GMC 702 V12 Engine

[litningrod74]

I don't suppose you have a good source for parts you can point me to with my own 478 v6?

[litningrod74]

@litningrod74 Oops, I'm retarded, forgot I had already asked :P

[litningrod74]

Beautiful engine! I need a line on parts for my 67 478 v6 if you can help any!

[DARTHNECRION]

One of the manliest engines ever built. God bless America!

[douro20]

Didn't EMD build a narrow-gauge yard switcher with two of these in it?

[boaterbil]

Looks like it has 348 or 409 valve covers on it... funny

[castirondude]

I have heard a lot of good things about these engines, except of course fuel consumption as with any big gas motor. They have dual distributors!

[gokartmaster123]

That makes my dads 440 look like a 289 :S

[bradarel22]

Can you fit this in a GMC Sierra pickup truck?

[oldcarkook]

Sure, not problem as long as you have a crew cab and drive from the back seat!

[Makemyday0126]

@oldcarkook you could always mount it In the bed and make it a mid enginer...some engine!!!

[mrspenn1611]

@bradarel22 I am sure if you try hard enough you will shoe horn the damn thing in

[RomeoReject23]

@bradarel22 Never in the front. If you have a Super/King cab and long box, you could hypothetically run it longitudenally in the back though (Firewall out the rear seats and let the engine take up the area).

[TheAzboy1987]

i have one of these sitting in my yard and i want to rebuild it but don't know ware to begin or sell it if some wants it more

[ryansiki]

Thats no car engine dumbass.

[TheCottonTop]

What with these old farts that have motors on trailers? Airplain motors I can understand. But car motors should still be in a car!

[oldcarkook]

Many of these motors were stripped out of the trucks they came with decades ago and put into service pushing irrigation pumps among other things. Because this is such a rare American designed and made V12 engine, they tend to have survivability in most wrecking yards. There are still quite a few of these around from Texas up through Utah serving as irrigation pump motors. When you find one, you work with what you have.

[fordhastalamuerte]

que bestiaaaa a pesar de ser gmc me encantaaa quiero unoooo

[supershy13079]

I've only seen one of those up close in my life.  It was in a scrap yard with the truck still intact not including the rotted flatbed. I didnt have the money to buy it but I always wanted to hear one run. Thanks for posting!

[MegaZsolti]

uh what's the engine's maximum rpm?

[Alisterwolf66]

2400rpm, governed. They'll safely handle 3,200rpm, and there's a company modifying them for up to 5k.

[popplewort333]

Wouldn't the sparkplug location, on the intake side pose a fire risk?

[AlchemistHawk]

me want..... @_@

[richardman1992]

are there any relations between these gmc v12 and v6 engines because the valve covers look a hell of a lot like the 348/409's

[oldcarkook]

Richard: They were setup to use the 351 "big block" V6 heads and the V12 block is essentially a doubled 351 V6 block casting.

[kennethrobinson123]

If it had only one exhaust manflow per. side it would sound totality deference sound

[kennethrobinson123]

The man your looking at told me when he bought the truck from me that he drove a road tractor for his father when he was a boy. he said it could run with a 250 cumm. all day long but the cumm. was getting 5 to 6 mpg and he was getting 3 or 4. he said that it would pull with a 250 cumm. up hill. V6 GMC were 305-351-401-478-V12-702 cu. in. thay did make a V8. of this style only seen one in a fire truck. it was on ebay not long ago.

[kennethrobinson123]

HP. on this engine was 275 the governors are operated by oil pressure. you could change the springs and turn3500 rpm. but more than that would brake the crankshaft because the crankshaft was so long it would flex. but @3500. will get you 400 HP. big bore short stroke. that engine would used 20 gal. gas a hour pumping 1000 gal.a min. of water. they had one man running getting gas at fires but it could run all night wide open and never get hot. Ign. timing had to be right or bust a piston.

[kennethrobinson123]

I sold this man the truck the engine you are looking at. 7000 GMC 1961. 9000 front 23000 rear 2 speed axle 5 speed tran. x fire truck. 30000 miles. came from boston. sold to camp dick fire dept. garrard county KY. in about 1980. top rpm. 2400. 58 mph up hill or down. I am a mechanic 62 years old worked on this truck for years. 702 cu. in. is two 351`s but one block one crank one cam 4 heads 2 manflows 1 dist. in rear with 2 dist. out the back of it. u time front 6 with one and back 6 with other.

