GMC V12 F.A.Q.’s

GMC Built the 702 from 1960 to 1965 as a gas V12.
The engine is a one-piece V12 with four heads that were also used on the GMC 351 V6.
The block is one piece, not two v6’s joined together.
Two different block castings were made. Some parts do not interchange between the two.
The cam and crank are 4 feet long.
The crank is a 180# forged steel.
The engines were produced for tractor-trailer semi-truck motors.
The 702 was never affiliated with the GM diesel line of engines. The 702 was always a gas engine.
The Factory V12 was rated at 630 Lb-Ft torque and 275 HP and was governed at 2,400 RPMs.
The engines were rare in 1965 and are quite scarce today Less than 5,000 were ever produced.
Virtually no parts are made for the 702. ThunderV12, LLC has had to custom make or contract the parts it needs.
The stock bell housing pattern is unique to GMC and not shared by Chevy. ThunderV12, LLC has made GMC to Chevy bell adapters.
The V12 does not use the 305 rods.
ThunderV12, LLC checks all cranks for straightness, magma fluxes for cracks, and balances the rotating assemblies to +- 5 grams.
Although we recommend the continual use of a ThunderV12 to remain under 4,000 rpm, ThunderV12’s have made dozens of dynos pulls in the 4,000-rpm area with max pulls to 5,000 rpm.

702 V12 INTERNET MYTH’s

It had a long flimsy crankshaft that breaks easily.

“True and False”.
True – It has a long crank!
But these cranks do not have a design flaw that causes them to break. Cracks are not uncommon as is the case with any high time industrial crank, but break from being flimsy? It just isn’t the failure mode of this engine. These engines are durable beyond belief. We know of one engine with 200,000 hours (9 million miles) on it in western Kansas.

This “flimsy” internet opinion was started with a single internet thread post. Maybe that internet poster has broken every 702 cranks he has found, but I have over 1,000 crankshaft database that has yet to show one crank in two pieces. All my contacts that have maintained these engines over the last 50 years and all the fleet operators I know that ran these trucks have all stated unequivocally that the GMC V12 crank is not flawed in any way. Overused and abused. Yes.

These cranks are short stroke (3.58″) 180# forged units with tons of overlap and (7) 3.124″ main journals. The fact is these engines routinely went 1,000,000 miles before the first overhaul.

Not many gasoline engines from 1960 can pull all day at full throttle for 1 million miles and never pull a valve cover. The term “flimsy” is unbackable here. Especially in the context of “I’d like to put a GMC 702 in my Packard, but I hear the cranks are too flimsy.” I have potential customers giving me this. If these engines can pull 40,000 pounds for a million miles, it will be fine in your Packard.

With that said, we do find bad cranks, much as you would expect with any engine used industrially for 50 years. The typical failure mode we have seen for the 702 is in a stationary industrial application where the engine is running continuously and checked once a day for the operation. It goes something like this. A cylinder breaks a valve and drops the valve head into the cylinder. The engine runs well on 11 cylinders and continues to run with the piston pounding the broken valve head on every stroke. This sets up a series of crankshaft vibrations that are unnatural and damaging. At 2,400 pm, the piston has pounded the valve over 3 million times before the maintenance man comes by for the daily rounds and hears the rattle. Typically, the engine is quickly rebuilt, without checking the crank and put back into service.

This engine will mysteriously fail 5, 10, or 15 years later. The failure mode is a crack that allows the crank to exceed its runout limits and ruins the main bearings. This is long before the crank has been cracked enough to break. All the cracked 702 cranks we have were in running engines. One crack was 8″ long and the engine was still pulling at full throttle all day long. Another way the cranks were damaged was from a faulty distributor that allowed one cylinder to run in detonation. We have some pistons that were run in detonation so long that over 1/8″ of the top of the piston is ablated. Simply gone. This is hard on any crank.

In our tech section, we recommend all cranks be checked for cracks and straightness during overhaul. ThunderV12 checks every crankshaft it uses for cracks and straightness. To keep the good reputation these engines, deserve, if you are rebuilding one, you should too.

Used and Abuse Yes…Faulty design No Way!

The GMC V12 is a slow revving industrial engine and not a very good Hot-rod engine.

True and False.

Boy, I hear this one a lot.
First off, in stock form, the 702 is a slow revving industrial engine. So, “True” on that part. But even in stock form, the idea that it is unsuited to a hot rod is simply sour grapes. I wonder sometimes if “Hot Rodding” is confused with “Racing”. The stock engine would not be an easy race engine. But Hot Rodding is about being different.

How can a 702 V12 fail that definition?!!! The changes that ThunderV12, llc makes to the stock 702 changes it from a slow revving industrial engine to a modern free-revving hi-power Hot Rod engine. Big time “False” for a ThunderV12!