Building a GMC 702 V-12
This is perhaps on the obscure side of hotrodding, but nevertheless 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, and because it is relatively simple to move the cab back and radiator forward.
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 in order to, by design, hit 1000 hp of very streetable power.  is a great source of parts for these engines 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 standard parts and tidbits I have found along the way.
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 the completed short block in the chassis to complete.
These cranks have four bolt mains and have a 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 that 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 is the problem? Cavitation! The suspended volatiles 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 designed a modification to the standard pump to dual-feed it. As a note here the V6 gerotor is 1.1" and the V12 is 1.37" tall. Make sure you get a 1.37" if replacing the pump.
I have complete pictures and will edit this Wiki to post this important modification eventually.......
Your larger gerotor oil pump may also have the hydraulic governor as part of it. If it does it has 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 in between the chamber and the main pump body. The good news is that you will 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. I had a photo but it 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). Please note that the next three photos are have been scaled to 400px. To remove the size limit so they can be used as scalable templates, enter the "Edit" window and remove "border" and "400px" from the image line. It now looks like [[File:Plate.jpg|border|400px]], to see it at full scale it should look like [[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, 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 RPM 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).
Notice the shapes are similar to the GMC.
Dual feed pump mod
OK you are almost ready now that you have some knowledge of what it is you seek to accomplish and the necessary parts for starters. There are a couple things to keep in mind; make sure you know the top of the plate (rotor side) from the bottom and where the true center of shaft rotation is. The bolt holes are not symmetric nor is the center of rotation located in the center. The best way I found was to determine the center of the circle based on wear patterns knowing that the inner gear was the shaft. I was able to make a paper template based of the Titan bottom and transfer that onto my steel plate. As you will see, I had a substantial investment into the Titan part so I modified it for use. I believe it is entirely feasible to build a steel port chamber and weld it to the plate. In any even it starts with the plate mod so here we go. One more large photo.
OK now that I have 3 megs of photos on here.... notice the smaller center circle wear. That is what I picked center from. I did not use the milling machine on the here but rather a dremel and a die grinder. The pressure relief side loos rough but its purpose is to trap oil and promote lubrication. What this modified plate now needs is for an oil feed be supplied. Rather than mod the tube feed that exists some how, this mod actually brings another pipe up from the strainer.
So the first picture is one of the plate with the appropriate sized block that would fit the AN fitting port. Again, I had the Titan lower assembly and I chose to modify that verses machining some sort of chamber with fitting port into the steel block. So picture directly above is the basis. I had to modify that and eliminate one of the side ports as well as the bolt pattern; if you look closely at the picture you can see scribe lines. The Chevy pump pattern did not align with the GMC.
The ID of the original pick up tube is roughly 9/16" and keeping with that I would have to say that you should seek to maintain a 1/2" ID on your design. I also incorporated a check valve to avoid having the pump drain after shut down. Not sure if this is necessary but Summit had the parts for it.
As I had a nice AN fitting to attach to, that is what I stuck with to get me to the oil pick-up.
These are the largest AN fittings sold by Summit, I would have to look up the order but I think they are AN-16 (1"). I had to modify the intake side of the check valve to keep it from protruding past the bottom of the strainer.
If you look carefully you can see the original tube behind the bottom feed assembly. This is, along with the screen strainer, nestled snugly into the oil pan. I did have to "custom form" the baffle in the bottom of the pan but not too much.
There you have it... a dual feed gerotor for the V-12. BTW any of the GMC gerotor pumps with the 1.37" tall rotor will have the cavitation problem.
You can see both tubes in this shot. The "white dot" is the bottom of the modified check valve.
Next, a trip thru GM automotive history as I try to get "new" lifters together...
This was sort of fun. I went to all the old fleet repair engine shops, you know the ones that have been there since the 1920's and have the old time engine mechanics who have actually worked on this stuff in the day. I also scoured the internet and parts books. Not to mention I do have the GMC heavy truck maintenance manual. Armed with all that I started with the notion that the Chevy inline six-cylinder passenger car from 1950-'52 with the Powerglide used the same lifters. That is supposedly identified as an HT-761 but the first ones had that hand written on the boxes. Not even worried yet as I needed 24 and only had two, seven months of eBay (these things are expensive too, just looked and saw someone asking $499 buy it now for a six cylinder set) netted me a large collection- in excess of 50. All of them for inline six Chevy motors. Now look at this:
On the left is the one marked for 1950-'52 there are HL-16, HL-18 and HL-31. The one laying in front has no such designation but carries the same part number as the HL-16. The HL-18 and HL-31 are different part numbers.
