# Thin Head Gaskets For 1975 400 engine



## beertracker (Oct 1, 2009)

I have a 1975 400 engine going into a LeMans. I am using 6x-4 heads (93cc) which have been milled .040" and have a valve job. My question is what are the available head gasket thicknesses that I can use? I have found a .028" thick head gasket and are there any thinner ones out there? I would like to get 9:1 compression ratio if possible. I am using stock flat pistons and a SUM2802 cam.

Thanks,
BT :seeya:


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## geeteeohguy (Feb 2, 2008)

I think a head gasket thinner than .030 like the one you found is about as good as it's going to get. .028 is pretty thin! You could have the heads milled another .020". It's early yet, I'm still foggy, but I think you lose 1cc for every .005" milled off. I have 87cc heads on my '67 400 (bored .030 for 406cid), and I am barely at 9:1. I can still run regular fuel. There are several good compression ratio calculators on line. If you can, you want to run around 82-85cc, I would think. maybe a tad less. Stock hi compression Pontiacs have a cranking compression of around 200 psi, which is not doable on today's fuel. Optimum compression these days is about 170. 175psi is on the edge, and 150 is as low as you want to go for good power. Good luck. You are diefinately on the right track and doing the right things!!


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## beertracker (Oct 1, 2009)

geeteeohguy:

I ran the numbers using a compression calculator and it comes out to 9.6:1 for my 400 using a .028" thickness head gasket and milling .040" off the heads. My shop guy told me using these numbers I would end up at 8.5 or so to 1. Those numbers are quite a bit different. I don't who to believe the shop guy or the CR calculator. The shop only recommended taking .040" off otherwise they would have to mill the intake manifold so it would fit to the head. Is there a way to measure actual CR?

I am not taking into consideration the valve job which adds volume to the chamber. Per Jim Hand's book a valve job adds 2-3cc per chamber. This seems like a lot? I am wondering if its accurate?

BT


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## geeteeohguy (Feb 2, 2008)

BT: I've found the Jim Hand information to be pretty much gospel. Your machinist is corect about the intake needing to be machined, but I think it's only if you mill the heads .080" or more. Could be wrong. Mill too much, and pushrod length becomes an issue, too. What I found out personally was that with my 400 inch motor, bored .030, I needed an 87cc head to get about 9.2 to one or so. That's what I did, and I am able to run 87 octane fuel still. (I had 114cc heads on it earlier--7:1) . If I were at 9.2 to one, I would think it would ping on 87, and that I would need at least 89 octane. Keep in mind, I have 87cc chambers, not 93cc like you have. I have run 72cc chamber heads on this same motor, and they were not streetable on pump fuel (at the time, it was 94 octane leaded).


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## beertracker (Oct 1, 2009)

geeteeohguy:

Thanks for the info and that's data good to know. I wanted to ask if your compression ratio number is based on a calculator or your machine shop guy?

One last point I used .015" for deck height in the compression ratio calculator. This number came from Jim Hand's book. Just so I understand is deck height the distance the piston at TDC is from the top of the block?

BT


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## geeteeohguy (Feb 2, 2008)

BT, yes, that's it for the deck height. And it can vary according to who made the pistons, etc. I used the calculator for my compression ratio, an not the machine shop. but I did cc my heads, and verified that they were indeed 87cc in chamber volume. I plugged in all the numbers, and got something like an 8.8 on one site, and 9.3 on another, same values, no changes. I have yet to do a cranking compression test. If I'm at 170pis, I would judge that I'm around 9.5 to 1. If I'm at 145-150psi, I would guess I'm really at 8.8 or 9. Here's the thing: I've driven these particular cars for over 30 years now, with all kinds of engines, compression, etc. A high compression Pontiac just feels different than a low compression one, in the seat-of-the-pants. Pontiacs love high compression, but they also love to run hot and ping. You sould be able to run full advance with your set-up, and on regular fuel, too. As I've said before, the next time I do an engine, it's dished pistons for me, and back to my GOOD 72cc heads. I've experienced dished pistons in two different motors recently, and they just plain fix the problem.


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## Too Many Projects (Nov 15, 2008)

beertracker said:


> geeteeohguy:
> 
> Thanks for the info and that's data good to know. I wanted to ask if your compression ratio number is based on a calculator or your machine shop guy?
> 
> ...


Was the block decked to true it up ? That would make a small difference too. It sounds like the engine isn't assembled yet and you could actually measure the deck height for an accurate input.


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