small block head gasket question

[str8up720]

I'm in the process of calculating the compression ratio gona 360 build I have in mind. When it comes to the head gasket bore, are there any negative effects to having the gasket bore being larger than the actual cylinder bore? I'm thinking that cylinder pressure might leak around the cylinder..? I'm planning on over boring to 4.030, and using a 4.060 bore gasket...any thoughts on how close to the actual bore I should get? What about thickness of the gasket as well...any inputs whether I should go thicker or thinner according to the 9.5 compression ration I'm trying to achieve.

 

[J5 GTX]

As for a larger bore gasket, you lose some compression ratio because of the gap created between the head and cylinder block. How much you lose, that just some simple measuring and calculating.
I don't think you will have any sealing problems.
Thickness of gasket: again you lose some of the compression ratio with a thicker gasket.
With the overbore, I guess you are expecting more power? Then I would use the same diameter bore head gasket and a thin, stamped steel head gasket.
Also, keep in mind deck height. To achieve a true 9.5:1 compression, you must factor in deck height: the actual measured distance that the piston protrudes above or is below the cylinder block deck.

 

[Cranky]

If you use a head gasket with the same exact bore diameter as your cylinder, you will burn the gasket's fire ring. There is a figure for how much you should go over and that usually depends on the build and how you use it and the gasket manufacturer should have those figures available but seems from what I recall, a mild engine should have a gasket that's around .015 larger at the very minimum.

 

[rumblefish360]

Find a good (on line) compression calculator. Enter in all the specs. Then change the gasket specs to see the differences. Each head gasket should have there spec'd from the maker. Fel-Pro, Mr. Gasket etc.... Should have them listed. You'll find there are slight variations but not big ones unless you go to the extremes.

 

[str8up720]

All sounds like good advise! I have actually been playing around with a compression calculator figuring out the best combination for my block. That is what led me to ask these questions. The only uncertainties I have are the crack centerline to block deck distance, which assuming my block hasn't been machined down, should be around 9.585"-9.599" as well as my cylinder head c.c. volume, which I will send off to the machine shop for measurements. Any advice on how thin is too thin of a gasket?