 305,s a 351 and a 478 Toro Flow diesel. In the PK,s I swapped in small block chevys and was much faster & happier and used a lot less fuel. Not to hurt anyones feelings but the V6 is a obsolete outdated design. there not practical in todays world. They should be considered antique oddities.
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Old 10-09-2014, 09:52 PM   #13
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Re: GMC 702 Twin Six V-12

Quote:
Originally Posted by Old Truck Man View Post
In the PK,s
What is a PK? Thx.
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Old 10-10-2014, 12:27 AM   #14
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Re: GMC 702 Twin Six V-12

OrrieG
That truck on the video was built buy a boad member cayoterun
http://67-72chevytrucks.com/vboard/s...d.php?t=254111
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Old 10-10-2014, 08:03 AM   #15
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Re: GMC 702 Twin Six V-12

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Originally Posted by Hutch View Post
OrrieG
That truck on the video was built buy a boad member cayoterun
http://67-72chevytrucks.com/vboard/s...d.php?t=254111
OrrieG: This link was posted over on the 6066GMC truck club. I thought I'd drop in and add my .02.
The V-12s were used in this area for power units on irrigation wells. I took care of the engine in the '62 video when it was new in '65 on a well. I always thought the engines were interesting and neat. They were fast disappearing, so I had a chance to fix the old motor up, so jumped on it. I donated that truck to the Cimmarron Heritage Center in Boise City, Okla, and it's on display there. They have said that a lot of people really enjoy seeing one, and had never known they existed.
I found another 12 and put it in the 47 Chev pu. in my avatar. It's all riding on an '84 GMC 3/4 ton chassis. I still have this old truck, and crank it up and go for a ride now and then just to keep it oiled up, and it's fun to hear the old critter scream again. I never abuse or turn the motor over 2400rpm, tho.
If you or others are interested, my son has posted other V12 videos on his Utube.

Haystack155 Utube.
Hop in and we'll take a ride.

Due to age and health, I don't tinker much anymore, so I'm glad I fixed those two up when I had a chance. I guess they and I are riding off in the sunset together. LOL

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Old 10-10-2014, 10:31 AM   #16
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Re: GMC 702 Twin Six V-12

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Originally Posted by Old Truck Man View Post
Back in the day the V6 was Ok for what it was designed for. They like a Y block ford where dogs when it came to high performance. Slow turning poor design and lots of internal friction. Ive owned several 305,s a 351 and a 478 Toro Flow diesel. In the PK,s I swapped in small block chevys and was much faster & happier and used a lot less fuel. Not to hurt anyones feelings but the V6 is a obsolete outdated design. there not practical in todays world. They should be considered antique oddities.

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


Uh, no. Slow turning, ultra high torque at low rpm, extremely low internal friction, stupidly high water/oil flow for constant cooling, and constantly lubed cam. If you can get the traction you can change the earth's rotation with that beast. SBChevy is a faster turning engine, (faster driving) but for economical high torque, longevity, and consistent reliability you won't beat that old 305V6. Swap the rearend gears and tranny and you'll get stupid high gas mileage too, instead of the normal 13-15 mpg on a good engine. I use my "obsolete" 305V6 for stump pulling, post pulling, towing, and even the occasional mud rescue with great results.

And to top it off, pop the hood and show them you run a 305....V6..... and watch their cartoon eyes fall out and roll on the floor. Talk about a conversation starter!




Quote:
Originally Posted by cayoterun View Post
OrrieG: This link was posted over on the 6066GMC truck club. I thought I'd drop in and add my .02.
The V-12s were used in this area for power units on irrigation wells. I took care of the engine in the '62 video when it was new in '65 on a well. I always thought the engines were interesting and neat. They were fast disappearing, so I had a chance to fix the old motor up, so jumped on it. I donated that truck to the Cimmarron Heritage Center in Boise City, Okla, and it's on display there. They have said that a lot of people really enjoy seeing one, and had never known they existed.
I found another 12 and put it in the 47 Chev pu. in my avatar. It's all riding on an '84 GMC 3/4 ton chassis. I still have this old truck, and crank it up and go for a ride now and then just to keep it oiled up, and it's fun to hear the old critter scream again. I never abuse or turn the motor over 2400rpm, tho.
If you or others are interested, my son has posted other V12 videos on his Utube.

Haystack155 Utube.
Hop in and we'll take a ride.

