Editing Building a GMC 702 V-12

== GMC 702 == This is perhaps on the obscure side but never-the-less the 1960-1965 GMC V-12 702 cubic inch motor affords the purest an opportunity to live the glory days of hotrodding. With ZERO aftermarket items for this 1800 lb behemoth that measures roughly 41 inches from the front timing cover motor mount to the engine flange on the rear of the block, you will need to apply your creativity to the chassis and metal design aside from how you are going to make this unique motor hum. I will continue to add as my progress continues. This engine is going into a 1926 International Speed Truck. The truck was chosen, because I had it for one,because it is relatively simple to move the cab back and radiator forward. [[File:V12sr.jpg]] V-12s in the "wild". The hunt is part of the thrill. Anyway, as it is a truck beginning I am staying with the dual disk set up for clutching and upgrading to the steel Lipe Rollway clutch for better durability. I will initially stay with a Clark or Fuller transmission of significant heft. The plan is to push 5000 rpm with 600 hp across 3000 RPM range. I will also run NoS as direct inject to, by design hit 1000 hp of very streetable power. [http://www.thunderv12.com]Is a great source of parts for these and if you want you can even get the whole thing. This WIKI is going to assume that the reader can handle the basic engine rebuild as far as pistons, rings, bearings and valve job. What I address here is the home builder issues that need to be addressed to get to my performance goals. Covered are the cam, lifters, oil pump, running clearances, magnum vs std parts and tidbits I have found along the way. == Prep Notes== Starting with the obvious.... YOU DO NOT HAVE A STAND THAT WILL HOLD THIS! It will take two engine stands, one on each end to do the initial build. Do not think the Harbor Freight 2 ton stand will hold this 1 ton motor. It will break the weld on the tube that the mounting plate is attached to. Buy two stands and build up your short block. I then mounted that in the chassis to complete. These cranks have four bolt mains and have the old nitride case hardened shaft. Remember they are an industrial motor and I would not be so concerned in going to an undersize as you will never get the hours on this to make the difference for a case hardened crankshaft. As always balancing is very important if you are going to be over-sizing the pistons. These are very low compression and I think the lp gas motors had a 9:1 while the gas motor had a 7.5:1 compression ration. You can of course get some custom pistons made to improve weight and I think I even read where someone turned the crank to run lighter weight V8 rods in their 478 V-6 GMC. My goal is frugal all the way on this one so most of what I do will be stock or adapting existing and readily available items. == Pay Attention 1 == So, you have figured out how to handle this anchor and have the bores cleaned up with new rings and the rods and mains are going together. These motors runs VERY TIGHT side clearances on the rod journals. Be sure to open them up to at least 0.007". If you are running at 0.003 and need to take .004 off well then that may necessitate the assembly being balanced so be sure to check this measurement when you are tearing down if possible. It wasn't possible for me to as the engine was stuck. == Pay Attention 2 == OK, Here is a problem the I was keyed into by Bob at Thunder V-12. The stock oil pump drops pressure around 3500 rpm. Now you can get a trick dry-sump set-up for a good bit of cash and work the plumbing issues OR here is the mod you will need to craft. I spent 6 months talking to the external dry sump manufacturers and also Titan who makes "beefed up" gerotor pumps for the nitro drag race crowd. The gerotor oil pump design has been around for a long time. Make a long story very short here, the 702 has the biggest gerotor of them all. The calculations put it at nearly 50 gallons per minute running at a crankshaft speed of 5000 rpm! It measures 1.37 inches in height compared to 1.10 inches that are run in nitro funny cars. So what gives? Cavitation! The suspended volitales and gases will cause a destructive cavitation issue under vacuum. Once your pumping cavity flashes with a vapor it looses the pressure on the compressing chamber (they are not actually chambers but "meshing" of the rotor and ring) and you have no oil going out to lubricate the motor. The design of the GMC gerotor used in these motors (both the V6 and V12) have just one surface of the gerotor fed. In this case it is the top and the bottom is left to take its fill from what is sucked from the top entry point. The owner of Titan Speed was very helpful in diagnosing the problem. I derived a modification to the standard pump to dual-feed it. As a note here the V6 gerotor is a 1.1 and the V12 is 1.37. Make sure you get a 1.37 if replacing the pump. I have complete pict and will edit this WIKI to post this important mod in a few....... Your larger gerotor oil pump may also have the hydraulic governor as part of it. If it does it is a stand-off chamber below the main pump that is fed oil thru the center of the shaft. You will see a spinner that open a valve in the hollow connecting shaft inbetween the chamber and the main pump body. The good news is that you shall remove all of this and toss it. You can not easily perform the mods to this bottom gerotor plate as it is a casting of sorts. Which I had a pic but that got tossed. So if you have this governor BS thingy then unbolt it and get a plate from another pump. You can also make one as they are only 1/4 inch flat mild steel. Here is what the one you want looks like (Ignore the "peanut cutout" as I will get into that in a minute here) [[File:Plate.jpg]] To help understand what we are trying to do, if you have your pump disassembled and look up into the housing you will see the port for the vacuum side of the pump. Notice its tapering shape and orientation. We seek to create a mate to that on the bottom side and provide relieve in the form of a lubrication pocket on the pressure side opposite the top pump exhaust (where the oil is under pressure and sent to the distribution. This force on the top side of the gerotor causes the bottom of the rotor to wear under, just look at the scoring on your bottom plate and you will see evidence. Here is a custom Titan lower plate (ignore the center bearing as that is intended for support at very high RPMs and I did not seek to replicate that for this project though it would not hurt it would take some doing to get a stub shaft onto the bottom of the gerotor). [[File:bare_titan.jpg]] Notice the shapes are similar to the GMC.

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