The Stovebolt Site Forums The Parking Lot Big Bolts GMC 702 - V12 - "Twin Six" - arrives home

Just pulled in with the 702 on the trailer.
Here are pictures of the engine and extraction process http://rides.webshots.com/album/560407755NLypFl

I'll take more in the morning.

Engine is from a 63 GMC 7000 single axle road tractor.

Grigg

grigg, how was the trip? you should have given me a call i might have been available to help. how was the pull?

Hi Mike,
The trip was good, other than the $200 for gas. I sent you a PM on the old GMC site with a heads up. I would have called, and tried, but I forgot to bring your phone number with me on the trip. We had plenty of help between Dan and his torch, and Dannie a friend I brought from home.

It pulled fine, it weighed 3,200 lb as it sat on the trailer. Engine alone should weigh 1,800 lb.
I ended up bringing my dad's truck and our trailer instead of the Detroit Ford, as I just sold it on Monday.

Grigg

Grigg, What are you going to do with that "Monster" ? You da man.

No quite sure what to do with it yet.

The immediate plan is to get it running. although it has been inside a truck the plugs were removed, so I will pull the heads first and clean all the dirt out of the cylinders, and hope it's not rusted to bad inside. Then I'll soak it with my favorite penetrating oil and attempt to unstick it. I am reasonably sure it is stuck, I could not turn it by hand with the fan, that's not surprising though. I'll try harder after I know the cylinders are clean.

I think I will get it running and on a stand or trailer to take to truck shows, make noise, and burn lots of fuel. At least that's the plan until I think of something more clever.

Grigg

Are you going to put that in something, or restore the engine and build a run stand for it?

Would make an interesting engine for a tractor pull rig.
All those cyclenders, and bone stock? Yeah, thats the ticket,...

you sold the ford! i was really fond of that thing even though im not a ford person.

i like the idea of taking it to shows and making lots of noise with it.

Yes, I sold the ford. I only bought it to play with for a while, it was, and is, a very useful truck and I will miss it. On the other hand, I will be glad to put the money into the 48 Chevy with the 4-53T Detroit. That's the one I'll keep for ever.

Here are some better pictures of the engine, the last few taken after a little cleaning of the valley area.
[img]http://thumb9.webshots.net/t/60/160/4/98/89/2270498890080251109nvTTZj_th.jpg[/img]

I did notice the pan is only held on with 4 bolts. Could be the dented pan caused oil pickup problems and it has bottom end troubles. I'll drop the pan in a few hours and take a look....

Grigg

Holy cow Grigg, thats the coolist thing I've ever seen. I didn't know that combination existed.How many years did they make them.

The 702 V12 was in GMC trucks from 60-65?.
They were offered in a COE and a stub nosed truck. Also in some Walter snow plow trucks.
Many of them were used on irrigation pumps in the Midwest.
I don't know if they were offered for sale as industrial engines any later than 65, but I am guessing not.

Some great info on the 702 can be found here:
http://www.6066gmcguy.org/TwinSix.htm

Grigg

Thanks Grigg, awesome info. I'm guessing there hard to find but my '51 GMC 640 could use one. 2500 r.p.m. max, the thing would last forever.

Dropped the pan,
There was a big handful of rust in the bottom of the pan, but looking up at the crank and cylinders they look OK.
It will take some work to get it running, but so far it looks like I may be able to without a complete rebuild.

The dented oil pan looks like it was from a forklift, and never tried to run like that.

Next is to remove the carburetors one intake and one of the four heads. I'll take a look at the cylinders and decide if I need to pull all four heads.

I'll also remove a main and a few rod caps to have a look.

Grigg

Surprisingly while out with my demo rig last week I called on an out of the way customer who has two of these beasts in storage. I was really surprised. Hope it is in as good a shape as you indicate. It's just two 351's running in tandem and a lot of them were used on irrigation pumps. Must ahve been some darn deep wells.

Dave,
Let's not get confused now...