Due to age and health, I don't tinker much anymore, so I'm glad I fixed those two up when I had a chance. I guess they and I are riding off in the sunset together. LOL

Cayoterun
Coyote, you'll be around forever. I've commissioned Umbrellacorp to upload your brain to the internet so you can stick around and provide friendship and advice well into 2050. After that my grant runs out

That truck is legend!
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Old 10-10-2014, 01:39 PM   #17
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Re: GMC 702 Twin Six V-12

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What is a PK? Thx.
Pickup truck.
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Old 10-10-2014, 02:05 PM   #18
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Re: GMC 702 Twin Six V-12

any time you have huge heavy pistons large bearings ect you have more internal friction than an engine that has smaller lighter parts. Even if you don't believe in the laws of physics they still apply. In never said the V6 wouldn't pull a big load. However a 292 straight 6 will do equally as well and get better fuel economy. There is a reason GMC stopped building those V6,s and went to regular Chevrolet designed engines. Like I stated ive owned several V6 engines. Ive drove truck tractors equipped with the V6,s and the pulled ok but used twice the fuel a similar diesel rig did. For example a truck would have a 478 V6 with a five speed and two speed. and a similar truck would have a 6-71 Detroit diesel with a ten speed road ranger. the Detroit was only 426 cubes. however there was no comparison. Taking off from a dead stop with the gas burner fully loaded it would take over a quarter of a mile to get up to speed. the two cycle Detroit would do it in half the distance and use half as much fuel. given a choice the drivers always took the diesel. If they got the choice between the GMC v6 truck and the chev 409 truck they took the 409. then we got a couple of GMC that where 427 engines with 13 speed roadranger trans and everyone liked them. I owned a truck tractor with a 478 toro flow diesel. that engine was governed a 3200 RPM it wasn't that hard on fuel however it wasn't turbo charged. it had less pulling power than a 478 gas engine. The fact is they where what they where. and today they are what they are.


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Old 10-10-2014, 02:43 PM   #19
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Re: GMC 702 Twin Six V-12

Quote:
Originally Posted by cayoterun View Post
OrrieG: This link was posted over on the 6066GMC truck club.
Cayoterun
Yours was posted on my thread, have not had time to look at both of the build threads but will this week end. The engine her looks to be in really good shape, but I did not have time to find out if its for sale or just show. Looking at my pictures realized there is a transmission hooked up to it so probably a truck application and not a pump as I thought. Do not let the age thing show you down, my father was still tinkering into his 80's...
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Old 10-10-2014, 09:38 PM   #20
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Re: GMC 702 Twin Six V-12

Quote:
Originally Posted by OrrieG View Post
Yours was posted on my thread, have not had time to look at both of the build threads but will this week end. The engine her looks to be in really good shape, but I did not have time to find out if its for sale or just show. Looking at my pictures realized there is a transmission hooked up to it so probably a truck application and not a pump as I thought. Do not let the age thing show you down, my father was still tinkering into his 80's...
The 12s and MM 800s were the sweetheart pump motors on the bigger wells for years due to their low operating rpms and durability, but the 12s lost favor in time due to outrageous fuel consumption.

A neighbor had trouble with his NG line, so set a 1000 gal propane tank for awhile, and got a pretty accurate check, and the thing used 22 GALLONS PER HOUR.
I've been told the road trucks with them never got over 2-3mpg.

Thanks for the age encouragement. My goal is to do my best to always be positive, and find something humorous every day.
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Old 10-11-2014, 06:03 AM   #21
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Re: GMC 702 Twin Six V-12

I like that Cayote. I also try to learn something each day. These forums help with that.
Here's something humorous for you today:



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Old 10-11-2014, 10:17 AM   #22
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Re: GMC 702 Twin Six V-12

Quote:
Originally Posted by cayoterun View Post
The 12s and MM 800s were the sweetheart pump motors on the bigger wells for years due to their low operating rpms and durability, but the 12s lost favor in time due to outrageous fuel consumption.

A neighbor had trouble with his NG line, so set a 1000 gal propane tank for awhile, and got a pretty accurate check, and the thing used 22 GALLONS PER HOUR.
I've been told the road trucks with them never got over 2-3mpg.

Thanks for the age encouragement. My goal is to do my best to always be positive, and find something humorous every day.
Ok 22 gallons per hour. then the 351 V6 would have got 11 gallons per hour and did half as much work? All I know is that every V6 GMC I was ever around was a gas hog. There was a guy that had one with a 305 and a 1 bbl carb, he claimed it got teriffic gas milage. I drove it and the speedometer didn't work so I pointed that fact out. He told me he twisted it off when he got it up to 120 mph!
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Old 10-11-2014, 03:16 PM   #23
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Re: GMC 702 Twin Six V-12

A lot of farmers around here ran the V6 GMC's when they were new because they did really well out in the fields at low speeds during harvest and a couple of neighbors still have two ton cattle trucks with those engines. The big issue around here outside of drinking fuel was the cost of repair when they finally wore out or got broken by inept farm hands driving them wound out to the max in too low a gear.
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Old 10-11-2014, 04:08 PM   #24
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Re: GMC 702 Twin Six V-12

Quote:
Originally Posted by cayoterun View Post
the thing used 22 GALLONS PER HOUR.
holy smokes Bullwinkle !!!
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Old 10-11-2014, 04:21 PM   #25
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Re: GMC 702 Twin Six V-12

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They should be considered antique oddities.
Some could say that about members here. Something about glass houses comes to mind.
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