From a top view you can see the differences in the push rod cup. Note too that the non HL designated lifter has seemingly larger ID bore from the outside looking in. Anyway, the V-12 has solid push rods and there is not any hole in the cup so I focused initially on the HL-18 and the HL-31. Problem is however that I only have 5 of the HL-18s and a dozen or so of the HL-31. I am forced to dissect them to see what I could do.
My plan is to have at least all new bodies and from the above picture you can see that two of the types have grooves and two don't. My research indicated that these grooves are intended to help keep oil in the lifter bore for longer life. I am not going to worry about that, if I get 1000 miles a year on this I will be smiling from ear to ear. Note the large galley hole in the one on the far right it has the same PN as the HL-16 but these are major variations.
I will start by taking the HL-31 apart and comparing it to a used V-12 lifter.
OK well I did get the HL-31 apart but the comparison is for an HL-16 to the V-12 used lifter. The used one is the oily mess and also note that other than the body external groove and push rod cup, a HL-31 and a HL-16 are the same internally.
In the picture the lower set is the used V-12 (by the way the wimpy little clip is removed with just a small screw driver and if the assembly does not easily come apart it will if you rap it against a hard solid surface like a vise or anvil). The GM manual wants you to clean them with kerosene. Pay attention here because this is where the difference is. Notice the ball and retainer in the upper new HL-16 is free floating and in the V-12 there is a little spring that keeps in contact with the plunger body orifice at the bottom center of the plunger. These parts are easily cleaned and put into a new plunger and body. All of these lifters I have been showing have the same external dimensions in case I did not mention that up front. In the example in the above picture I simply used the old push rod cup, ball spring and ball retainer.
Here are the parts I shall put together including a new plunger, ball and body. Oh yes and a new wimpy retainer. I think I may regret not putting some high quality snap rings in place of these. The original motor had plenty of these broken and or well worn.
What is the purpose of that spring? I did find that the HL-18 is the lifter that matches 100% including this spring. I summarize that it to help prevent bleed down of the valve train when sitting for long periods. It would take a while to fully pressurize 24 empty lifters this large. Comments are welcome and I will also get a full PN listing though I tell you that I have three distinct lifters with the same part numbers as the HL-16 and older not HL series are by the book of numbers replaceable. Back to that spring thing, what would happen if one uses the HL-31 without it?
On with the lifter assembly: the "keeper" of this ball and spring is a tight fit on the HL-18 (and originals) to the point of requiring a small screwdriver to drive the "hat" edges into the back side of the plunger piston. I noticed that while disassemble of originals that some were not fully seated and that the other part numbers did not have any means of retention as the "hat" is of smaller diameter. I would guess that not using a spring would make it difficult to insure the ball did remain in proper position relative to the orifice at the plunger bottom. Hence a malfunction during operation as this spring would not interfere with the proper operation of the tappet.
The photo above shows the keeper properly seated (ball in orifice, small spring and keeper) in the plunger bottom. Now we are ready for assembly of the lifters. By this time all parts have been thoroughly cleaned in solvent based parts washer (remember when it used to be cheaper to use gas to wash parts, invest in a commercial parts washer service... it's worth it) and then rinsed in kerosene. That's right, kerosene for reassemble. Oh, actually prior to the setting of the orifice ball it is a good idea to trial fit all the plungers with the mating lifter bodies. DO NOT MIX these up as there are factory matched. Generally if they bind you will need to take some crocus cloth and polish the spot of interference. Remember that if you force them then they will scratch which will make the whole thing worse. Your gonna want that plunger to slide smoothly into the body. While I am on this whole smooth thing, if you bought NOS lifters on evil bay or somewhere else they still need to be disassembled and cleaned. Several I found were rusted from age and moisture. To get them apart I simply removed the top retaining ring, squirted Kroil into the side port hole and taped it vigorously against a heavy vise, pushrod end doing the impacting as the plunger came to the edge of the body.
