Head Gaskets
Thread Starter
Race Director
Joined: Jun 1999
Posts: 14,924
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From: Merritt Ils Fl
Head Gaskets
Been building small blocks for a lot of years but am stumped,more or less.
It is possible to fine tune compression ratio by the thickness of the head gasket. As a general rule the thicker the gasket the larger the diameter of the piston opening: 4.060,4.080,4.0125, for example. I have .030 overbore; 4.030, what is the maximum safe size for for the piston opening? Can I safely use copper gaskets in a purely street engine if that is the only thickness/opening that will give me the CR I am looking for? If it matters this is a 383 build. The fine tune of the CR is about .5.
It is possible to fine tune compression ratio by the thickness of the head gasket. As a general rule the thicker the gasket the larger the diameter of the piston opening: 4.060,4.080,4.0125, for example. I have .030 overbore; 4.030, what is the maximum safe size for for the piston opening? Can I safely use copper gaskets in a purely street engine if that is the only thickness/opening that will give me the CR I am looking for? If it matters this is a 383 build. The fine tune of the CR is about .5.
Burning Brakes
Joined: Sep 2005
Posts: 993
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From: Iowa
A copper gasket will work on a street car but I would recommend getting a set of self sealing ones. A company called SCE hit the market a few years ago with a self sealing copper gasket that can be had with our without a combustion ring. My personal hot rod has a set of these in it and has zero issues. In the old days we had to put sealant of some sort on the gaskets and we did have long term problems but with this product we haven't had any (the engine I mentioned has been running these for around 3-4 years). Hope this helps.
Burning Brakes
Joined: Oct 2004
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From: fighter pilots make movies, bomber pilots make history
Yes you can set the CR with the thickness of the gasket. Most gaskets can be purchased with 2 or three different thickness within the same style.
Have you cc'ed the head chambers and confident with your CR math ?
Have you cc'ed the head chambers and confident with your CR math ?
Thread Starter
Race Director
Joined: Jun 1999
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From: Merritt Ils Fl
Machine shop and I agree they are 64 cc heads. My concern is the size of the piston holes in the gasket.
Race Director
Joined: Sep 2000
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From: The Top of Utah
A large gasket bore can contribute to the crevice volume and gases of the cylinder and as the bore gets larger, the amount of gasket material available to seal between the cylinder and the water passages and bolt holes is decreased. I'd have to do the math, but I can't imagine there is half a ratio variance in gasket bore selection. Small variations in the compression ratio are less important than a reliable seal and an effective quench area. If you feel an adjustment to the CR is necessary, remove some metal in the combustion chambers by unshrouding the valves. I would use the smallest bore gasket that didn't threaten to over hang the bore and adjust to it.
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Safety Car
Joined: Sep 2000
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From: Hinckley Ohio
Results will obviously vary depending on head volume, engine displacement, gasket sizes etc...as you said you need to do some math to see where you are at for a particular setup..
Last edited by LT4BUD; Sep 11, 2008 at 12:39 PM.
Burning Brakes
Joined: Nov 2006
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From: Portsmouth Virginia 396LT4 435RWHP/400RWTQ Best so far 11.26 @ 123mph
Muffin,
The difference between a 4.060 and a 4.125 gasket I.D. on a convential 3.5 stroke 350 with a 4.00 bore is less than 5/100th of a point based on various web site calculators meaning for example 10.69 SCR versus 10.65 SCR when comparing a 4.060 versus a 4.125 gasket. Not enough to really make a significant difference in compression. Just use the smallest ID possible staying just above bore diameter dimension.
This is probably more of a concern when dealing with quench area than a real compression issue.
Rick
The difference between a 4.060 and a 4.125 gasket I.D. on a convential 3.5 stroke 350 with a 4.00 bore is less than 5/100th of a point based on various web site calculators meaning for example 10.69 SCR versus 10.65 SCR when comparing a 4.060 versus a 4.125 gasket. Not enough to really make a significant difference in compression. Just use the smallest ID possible staying just above bore diameter dimension.
This is probably more of a concern when dealing with quench area than a real compression issue.
Rick
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Thread Starter
Race Director
Joined: Jun 1999
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From: Merritt Ils Fl