My LT1 block is bored 10 over. LT1 Cometic Head Gaskets come in two bore sizes 4.040 and 4.100 both at .040 thickness. Which should I use?
Seems like the 4.040 bore size would be better for quench (zero decked block) but it only leaves .015 on either side of the bore. If the gasket creeps into the bore, it could create a hot spot leading to detonation, right? But the 4.100 bore size seems a bit large.

 

If you get the 4.04 gaskets, you can lay them on the dowls on the deck and on the heads with some other loose dowl pins and see if they intrude, and if they do you can carefully trim them as long as you dont leave a burr or other raised area. They are kind of hard to trim because they are 3 layers and they can get ruined if you aren't careful. Mine needed a little trimming for valve relief and they protruded a couple spots in the valves side of the combustion chamber and I took them apart and trimmed them which was a PITA but it turned out well and I used a light coat of Hylomar spray to seal them up when I did final assembly. I see a lot of gaskets, including the GM and FelPro ones are a lot bigger diameter than the bore, so don't know how much that really matters besides losing a little compression which you can calculate out if you care to. That combustion chamber shape doesn't match the bore nohow, so unless you cut the valve reliefs int he cylinder, it isn't going to match up anyway. I'd like to know how much it really matters to have them intrude a little or be set back a little, cause I'm sure most guys just plop them on there & torque the head down without ever checking it.

I should have stated that I used the larger size, and still had to trim a couple spots.

 

I would be worried about the 4.040" edge being quit a bit inside the chamber...then it will not seal at all. Depending upon how much your chamber walls have been relieved for unshrouding the valves...the bore would not be my concern at all, but I see a lot of people come in with mismatched head gaskets to chamber size.

 

M1 - These don't have a fire ring, just an embossment, which I assume, gets flattenned totally when they get torqued. It might still seal, but might also be sticking into the combustion chamber. So do you think it's fine to use the larger size with no ill effects? That was my choice, based on measuring several gaskets, including the stock ones, but I was a little unsure of my choice.

 

Call them and see what they offer. They will make anything.
.015 to the side will have the gasket in the chamber when torqued down.

 

I know what an MLS gasket is, and never said anything about a firering. Having the edge inside the chamber crimps it and hurts the embossment causing a leak, like I said, I have seen it before.

 

OK, I figured you did - on my setup, even with the larger gaskets, they protruded, but just the flat part before the embossment, not the embossed part. Might be worse for the .040 size though, but hard to tell - So if Cometic makes you a custom gasket, do they move the embossment around or just trim back the flat part back a little per your drawing?

 

It's a different stamping. The 4.100" gasket should be good, don't worry about the extra .045" per side of the bore.

 

I had planned on using the 4.04 x .036 Cometics on my build, but mine is a standard bore, 0 decked. Is this not a good idea? Should I order the 4.1 x .036 instead? I'm glad I saw this thread because I had planned to order my head gaskets on Monday...

 

4.040 is a bit small even for a standard bore.
Call Cometics and see what they recommend. They will know how much their gasket spreads when crushed.

 

Good Idea, I'll do that Monday and order the ones they recommend

I can't imagine them offering a 4.04 gasket specificly for LT1's that would protrude into the chamber knowing that these motors don't have any less than a 4" bore stock

 

Let us know their reply - one way or the other, OK? and ask them if you had custom valve reliefs cut, do they move the boss over or just cut the edge near the chamber a bit. - Thanks.

 

will do

 

Ok I called them this morning and the rep said in my application (standard bore) 4.04 is what they recomend. He said .015 to .020 larger than the bore is fine so thats what I should run instead of the 4.100. So I'm sure they would not recommend a 4.04 for anything other than a standard bore or a .010 over bore.

BTW, I forgot to ask what they do about the valve relief situation, sorry.

 

That is the important part to know. The widest part of the chamber is offset from the widest diameter of the bore, so even if you chamber measures 4.030" across the widest part, the gasket will be quite a bit into the chamber at that point becuase it is not using the same centerpoint for the imaginary circle as the bore does - you get the picture I'm trying to explain?