Oh boy! We have seen and heard it all!

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 rpm's.
• ThunderV12 moved the torque curve from idle to the 2,000 to 3,000 rpm band. The HP figure jumped to 425hp at 4,200 rpm.
• HP is money. The ThunderV12 can make substantialy more power. How fast $ do you want to go?
• 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 bellhousing 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, magnafluxes for cracks, and balances the rotating assemblies to +- 5 grams.
• ThunderV12's have made dozens of dyno 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 crank he has found, but I have over 1,000 crankshaft data base 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 un-equivically 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 first overhaul. 

Not many gasolene 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 actually have potential customers give 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 ran continuously and checked once a day for 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 sroke. This sets up a series of crankshaft vibrations that are unnatural and damaging. At 2,400 rpm, the piston has pounded the valve over 3 million times before the maintenace 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 ditributor 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 ablaited. Simply gone. This is hard on any crank.

In our tech section we recommend all cranks be checked for cracks and straightness during over haul. 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 NoWay!

The GMC V12 is a slow revving industrial engine and not a very good HotRod 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 make to the stock 702 change it from a slow revving industrial engine to a modern free revving hi power Hot Rod engine. Big time "False" for a ThunderV12! 

And for the final Myth.....

Drum roll please !

ThunderV12 llc is good at web site design.

FALSE!

We are engine builders. This web stuff  is not our bag!

Suggestions accepted and appreciated.