Next thing that happened, I found out what those springs are for. It is an attempt to prevent bleed down during period of non operation. As this thing is so large and the oil path long, I got an idea that the engineers were trying to prevent premature wear (like poor lubrication of the cam lobes) by holding a good amount of oil in the lifter body. At least it would mean having pressure come up faster if you did not have to pump half a quart of oil into the lifters. Anyway, you can not assemble anything unless you relieve the trapped air under the plunger.
This is how I did it with an awl.
That's a lot of lifters....
Of course there are a lot of holes to fill. As mentioned in the start of this article, I am only going to cover the "odd things" that one needs to pay attention to and not normal assembly. If there is interest, I can post another section or make a new Wiki article on the run of the mill tear down, check, and rebuild.
So here is the short block... Motor City's greatest RPO gas motor of all times.
Note the difference in design of the lifter galley over a GM V-8. More to come... heads next- and they are just as interesting, right down to the sodium filled valves.
One caution on assembly of flat tappet lifters with a new cam lifter assembly. This Wiki is great because I can share my BS stories so here goes... When I started my HP addiction in the late 1960's, the EPA was worried about rivers catching on fire from the toxic/flammable stuff floating and also trash on the side of the road. They really did not care what was added to the gas or oil that we used. Well we all now live successfully with unleaded gasoline (though this alcohol craze is killing stuff in storage and robbing HP from our street cars) and- unknown by me until last year- no more "heavy metal" anti-friction additives in our oil. Not exactly sure when this happened but somewhere after the last flat tappet 350 chevy I built in 1998 (I had switched to roller cams after that, again HP addiction) and 2009 they pulled the plug. I was putting together a big block 396 with my son and wanted to run a flat tappet for cost and did not really want a HP monster in his car. I assembled using cam assemble lube (Comp Cams camshaft) and added by Castrol 20W-50 oil as I had done so many times before. It was a dual spring matched to the cam and the directions sort of said remove the inner springs on break-in (the 20 minutes between 2000 and 2500 RPM right?) and I thought that was stupid as I had never done that and did not want to spend the extra time on it. So, 20 minutes of running in the garage and we were good to go, at least I thought. Within 15 miles the car started to backfire, I drove it to verify and get a NYS inspection (another BS thing) and within the next 10 miles it would not idle and continued to backfire no matter what I did with the timing. Long story short, by the time I got it back home and pulled a valve cover the heads had up to 1/8 inch of what looked like gray moly past on them. Rockers were all loose and when I pulled the cam EVERY lobe was wiped. The engine had to be completely rebuilt, every bearing, pistons, rebore, crank turned... Expensive lesson. So, get yourself a bottle of MPZ (Magnesium, Phosphorous, Zinc) assembly lube and use liberally on all lifters, cam lobes and lifter bores. The add a break-in additive to your oil.
Heads, or rather head ache
It is said that all the power is developed in the heads and the heads GM put on this engine just do not equate to any performance rule. Small valves, flat runners, large stems, poor angles, and the list goes on. They do have machined combustion chambers so that makes for some consistency vs. standard as-cast surfaces. Of the heads available, I believe that anything from the 305, 351, 401, and 478 V-6 aside from the ones that are V-12, will bolt up BUT there is significant variation in valves and in what was called the magnum heads. Right off the bat, the magnum heads require magnum exhaust and intake manifolds (hence I am now building custom stainless steel headers from custom plasma cut flanges). I will get into that in a bit but my advice for cost and simplicity is to stay with the stock stuff unless you are addicted to HP like me.
The basic V12 heads have large stem diameter valves which are sodium filled. The associated intakes have all the right cylinder numbers on them so you get the cool 1-12 numbering on the rear manifold. I went to magnum because the port sizes are significantly larger and to me that means better flow. According to those in the know, a good ported standard head will flow as well as a magnum. I believe this has a lot to do with the magnum exhaust port design (I have a neighbor who has been porting heads since Harvey Crane opened shop in Orlando and Daytona was still run on sand) and the fact that the runner is flat and almost in the same plane as the exhaust valve. I will take some time here to document what I have collected as I am by no means in a position to recommend what will be best for you other than how to be cheap.