The Pitfalls of Restoring Your Own 702
Re-Birth of the GMC 702 ci V12 Engine
So ... You want to rebuild a GMC 702 engine.
First off, congratulations on the rare find!
Now for the reality check.
A vast majority of these engines that are "found" have set outside and had bores that are ruined from rust.
The distributors were usually yanked to use in another engine and rain has ruined the exposed cam. The distributor has 4 gears in it that are not available.
Or worse yet, water was left in the engine and it froze causing the block to bust. Many of the stationary engines were used seasonally and were drained at the end of the season. In 50 years, it only takes one time to forget to drain the water and that guy has a 702 he wants to sell you cheap. Another failure mode from freezing is a small crack in the head that allows water to enter the cylinder while sitting. the next time the engine is started the piston hits the water (which is not compressible) and bends the rods and cracks the main bearing webs. From all outward appearances, this engine could be unscrupulously sold as a good engine.
Another pitfall is the used fire truck engine advertised as low miles. Pumper trucks don't age with miles but rather hours. A pump truck with 50,000 miles is NOT low miles! If a typical run is a mile or two from the station and then the pumper truck runs at full load overnight providing pressure for the other trucks, you can see how the truck miles and engine miles are vastly different. With any gas engine but the GMC V12, a 50,000-mile fire truck can well be on its second or third engine.
The list of parts no longer made for this engine are staggering. When trying to get parts, there were 2 different blocks made and some parts don't interchange between the two. Also, there are two different cylinder heads, oil pumps, filter housings, bell housings, and flywheels. Three different starters were used by GMC and they don't all interchange.
And don't believe the online seller that tells you the water pump they have for a V6 also fits the V12. It doesn't.
Was your engine missing any parts? Did it come with the water pump, flywheel, starter, distributor, or manifolds? I hope so as these parts are no longer available. Rod and main bearings have been discontinued by all three companies.
Are the bores good? Are the pistons and skirts good? Are they within weight tolerance? Hope so. Oversize pistons haven't been made for over 25 years. Are all the lifters good? Yup, hard to find. They are only made in small batches and fetch $33 per lifter now! Valves? Main and cam bearings? You got it. No longer made. And as for the distributor.... don’t ask.
Have I mentioned the v12 special tooling yet? All seven cam bearings are different sizes, and most of the installation tooling is custom-built.
ThunderV12, LLC has jumped these hurdles to bring these engines to market.
Here is the formula to build your V12
Buy three core engines at $4,000-$10,000 ea. (per eBay listings).
Tear all three down and pick the best block, crank, and heads.
Perform $5,000 machine work on the block, crank, cam, (4) heads, rods.
Buy 12 custom tall CH pistons ($4,000+)
Custom make all the unavailable parts. ($8,000+)
Spend hundreds of hours at $100 per hour shop rates.
There, you did it! And only $30,000-48,000!
All we can say is, if you plan on restoring a 702 V12, or buy a "rebuilt" 702, check its condition before you buy and be careful out there.