In my 509cid project, if i want to raise CR a little what is the thinnest head gasket and what brand should i use. part numbers are very very helpful. thanks, dave

Mill the head.

its a brand new dart big m block. dont really want to mill the head unless that is the best thing to do.

Milling the heads .030 will give you about a .25 increase in comp ratio--assuming 118cc chambers.

You can get Cometic head gaskets in .030 thickness

How thick is a normal head gasket? I have GM 188's that have been milled to 114.5cc, I am now putting Merlins on that I believe are 123cc. I was told that I would not want to mill these heads. What would be a good way to go about raising my compression. 454's gen 4

sorry for hijack of the thread but I was just going to ask this.

Rookie, you're going to lose almost a point of compression with that swap. If your block has never been zero decked, you can get about 2 cc per cyl. back by doing that. If you can mill the heads .030, you can get about 5 cc back from that. Or you can use a different set of pistons with a larger dome. I think you're going to notice the loss of power through the whole rpm range including idle otherwise.

with the inefficiency at lower rpm 188's I have on my 425's, and the flat tappet cams. I was told that with the higher flow #'s of these new heads and better ramps and profles of the roller cam it would be a mute point. I am only looking to get it to 118cc. would this be achievable with just gasket? I will check about how much I might be able to mill these.

Rookie, you need to have .040 min distance between piston and head. If you deck the block or go to a thinner head gasket, that distance gets smaller. You will have to measure it to find out what the thinnest gasket is you can use.

Are your new heads oval port or rectangular? The flow numbers for the rect. port Merlins are pretty close to the 188's. The cam change will likely do some good.