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

Grigg if you can no longer get the pistons measure pin height pin size and check for any offset, check ring size and placement, skirt design and length. You may even send one off to some one like TRW or Keith Black there may be a close enough piston from something else that will work fine.
You may even find that there are options that no one has thought of yet like a hypereutectic piston. If you can’t get something off the shelf you could always have them made.

I sent a piston from a Touota to Ross Pistons. Durable enough for turbo/nitrous use. Pretty pricey though.

I'll give you a Felpro part# for head gasket/seal kits. I called a local Auto Zone searching for head sets, they have them in stock at the warehouse. It was a listing for a 351 V6, so multiply that by 2.
A place called Egge machine produces obsolete engine parts. I'm sure they could produce a set cheaper than anyone else. I think they only offer cast pistons. TRW might be a good choice for something more durable (if it's on a stand, how durable does it need to be???????).

If I ever have mine torn down to the bare block, I will surely have the internals balanced. Then there would be no fear of turning up the RPMs. I plan to eventually put mine into a vehicle, I'm just not sure what kind to look for.

Jay Leno has a fire truck, and he said it had a V12. He didn't go into any detail about the truck, he was in an interview on one of his other vehicles.
I've heard that the bigger old fire trucks were powered by the Twin.

I just noticed that it has 2 bolt mains. Is the center cap the only one that has 4 bolts? What location is the thrust bearing in?

Stupid me, I could have looked in my Twin-Six service manual. Oh well, just thought I'd ask.

thrust bearing is second to last, near the flywheel, but not quite.

Jay Leno has an American LaFrance firetruck with their own V12. I have seen one before, very nice engine. Chain driven overhead cam. The engine was designed by Brush, who had his own brand of cars early on, before the teens.

I would not mind having one of those either, the V12 model "J" engine.
Grigg

As previousely stated, you may be able to measure your pistons & find suitable replacements, .... However; the biggist obstical may be the piston boss where the pistin pin seats. I read that the 702 piston receives a larger piston pin than a 351 6 cylinder engine. This translates into; Finding a piston suitable may not be difficult provided that the replacement piston's boss area can be safely bored out to receive the old larger piston pin. My guess is that finding a piston w/ enough meat in the boss area to bore it & still have enough metal in the boss area to run the engine safely is probably unlikely. If you find you're nailed to the gun in this situation, you may be able to improvise by installing brass bushings in the top of the connecting rods, then bore & hone them to accept the smaller piston pins. pressing brass bushings in the top of connecting rods is easy & only requires threaded rod, 2 sockets & washers to make a tool to install/ remove them. The "new brass bushings" may need holes drilled in them to accomidate any oil passage that may needed to lube the piston pin.
Does anyone know how much larger piston bosses on the 702 are, than bosses on the 351?

My literature shows that the 702 and 351 have identical wrist pins at 1.240".
Bore is a nominal 4.562 (4-9/16) with standard oversizes at 0.020 and 0.040.

Rods are the same as a 401 V6, and I believe a 351 also.

Looks like Kanter sells parts, I have requested their catalog.

I think hydraulic Lifters are the same as a 235 / 261

I have seen Egge pistons before, while suitable, I don't think they are as good as the real thing.
I too think Ross could make them, but at what price?

I am still thinking that there will be some genuine GMC pistons out there somewhere, or some NOS replacements? Or even new ones from a regular engine parts store.

Grigg

My bust. The 702 con-rods receive larger "crank pins", ... not piston pins.

If you buy from Kanter, figure on opening your wallet wide, & waiting a while to receive the parts. Personally, I'd use Kanter as a last resort when all else has failed.

Not to be mean, but I need to disagree again.

The main and rod journals on both the 702 and 351 are identical.
The crank pin diameter, where the rod sits, is 2.8107", and the crank main journals are 3.123"

Grigg

Hey Grigg.
It looks like your thread is a big hit. I never thought you'd get this much action. Then again, I can count on 3 fingers, how many of these engines I've seen in a forum.

I went through my file and I got the part # for the head sets.
Some counter person researched the application (351 V6) for me and came up with a Felpro HS7962SB1 . Priced just under $52.00 each. I guess $104.00 isn't bad for a complete top end gasket/seal set.

For a Felpro true V12 complete gasket set with crank seals, and oil pan gasket I was quoted just over 200, this includes the V12 specific gaskets that can't be had by doubling a 351 set.
I can reuse my head gaskets, I may end up making the special gaskets, and just buy intake exhaust and valve cover gaskets. It would be nice if I could buy each gasket I needed separately, but not all of them can be had outside of a complete set.

The big hold up in parts are the pistons, I have yet to locate an easy or reasonably priced source for them.

I did get a military 1.5 ton trailer, M105 I think, to mount the engine on. I'll remove the bed and make some brackets to bolt the engine to.
Also look for some 19.5 6 lug Budd wheels and low profile tires to get it closer to the ground.

