ChrisWhewell

Bored my 350 LT1 block out to 0.030" over and confused as to what diameter head gasket I should use ? 4.125" ?

merlot566jka

your bore is now 4.030....

ChrisWhewell

Thanks.

I've seen new stock gaskets when placed against a new stock head, metal adjacent to the comb. chambers are't perfectly matched to the diameter of the big ole holes in the gasket - typically some head material extends beyond the dimension of the big ole hole in the gasket, in an inconsistent fashion.

I suppose I should have asked, does the bore in the gasket always have to match that of the bore, i.e., are there folks who run a gasket with larger diameter than the bore ? If so, why ?

I've seen so much talk about quench & other factors, it wouldn't surprise me if there wasn't some obscure reason to go slightly larger.

96GS#007

You are waaaaaaaaaaaaaaaaaaaaaay over-thinking it. You use a stock 350 head gasket.

Here's a shot of the stock gasket on my 383...

PLRX

ChrisWhewell

You're probably right, but.....

If the same stock gasket is good for a hole that's 4.000 and a hole that's 4.030, then when used on the 4.000 hole there is extra deck within the interior circumference of the gasket, between the head and the deck.

Some discuss the effects of the size of the volume between the top of the upper compression ring and the top of the piston, between the piston wall and cylinder bore (sort of a dead volume that doesn't burn much). Others discuss quench distance, which is between the top of the piston and portions of the cylinder head. Each of these are tiny volumes, the quench having a profound impact on detonation / knock resistance. The amount of deck that's within the inner circumference of the gasket should be ignored no less than the dead volume and the quench distance. It seems likely that some really serious engine builder somewhere would optimize or eliminate it. If I could, I'd make the i.d. of the big hole in the gasket to be less than 0.005" larger than the cyl bore, maybe 0.002. I meticulously grind sharp edges out of the chambers, around the seats & even the plug threads, and after all those details, considering other factors like gasket diameter are a heck of a lot easier than putting a seat at the right place on the valve & getting everything else right !!