Building a GMC 702 V-12
From Crankshaft Coalition Wiki
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== GMC 702 == | == GMC 702 == | ||
| − | This is perhaps on the obscure side of hotrodding, but nevertheless the 1960-1965 GMC V-12 702 cubic inch motor affords the | + | This is perhaps on the obscure side of hotrodding, but nevertheless the 1960-1965 GMC V-12 702 cubic inch motor affords the purist 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. | 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. | ||
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| − | == Prep | + | == 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 the completed short block in the chassis to complete. | 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. | 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, part 1 == | |
| − | == 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. | 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, part 2 == | |
| − | == 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. | 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. | ||
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| − | 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. | + | 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. | 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. | ||
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[[File:702 Short Blockr.jpg|border|400px]] [[File:halfr.jpg]] | [[File:702 Short Blockr.jpg|border|400px]] [[File:halfr.jpg]] | ||
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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. | 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 | + | 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 1960s, 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. |
[[File:Mpz.jpg|border|400px]] | [[File:Mpz.jpg|border|400px]] | ||
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== Heads, or rather head ''ache'' == | == Heads, or rather head ''ache'' == | ||
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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. | 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. | 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. | ||
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| + | == OK Time for the "ache" == | ||
| + | I am back and have a bunch to share; '''FIRST (PAY ATTENTION), though all the BB v-6 heads will work ONLY the V-12 intake manifolds work'''. As I don't have a V-6 apart to measure the bank to bank spacing, I can not explain why but the V-6 has a greater left and right bank offset that the V-12. This results in the V-6 intake manifolds having a different port and bolt spacing between the odd and even bank sides when compared to the V-12. It means that you can FORGET using magnum heads unless you are going to fab your own intake. So, I will walk you through this in a series of pictures and document what the differences are as well just in case any V-6 builders wander onto this and want to swap to magnum heads. | ||
[[Category:Engine]] | [[Category:Engine]] | ||
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