Grigg

No offence taken & thanx. That's good news for anyone looking for 702 con-rods. http://www.6066gmcguy.org/TwinSix.htm
boasts "6 extra large crankpins", which led me to believe they apply "exclusively" to the 702.

You said that you can reuse the head gaskets.
What are they made from.

Head gaskets are a copper sandwich of asbestos, or something like it. The ones on my engine are original, best I can tell, and in perfect shape. I'll use some copercoat head gasket spray paint just before I reinstall them.

Making progress on the trailer, I got the one wheel to turn free, ended up removing the drum with backing plate still rusted to it, then put just the hub back on. I tried to break the stuck drum loose with an 8 foot bar, 2" solid steel. Had it chained to the spokes on the wheel trying to turn it. Even had the truck hooked to one end of the bar with a chain, four wheel low and all I did was bend the 2" bar.

I am having poor luck finding pistons.
Sounds like most people who need them have them made, mostly by Ross.
I have started calling obsolete engine parts places, one has bearings rings and stuff, but could only find 2 pistons. I have a call in to another similar place about pistons.

I did find a set of 6 on ebay, the auction is passed, but they are still for sale, I called.
Only thing is they are listed for a few years newer engine.
Can anyone explain why 351 pistons changed over the years, and what makes an early one different from a late one? The measurements I have found show they are identical, bore, rings, wrist pin.... so I am missing something, but what?

From my local auto parts store I did get a copy of the engine parts catalog covering the 351. they can't get pistons any more, but there is a listing for two different ones, a low compression, and a higher compression.
See any reason I should not build my engine with the higher compression, about 9:1 I think, if that's the only pistons I can find?

Grigg

The 351 came in both heavy & Light truck versions, the light truck engine came out in 1966, it's the 351E. the heavy truck engines are the 351 with no letter, 351C & 351M. the diffeance is in the number of pison rings, light truck engines have 3 while heavy truck engines have four.

do you have any of the pistons out yet?

I know the management here dose not like it when I tell folks they should be asking these questions in the 6066 GMC Trucks Group on Yahoo! Groups, but that is the best place to find help with this or any of the other GMC Big Block V6/V12/V8 engines, we have a lot of guys that are more expert on these engines over there than what I am... And you are a member there now.

Swung over to Kanter today & asked if they had any pistons for GMC V-12 702 engines. Nothing at all in stock. Asked if they can get them. Can't get them. "Truck Trader" & "Hemmings" were the only suggestions offered. For the heck of it, grabbed an updated copy of their catalog on my way out. Just got done looking through it. 702 not even listed.

Did you guys see these pictures? Thats a lot of cool iron.

[img]http://thumb10.webshots.net/t/36/36/1/82/34/2106182340033750270njhTgA_th.jpg[/img]

Those "Texas Twins" were the other 7 engine they found before I got down there to pick up my 4 engine I had bought, and if I had the extra money I would have gotten them all. heres the kicker, he only wanted $100 Each!!! last time I talked to him they were all gone and he had not found any more.

So now that you all know how dumb I was on that deal, have I pointed out this one yet? GMC V12 Frankenstein Truck , its running around in BC Cehnehdeh (oosp Canada).

Hey, I used to drive by that truck everynight on my bus route. Heard it running a few times. Hadn't seen for some time, then it popped up back in town. Don't know if the owner would appreciate the Frankenstein name though.

How did it sound, Freedo???? I can only guess on the music that can come from its pipes.

I'm stll searching for an old body to put mine in. Whether it's an old car body or an old truck, I'd like to channel and chop the thing. Then stretch the front end to make room for the Twin (one can dream, right???).
Eventually I'll do whet Grigg is doing, and start the tear-down.

One thing I did notice. The intake ports are fairly large, the valves look big (1.94???) and it seems to have respectable valve lift. Should be a nice performing engine. Worthy of forged pistons and a good rebalancing of the internals.

There was a V-12 on Youtube a while back. Probably is still there, I haven't checked in a while.

It sounded good.

There are Two Videos from the same guy, he bought two GMC L7000 fire trucks, one was not much beyond the cab, so he cut it off just behind the trans and ketp the cab end of the truck for spare parts for the other one, which is the whtie convetable.

anyway the are ~~~> RIGHT HERE <~~~

enjoy.

Be careful with rebuild parts- - - - -The big V-6's, and, I'm sure, the V-12's had balance issues. I worked with an industrial engine machine shop some time back that did a lot of work on the old big 6's, and a few V-12's for customers who were trying to keep them in service. It was important to keep the rotating assembly for a particular engine pretty much intact, as they were extremely sensitive to balance. If you bore it and install different pistons, you might create some vibration problems unless you match piston weights exactly.