That is only partly true, or all wrong, depending on your understanding of tandem...

A 702 has a one piece block, crank, cam, and oil pan. Although many people think they are two 351 V6 engines bolted together they are not.

They do however share several components with the V6, to include rods, pistons, heads, and most carburetor parts.

The list is even longer for the V12 specific items, including:
block, crank, cam, flywheel, oil pump, water pump, oil pan, water connections between heads, thermostat housing, and even intake manifolds.

Although V6 intake manifolds will work there are dedicated V12 manifolds with the complete 12 cylinder firing order cast in them.

There is a chance some parts that are on a V12 that don't fit a 351 may fit a larger V6, perhaps the oil pump? and I know the rods fit several other V6.

Grigg

That is freakin sweet.
You got lucky finding a complete package. You found one w/air intakes, carbs and governor units. Mine was an irrigation pump engine, which was run on propnae. Mine is spotless on the inside, but lacks carbs and governor pods.
Too bad about the oil pan. Mine was laying in a pile of other engines, it has a perfect pan, but 3 valve covers were mashed in.

I used a mixture of diesel, ATF and Mystik 15W-50 oil down the spark plug holes. I could rotate mine 1/4 turn on the first attempt, the more I rocked it back/forth, the more it would turn. I was amazed too at how clean all of my spark plugs looked, they looked brand new.

I found an old Dodge truck a while back. I can remember pulling out the old Hemi using a wench truck and a cutting tortch. Looking at your pics sure brought back those memories of how hot, dirty and nasty the job was.

Glad you finally got it. I hope it's ok internally. Looks like using the old truck frame would make a cool display stand.

Keep a post on the progress,
Gene.

Grigg, Was this a a find that you had in the works that you posted about before? If so I missed it. I have a ton of questions about the engine but maybe you can tell us all about it in Winchester. Its a very cool engine thats for sure and I think seeing it run just sitting on a stand would be pretty cool.

Just pulled a pair of bearing caps.
The a main bearing and journal looks good as well as a rod bearing and journal. I'll assume for now that the rest are fine too.
The whole inside is as dry as a the proverbial popcorn fart, but does not appear to be damaged by the little bit of surface rust.

Next is to pull a head and take a peak. Then I'll either pull all of them to clean the cylinders, or clean them the best I can with the heads on. Once I have all the dirt and rust out of the cylinders I can start soaking the pistons to get them unstuck. I have already applied PB Blaster from the bottom side.

I also have a fair amount of work to do on the oil pan, rust removal and straighting.

Anyone else with a V12, can you please send me a picture of the bottom most sump on the oil pan, it will be helpful for the reconstruction of mine.
In the literature I have on the twin six it shows a bolt on cover on the very bottom of the oil pan, anyone have one like this? Mine is just stamped steel for all of it. I will try to heat and straighten, but I may end up cutting it off, making a new sump, and TIG weld it to the rest of the pan.

Also, any ideas or leads on a sturdy but cheap trailer or axle, around 3,000lb + capacity, single axle, preferably drop axle so I can keep the weight low. I would not mind finding just an axle and springs and I would make the rest.

This is interesting,
Grigg

look at all the junk in the oil pan.
[img]http://thumb9.webshots.net/t/59/59/2/54/2/2192254020080251109AfnFuu_th.jpg[/img]

It had been removed and cleaned, then reinstalled with only four bolts, so I don't think any of the junk in the pan has damaged the engine.

Bill,
Yes, this is the V12 I may have mentioned earlier, but I tried to keep it quiet until I got it home. I found it (actually a picture) at the show in Macungie, and agreed to buy it the following week, but I was not confident they would hold it till I could get there, even with my $100 down payment.

Glad it's home now, although still wondering why I really need it, as I don't, and/or shouldn't have the space, time, or money for it...