[ramblinmantoo]

do a search on thunder V12. They turn them over 4000rpm. The crank is factory forged and has plenty of support from seven main bearings combined with a very short for its size 3.58" stroke makes the crankshaft very reiable.

[DRNEGOLICIS]

I would expect it not to sound like a v6 being its two in one block but it does indeed sound like a big block v6 same firing order i think have to look at my chart i think it was the same

[SaGalv]

i think the seperate manifolds make it sound that way...its bascially have two v6 cars next to each other, it doesnt sound like a v12, just a like a louder v6

[600ThumperRider]

The one I have dispalyed in my shop makes for a good conversation piece.

"Is that based on the 409?".

"Are those 2 blocks bolted together?".

The usual comments.....Cool old GM history. They were produced by the train loads for the agricultural industry. Mine has the P.T.O. to connect to an irrigation pump.

Glad to see one running....cool old stuff.

Thanks for showing it off.

[torchmann4801]

they were fuel pigs from overlap.fuel passed right out the exhaust but that is how you fill the cylinder with a 2 barrel carb at high rpm's.the low compression helped avoid pinging on the old pre leaded low octane gas when you did lug it and at high rpm's it gave you more chamber to fill with boom boom. the pressure worked on the crank longer and harder over more degrees of rotation because as the piston moves the %pressure drop/piston swept area is less in low comp than with high.

[torchmann4801]

I could lug my 305 but i found out you could really wind it up and she would fly. there was slack between where the torque leveled out and the hp wound in. I didnt care if I blew it up so I wound her up and she sung through those dual glasspacks so sweet at 6,000rpm. you HAVE to run good points with enough spring pressure and have it timed right with enough advance. low compression and higher than car overlap on a solid cam with a 2 barrel isn't tuned for lugging

[torchmann4801]

No, not based on the 409. I had mine apart. it had big flattop pistons with 4 rings. the sparkplugs were on the intake side of the head. The heads are very different from the diesel heads. i think the plugs on top was so you could access them reaching down into the engine bay on a big truck. someone mentioned on the hamb that they were low revers. i have to say my 305 v6 would wind up like a 327 chevy and that old gmc would get over a hundred like it had a hot 350 in it.

[hmburner]

I have one of these engines for sale complete with tranny call Bill at 905 834 3027

[oldcarkook]

This engine is based on the GMC 351 V6 where the heads, manifolds, other components were used on a 'doubled' 351 block which gave GMC the 702 cubic inches for the V12. So it's basically two 351's put together.

[bulletman100]

is this engine based on the 409 design? valve covers look like the old Chevy 409. ?

[enginecontrol32]

I want that in my life

[DRNEGOLICIS]

me to its one of my items on my bucket list to build or restore a truck and put this engine in it

[WARD5KUSTOMZ]

looks like its assembled like the 12v detroit diesel

[DRNEGOLICIS]

i did here that there was a conversion for this engine to diesel but it was a fuel hog

[Joshie225]

You're thinking of the Toro Flow 4-stroke based on the same GMC V6. It was a factory job.

[DRNEGOLICIS]

not quite but yes i know about the toro flow seen a few weird sounding engine it is but ive herd of a conversion for the 702 v12 from gas to diesel don't know if it was successful in reliability probably lots of problems like them old 5.7L wrist pins falling out and stuff thats why u don't here about them they didn't make it

[jetmir82]

American Engeneering... Bad Thing...

[spencnaz]

Learn how to spell 'engineering' first before you criticize 'American Engineering'. The rest of the world has had its full share of engineering duds too.

[propdr]

Shows what you know. The United States military used that engine to power the primemover for the LGM-30 Minuteman nuclear missiles. They had to be reliable..

[DRNEGOLICIS]

not many people know that lol i thought i was the only one lol

[Jcashrulez]

just becuase its german dosent mean its good. are bmw has many a problem while are old tank of a blazer dosent.

[25tab]

that sounds like no v12 i've ever heard... I love it.

[USA6x6]

would you consider selling it?

[diamantegerardo]

Awesome engine!