If the pistons are serviceable after a cleanup, it might be wiser to sleeve a pitted cylinder and go back with the original standard-bore pistons, instead of boring the cylinders oversize and replacing all the pistons.
Jerry

Try Egge Machine co on google

The King of Obsolete has one of these V12s also, I think the plan was to try and put it in a dozer. You can check it out on his site.
www.kingofobsolete.ca He's got an interesting lifestyle and has some neat old trucks as well.

Grigg,

you could build one of these around it:

http://www.blastolene.com/B702.html



-W

It looks to me like the old fire truck took off down the street rather quickly. Even 2nd gear looked impressive. I've driven some old C50/C60 trucks, and they were real slugs.
Too bad the fan made more noise than the engine.

I've seen the Blsato's site. I hope the car looks better than the drawing.

The old Dozer is cool. Good for him that he's putting it to actual use.

Twin 6 .. That Frankentruck sounds just like the fire truck in the video.

I'm headed out for Oregon in the morning, I have updated the " Powered by GMC Big Block V6/V12 Engines " page of the site, to include the new pics of the finished B-702 that Randy Grubb built. This is a beautiful car and it will be a must see item at the "Petersen Automotive Museum", I'm hoping Randy will hold on to the car tell I can see it in Grants Pass, before he sends it to LA to be on display at the Petersen.

Yanno, I'm about sick to death of seeing same 'ol lame-o duece highboys at shows. Setting that beastly on a modified truck frame with a duece body on it would definately be a step away from the "typical hotrod".
Then again, mounting it to a trailer and dragging it to car shows would be fun too. Couple sets of headers with short glasspacks, open 'er up & it would sound like a day at the races.

Hey Grigg,
John Milliman, Sherry (my wife) and I met a guy at the Kansas City truck show last week end that has quite a collection of trucks, tractors, crawlers and equipment with a few cars thrown in. Several mid 60's era trucks, one of which was at the show although it was 409 powered. He did say he had a pristine tractor with a sweet running unrestored 702 "twin-6" that runs like a swiss watch. He has invited us as Stovebolters back next year for a special tour of his facility (his son does his restoration work) so I plan to keep in touch with him until then. I may even have to go down there before hand if I can't control myself, LOL. Keep KC in the back of your head for next year, it's gonna be quite a show,
Scott

I finally got the V12 situated on a trailer.
I started with a military 1.5 ton trailer and removed the bed. I am looking for some 19.5" 6 lug bud wheels and some low profile tires for the trailer. As it sits now it's really high, partly on purpose, so I can remove and install the oil pan.

[img]http://thumb9.webshots.net/t/57/757/6/75/43/2223675430080251109aJqKCr_th.jpg[/img]

I have placed the heads and valve covers back on the engine, and strapped the oil pan back in place. Now it's on the back burner for a while while I do some other things. I'll walk by and soak it with PB Blaster every now and then.

In the mean time I'll be searching for some oversize pistons at a reasonable price, as well as a few other parts.

When I have time again I'll try to unstick it, reassemble and get it running. In time I'll probably give it a complete rebuild and detail it as if new and original in a truck.
One day I may have a project for it, but an ice cream maker power unit is high on the list... I do like ice cream....

Grigg

If anyone wants one of these, there is a 63 GMC firetruck listed in Kenny's Klunkers in the current issue of Old Cars Weekly. It is priced at $950. Phone #585-728-2830. It says it has a 5 speed. Joe

It is a V12. Joe

I just spent three hours following this thread and the links. Fabulous.

Does anyone else remember an article in Hot Rod magazine, many years ago, about a man that put a Liberty V-12 in a custom frame with a FIAT Topolino body behind it? No hood, naturally. Those big engines are too pretty to hide.

hey grigg,

i am only 35 minutes over the mountain from you. where did you find a V12 around here?
do you know where i could find one?
i saw the one posted thats in new york, but how to get it to virginia would be the trick.

Hi Grigg,
Is the crank all one piece or is it joined. Do you know if is iron or steel, there has got to be quite a bit of torsional stress going on in it when under load, seems like it would twist if it wasn't steel. Quite a project you have going, good luck!
Rick

The crank is one piece, forged steel.
It looks really strong, it has huge journals, the same bearings are used for all the V6 engines, from 305 to 478, and the V12, and I believe the 637 v8 (it's also in the v6 family, as it is 60 degrees and essentially a 478 with two more cylinders and a balance shaft).

Again, the most info on the net about the V12's can be found here
http://www.6066gmcguy.org/TwinSix.htm

Grigg

Just to update, the fire truck listed for sale a few post above was sold, I just called and inquired.

Grigg

yup also just called, sold to a guy in the midwest they said.

guess i will have to keep looking

i've been following this post and just haven't had time to type in a bit and i have to say, Grigg you ceise to amaze me with all the projects that you have going on looking at some of the pictures that you have would have put me into a world of stress cause if it was me pulling those heads off i would have gone alot further then what would have been needed.... can't wait to see the finished project
Nate