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GMC v6 big cubes

H

Hay hay hay

Well-known Member
I saw the post about the GMC V12, and recalled that at one point back in about 1970 GMC produced a very big cube V6. It seems like it was about 600 ci??? My memory has faded, but they were great for cold start snow plows and yard tractors in the frozen land of upstate NY.
Got abut 2 MPG, but it was a lot better than a no-start snowplow.
 
I saw a new one back then. The Driver said it was two 401's bolted together. I do remember it had two carbs and 4 heads.
 
Brother-in-law had a '62 3/4 ton 4wd GMC with a V-6. It was a 307 (?), I think. It was the best lugging engine ever, at about 1,000 rpm, it would just pull! Drove it more than 100,000, before it spun a bearing & ruined the crank. His son would probably be still driving it if he could find a crank. He would restore the body in a heartbeat. No matter what you loaded it down with or how hard you drove it, or pulled behind it, it got about 10 mpg. jal-SD
 
Here's the history on them.

Never knew there were that many!

I did get into a mess on an engine swap once. Put it in, started it up, it shook something awful! Seems the larger ones were externally balanced, used a different flywheel. Don't remember the outcome, that was a long time ago.
V6 GMC
 
a long time ago i worked for a township hwy dept. they had 2 single axledump trucks with the v6 and a tandem axle dump v6 with a 3x5 trans in it. they were good runners.
 
Used to have a customer with an early 60's 1/2 ton pickup with a 305.

He was in the drywall business, kept that poor truck loaded to the max!

The biggest problems were the 3 speed transmission and rear wheel bearings.

I don't know what the final disposal of it was, but the engine outlasted the rest of the truck!
 
Dad talked about one at the feed mill he worked at, wouldn't win the race but would pull anything you hooked to it 3/4 ton 4 speed. Then I almost bought one in the late 80's early 90's, local plumber had it, then his SIL bought it, then they tried selling it to me, but the cab and utility box were rusted though enough that I was afraid of falling out of the cab floor 1/2 ton 2wd.
 
Our 702 New Idea Uni-System corn picker has a 401 GM V6. It is a smooth running engine. I think it is governed at 2100 or 2200 rpm, and has a big appetite for gasoline.
 
We had a GMC 6 wheeler at the power company, with an 80 ft. man lift on it, 401 cubes. It was a gutless wonder!! Had an IH with a 396 gas pot, that would run rings around it.
 
In 1964 my family bought two new semi tractors to haul logs. One was was a C60 Chevrolet single axel with a 327 5 speed with a two speed rear. The other was a GMC tandem axel with a 401 V6 and a 4 by 5 trans. Max speed was 59.9 MPH, but usually was lucky to get to 45 MPH when loaded. Front tandem with a 4 wheel pup. My cousin and I spent a summer taking turns riding with my uncle when he broke his wrist to shift that 4 speed aux trans.
 
They used a relatively large bore and relatively short stroke. They did that to reduce piston speed. Lots of heavy duty engineering; cam lobes dipped in a pool of oil as they ran, water pump and oil pump moving large volumes for the cu. in. size, huge bearing surface area. known for reliability but not fuel economy.

Lot of information on this guy's site.
http://6066gmcguy.com/gmcv6a.html
 
Actually, it was two of nothing welded/bolted together.

It has a single, unique block, crank, camshaft, etc., but did use the same bore and stroke as the 351 V6.

Dean
 
I used 401 GM V-6 Uni Choppers for years and they were reliable and didn't use much more fuel than the diesels. There was a larger 8 cylinder that had a balancer shaft because it was a 60 degree V and of course the V-12. They were also built as Toro-Flo diesels and not as popular or durable.
 
My neighbor had the 401 in his pickup. He always said it was the best engine ever made. He claimed it had almost 300,000 miles on it. He claimed that it was because they used a lot of nickel in the block. He also had a 5 speed transmission put in it. When his radiator went bad, he put in a radiator from a John Deere R tractor! He had to cut a big hole in the top of the hood for it to fit!
 
First truck I ever owned had that V6 GMC engine in it. I think it was a 305, but not sure. It was a workhorse. Somehow I got it stuck in 3rd gear, and drove it home like that, had plenty of torque to start in 3rd gear, just slipped the clutch some.
 
I remember as a kid in the early/mid 60s an ad on tv for the V6 Chevy engines they used in the pickups. They had had some cooling problems on them so they put a bigger water pump on them.
The commercial showed how they used the water pump to drain a swimming pool. They ran the engine WOT and drained the pool in just a couple of hours. Water was shooting out of the hose almost like a fire hydrant. Ad man had to shout over the sound of the engine to get the message across.
I would have been 10 or 12 then and was REALLY impressed.
In the mid 70s I worked with a guy who drove about a 64? Chevy 3/4 ton with that big 6 as his daily driver. He said the best mileage he ever got with it was about 7 or 8 mpg.
He bought a much newer C10 and was very happy to let that 6 banger go.
 
We had a couple of GMC trucks with the 305 V-6. One was a half-ton pickup with a four-speed automatic. It had dog-slow acceleration, but plenty of towing power. The other was a fuel truck; you couldn't tell if it was loaded or not as the acceleration was the same either way.

The thing I remember about the GMC V-6s is how easy they were to work on. The distributor was up front and the spark plugs were on top of the engine, unlike the small-block Chevys whose plugs and points were hard to get to. I once changed out the water pump on one and it took me about 20 minutes. No hoses to remove; just take off the fan belt and fan, then there were just four bolts that held on the impeller/shaft/bearing assembly.
 
Had a half ton 305 V6 and put a 4 speed tranny in it.
That thing would pull Hell off it's hinges.
Cut a big pine tree one day and it drove a big limb in the ground when it fell.
Backed that bad boy up and chained the limb to the rear bumper.
Took the slack out and it would just spin on the grass.
Backed up to the limb and put it in 1st and told my son and my friend to get back.
Floored it and saw the vee shaped bumper flying through the air and may son and my friend were rolling on the ground laughing.
Straightened the bumper and put it back on.
Richard in NW SC
 
Dad bought a new 69 3/4 ton GMC with the 305 V6. Like others said it could lug like no other gasser but being a 2wd it did t have the traction to utilize the torque in the two bottom gears. The gas mileage was not good but it would start right off at -20F if the starter could grind it over. If I remember right we had to crank awhile to get it started hot.
 
That is what stated my thread below about the V 12 , I have a 1966 2 ton with a perfect running v6 , and that made the guy get interested in the v 12. Right now this motor is for sale.
 
Must have been GMC PU trucks, UD.

Chevrolet never offered GMC 60 degree V6 engines in their PU trucks.

Dean
 
I believe that the V8 version was a 637 CI.

It replaced the 702 CI V12 in the mid/late 1960s.

Dean
 

I n the early 60s Sun oil, "Sunoco" had double tankers hauling gasoline that had v12 gas jobs. They called them double v6s, but were on one block with a total of I think 702 cubic inch.
 
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