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By 55blacktie - 2 Years Ago
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Has anyone used the Cometic MLS head gaskets vs, Best? The Cometic gaskets are expensive, but they come in various thicknesses and the bore size is smaller. I calculated the Cometic 3.86-bore gasket/w .040 compressed thickness to have a volume of 7.67 cc vs 10.00 cc for the standard bore Best. Wouldn't the Cometic 3.86 bore size be more than large enough for a cylinder bore size of 3.81? Generally speaking, shouldn't the gasket bore size be 0.030-0.060 larger than the the cylinder bore?
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By Joe-JDC - 2 Years Ago
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I would ask if the Cometic gasket if made for Mummert heads or the stock iron heads. The Mummert heads keep the combustion chambers almost completely in the bore on the larger size bores. You don't want gasket material in the combustion chamber. Just a thought. Joe-JDC
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By 55blacktie - 2 Years Ago
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Good suggestion, Joe.
I'm not concerned about high pressure from supercharging (NA application), but I don't want to leave compression on the table, particularly with flat-top pistons.
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By DryLakesRacer - 2 Years Ago
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The surfaces on the MLS style gasket needs to be to a specific “smoothness” . I have used them to lower compression on an Alcohol engine to run racing gas and it’s been trouble free.
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By 55blacktie - 2 Years Ago
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DLR, do you remember the bore size of the gaskets you used and specific application?
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By Ted - 2 Years Ago
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55blacktie (7/1/2023)
....Wouldn't the Cometic 3.86 bore size be more than large enough for a cylinder bore size of 3.81? Generally speaking, shouldn't the gasket bore size be 0.030-0.060 larger than the cylinder bore?
It’s desirable that the actual bore size of the gaskets exceeds the bore of the engine by at least 0.030-0.060” just to ensure that compression of the head gasket does not push the sealing ring into the cylinder bore. That brings up the point of whether the bore size of the gasket is actually 3.860” or if designed for a 3.860” bore. If designed for a 3.860” bore, then the gasket will be slightly larger than that. If you do not have the gaskets already on hand in which measure, then that would be a question for the gasket manufacturer.
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By DryLakesRacer - 2 Years Ago
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55Blacktie..the bore was 4.045” and the gaskets were for 4.060”. The compression was 14.1-1 and with the help of the the compression ratio calculator I use on the Keith Black piston site here on the net I lowered the compression ratio to 13-1 by the thickness of the gaskets available from Cometic. I hand stoned the surfaces on the block and heads as the engine was done for Felpro blue gaskets. May not have been the exact correct way to do it but it was pretty close. The engine is a back up for our Super Stock dirt track car.
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By 55blacktie - 2 Years Ago
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Those are some serious numbers, DLR. You must have sleeved the block to get 4.045.
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By Joe-JDC - 2 Years Ago
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Those dimensions are from another engine family. A Y block will not accept 8 sleeves that size and still have any strength to stay together without leaking everywhere. Joe-JDC
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By 55blacktie - 2 Years Ago
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I did wonder about that because Ted didn't have success with a 4-inch bore.
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By DryLakesRacer - 2 Years Ago
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55 … from what you asked I was just saying what we did with another manufacturer and the gasket sizing. I would have started with that if I had thought of it as I didn’t think it mattered. My main reference was for the 2 mating surfaces are different for MLS gaskets than Felpro blue. I also wanted to show we lowered compression the gasket was .025” thicker. Cometic offers gaskets to .018” and a std Felpro blue is about .041 compressed. If an .018” was offered for the Y-block in the proper bore and I needed it, I would sure use them to raise compression…
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By RB - 2 Years Ago
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AFIK the original shim head gasket is still available. It is .020.. I have had good luck with them
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By 55blacktie - 2 Years Ago
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Although I'll be running iron heads (less movement), there's still the possibility that MLS head gaskets will require re-torquing. Ted prefers Best composite gaskets, but I've found that they are offered in just two bore sizes for Y, both of which would be greater than cylinder bore size + .030-.060 that Ted recommends. I know that I would like to avoid having to remove the rocker assemblies to access the head bolts, after the engine has been built and running.
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