gwhammy

Well, I took the head to the machine shop to get it o-ringed. Probably a good thing as the head was worped .008 in the middle. It took .013 to clean it up with the pits around the gasket seal. The big question is does it need a thicker gasket now with that much off the head or will a standard gasket be alright. They did a valve job to which moved the valves a little deeper. It's amazing the old gasket was holding with the timing and boost I was throwing at it. Thanks, Gary

shoe

My book says for the cylinder head flatness
end-to-end max.012
side-to-side max .003
I would would also check the bolt free length for no more than 5.20 inch if you are not replaceing them.
and to answere your question I would make sure the proper thickness head gasket is being used for the amount the head has been milled your machine shop should be able to get you the right size, also when measuring the protrusion,the head gasket is not in place,so any increase in head gasket thickness over the stander head gasket must be subtracted from the injector protrusion measurement. min .150 max .171 this is very important!

-Rob

gwhammy

The machine shop book showed valve depth and mine was still in spec with the head cut. I hadn't thought about the injector protrusion,I was running thick coppers but was going to go to thin copper washers. This might be the wrong thing to do. I am running a decent amount of timing so which way would be the best way to go on the washers? I think I'm going to try the standard head gasket with the o-rings this time. I am running studs so bolt length shouldn't be a factor. Thanks, Gary

shoe

Which ever washer you go with just install the injectors then check the protrusion foro no more than .171 then if you use stock thickness gasket you will be fine. head studs are a good idea sounds like you got a handle on it.what other mods you have done? what boost levels you running?

-Rob

gwhammy

I'm running up to 48 lb. of boost at times but I usually hold it down to 35 with peddle control. I'm hoping to get the twins together on the bench soon and then put some fuel to it. You can see my other mods under my 97 description. It don't run bad now but it needs more air. Thanks, Gary

Fueling around

With the boost levels you run and the advanced timing, I would use the 0.020" over gasket.

gwhammy

Doesn't that lower the compression some and make them smoke worse when cold. I'll probably use the stock gasket I bought, I really need to get my work truck back together. With a flat head and o-rings and studs it ought to hold. I just abuse it when I have to!

Fueling around

Yes to all the concerns. But a stock gasket is going to increase CR more (-0.013) than a 0.020 gasket (+0.007). Missouri is not what I consider a cold place.
Go for the middle and use a 0.010" over gasket. All are common stock at Cummins.

Mine gasket let go without much boost and higher timing, but not extreme. Milled the head about 0.005 and used stock gasket with studs. Added 60 lb valve springs being the head was already apart. 4k GSK go in this weekend (already have 3k).

-John

gwhammy

But the only compression you gain is from the valve pockets getting smaller, the rest of the head is flat and won't change the compression no matter how much you cut off of the head. The gain from the head cut can't be over a cc per chamber. I bet you can't hardly measure the compression change but I could be wrong. With a valve job the valves get sunk a little in the head too. Does anyone know the compression change per .001 of head shave?

GO 4LO

I think your stock compression ratio is 17.5:1 on a '97, right? -.013 would leave you with a compression ratio of 18.39:1, while +.007 would leave you with a compression ratio of 17.00:1. I'd run the thicker gasket, personally.
Chris

gwhammy

The head is flat! Where are you coming up with the cc's to raise the compression that much? I just can't see the compression raising that much with the cut in the cups of valve pockets. I guess I'm dumb but I go back to it being a flat head and when you cut it, it's still flat! When you make the gasket thicker or thinner it does raise and lower the volume of the the total cc's in the chamber. Compression is the swept cc's divided by compressed cc's, right. The compressed cc's is the volume of the dish in the piston plus the volume of the gasket plus the volume of the valve plus the depth of the piston plus or minus depending if the piston is sunk or sticks out of the hole. Show me where it changes the compression that much and I will agree. I'm not doubting you're word I just can't see the change except in the valve pockets in the head. Please convince me one way or the other before I put this thing together. Thanks Gary

GO 4LO

My calcs are for a flat head with a gasket either -.013 or +.007 compared to a stock gasket. It's easiest for me to think about it as one big cylinder with some certain % being the swept volume and then deal only with the height numbers. Our height is 4.72" for the swept height of the cylinder, so with a compression ratio of 17.5:1, that means our unswept cylinder height is 0.2697", right? Now add .007 to .2697 and you get .2777. 4.72/.2777 = 17.06:1 (I made a mistake in my post above - that 17.00:1 is for +.008 - sorry.) Next subtract .013 from .2697 to get .2567. 4.72/.2567 gives you 18.39:1 compression ratio.
Chris

gwhammy

You are right if I change the thickness of the gasket but I'm not the gasket is the same thickness so the compression is the same. If we change the gasket to one .013 thinner or .oo7 thicker you are exactly right. The thickness of the head does not change anything if it is flat. I really think we are both right in this instance. It will still change with the difference in the valve reliefs in the head but not much. I was hoping someone on here had cc'ed a head to see how much volume was in the valve pockets and how much they lost per thousands of cut. I might be wrong with my numbers but I don't think so. Thanks Gary

Haulin_in_Dixie

Bet the valves lost more than the head cut in material. It's still stock. As was said, a flat head is a flat head.