Grigg

Well, all I can say is that people toss around the old cliche', "You only live once". The reality is that we are only here for a relitivley short amount of time so we might as well do or in this case buy things that make us happy. Hey, you could always sell it. ( this is how I rationalize the purcahse of the b model Mack I bought). It is defineatly a cool engine. Do you think you can fit this on the trailler with the 48?

I think you should build a sister truck to your 48 with the V-12 for power. It would be a lot of work but it would definately be different, but still GM powered,
Scott

I don't know if I can bring it to Winchester as well as the 48 or not.

I can put on top of the 48 with both on the trailer, but I don't want to hurt the powdercoated frame, so for now that option is out.

If I have it back together by the time of the show I'll figure a way to bring it, probably a friend will bring it on his trailer.

If I don't get it back together in time I'll just leave it at home.

Any ideas on how to start trying to turn it over when I get that far?
I know for a B series Cummins they sell an "engine turning tool", Snap-On part number YA9565A, it's basically a pinion with a 1/2" square drive that fits a hole in the flywheel housing and meshes with the ring gear. I could make something like that from an old starter? Or just pry with a pry bar on the ring gear through the starter hole? or a big strap wrench on the crank pulley? Or?

Grigg

I guess have have been away for too long, I have totally missed all this Twin-Six talk.

As you may recall I had four of these at one time, but Now I don't have any, so I'm looking for one again, and hopefully to find one in a truck.

After I sold my last V12, I changed this page of the site to cover GMC Twin-Six V12 Myths as there are a lot of folks that think the Twin-Six is just that, two engines bolted together, not so....

Nice find, I'll have to check the pics out later, I working a deal on a new old truck right now...

That's awesome, what an interesting find. Maybe you should build a COECC (COE Crew Cab).

If you dream up a plan to somehow utilize it, this could turn into one those Stovebolt sagas.

Keep us informed

John

Aw but another way cool project that Grigg steps to the table with.
Grigg that thing is just way cool when you get that thing going you'll have to post a movie with sound so we west coasters can enjoy it.

Grigg, you asked for oil pan pics, I'm uploading some right now, check out my Twin-Six Albums here:

Jolly\'s Twin-Six & other photos

I hope that helps. BTW I have saved all your Twin-Six photos in my collection....

Even more Twin-Six photos:

Three more albums here.

Jolly,
Thanks for the pictures, I had just looked at all of them again this morning, now you have many more, they will be helpful.
I'll make a second album for all the pictures of the other V12's I have seen.

There are a few things I have noticed that is different in some of the engines, and literature I have seen.

My engine is a 63, and I have a 60 preliminary instruction book.
The book does not show the crossover/ balance tube under the carbs form one intake to the other. My engine has one, as do most, if not all, of the engines I have seen.

My oil pan is different than the book. Mine is plain, and the picture in the book shows a removable bottom plate bolted on.

I do realize they changed rear motor mount design at some point, I have the later ones.

Jolly, another question, one of your engine pictures shows only a 6 cylinder distributor, what's going on with that?

I pulled the carbs, and got a better look in the cylinders, not good. Water in most of them, if only the spark plugs were left in the heads it may be running now.
I'll be pulling the heads soon, but I better make a stand or trailer for it first, as it may be a while before I can get it running, and I'll need the trailer it's on before then.

Grigg

The V6 distributor was installed after I removed the V12 distributor to plug the hols and keep water form getting to the bottom end....

I had my engine sitting on a old houres trailer I cut down, those things weight in at over 1500 lbs so you'll want a Heavy Duty trailer, two axles would be better than one, then you can ballance the engine over them and it won't be so hard to move around when not hooked to your truck. I could raise the tounge jack off the ground and just push my trailer anywere I wanted it to go....

Grigg,
I guess I was speaking in a sort of generic fashion about being two 351's but that is the essence of them. It shows my extremely limited knowledge about these monsters which is shared by nearly everyone else. I've seem them, heard them run and know what they are ...that's about it.

Dave,
I was pretty sure you knew the difference, but the way you worded it could have been confusing to others, that's why I gave a better explanation.
Grigg

cool

I took a few pics of mine, with a shot of the oil pan.
It's a 4-stepped pan, but I don't see a removeable base plate.
http://good-times.webshots.com/album/560430195hzVgsx

Grigg you're a mad man 4 sure. You mentioned 12 cylinder firing order cast in the block. Sounds like it fires 12 times per 2 crank revolutions (& seems most sensible), but I have to ask if it fires 6 times, (via: 2 plugs fire @ the same time)? Sure hope you come up w/ a feasible way to put that dog to work. How many horsepower do these powermills put out? For a laugh/ cry; How many mpg is typical for the 702?

The Twin-Six will fire all 12 cylinders just like any other 4 stroke engine,

Firing Order: 1-4-9-8-5-2-11-10-3-6-7-12 This order is cast into the intake manifold of the V12, not the block.

A side note, these engines will run on just one bank of cylinders, I have been told by a few guys that found their V12 running raty and it was because of bad points of coil on one side or the other.


Rating of hourse & torque can be found here: GMC V6 & V12 Engine Data


MPG were vary low, mite have be GPM, but back in the 60 gas was what? $.25 a gallon...

Thay are rated really low on power, but make a ton of torque.
I was talking to one old-timer. He said the governor can safely be turned up to 4K rpm w/o any ill effects. That would bring the power rating way up. These were made to run all day, every day and last for 10 years in the process. A true industrial engine.
I know an old truck mechanic, he said that the V6s are almost indestructable and capable of high RPMs. What holds true to the V6, can usually be applied to the V12.
He also said that even if water gets into the engine, it usually doesn't damage the cylinders. The blocks have a high content of nickel, making the cylinders really durable.
These guys can (and have countless times) take an old Poppin' Johnny that's been sitting in a field for 50+ years, break the engine free, bring it back to life and it'll continue to run for a long time. Old iron is really durable.

Funny you should mention turning the governor up.
I took the governor oil line off at the first carb, and removed the carbs. Then later I was looking at the line, and it looked like something was stuck in it, turned out to be a 3/16" flat head aluminum rivet, a perfect cap for blocking off the line, it is clamped in place when the line is re installed in the fitting, you could never tell it was there from the outside.

I just about have a head off to inspect the cylinders....

Grigg

Pulled the right rear head off, not good.
[img]http://inlinethumb27.webshots.com/11802/2358332920080251109S200x200Q85.jpg[/img]
I'll take the rest off soon.
Does not appear that the heads have ever been off before, and the ridge at the top of the cylinders is very small.

Surprisingly the cylinder walls look decent after scrapping the gunk off, still a small chance it will run without a complete rebuild.

If I could find a set of over sized pistons at a good price I wold be tempted to bore it...

I have uploaded more pictures several times now, so if you looked at them earlier, there are a bunch more now.

Grigg

Just to add some guesses on MPG, the best I can remember, the old 478 V-6 I would drive would get about 8 and the 637 V-8 would get about 6.

My guess on a 702 would be about 5.

Hello!

Last week I did not not know that this engine exsisted, and today I attended a local car show and there was a semi-tractor there with this engine installed. It was originally used by a fire deptment to pull a tanker trailer. It was definately the center of attention!

Randy B.

Jolly,
Where do you get internal engine parts for a 351 or 702?

My original plan was to get it running, not being concerned with tolerances so much because running occasionally on a stand is different from in a truck.

I am now considering a full and proper rebuild, but want to get an idea of the cost. I have time to shop around and wait for good deals on stuff if possible.

When I got this engine I knew I would have a hard time fixing it on the cheap. Not because of the price of parts, but because by nature anything I deal with that is mechanical I have to do just right or I am not satisfied, no shortcuts.
The illusion that I could get it running without worrying about proper procedures was short lived, about 4 days....

So, if I think I can get a good deal on pistons and such I'll probably bore it and do a complete rebuild.

I have found main bearings really cheap already.
Looking for pistons and rings next.

Grigg