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Cometic head gasket

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Old Oct 23, 2023 | 06:34 PM
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Default Cometic head gasket
Hi,
If you use a KB piston (96"-103") that is STD 103" bore + .040" equals total bore size of 3.915"

If you have a bore size of 3.915"..... What size .030" Cometic head gasket would you use? Seems like they only make them for either 3.875" bore or 3.973" bore?

Thanks

 
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Old Oct 23, 2023 | 08:04 PM
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Cometic MLS head gaskets for 103" and 95" bore of 3.875" use a C9721 which is 0.030" thick and has an ID of 3.910". The C9894 is 0.030" thick and has an ID of 3.950". The bore size in the catalog is just that, and not the gasket ID.
 
Last edited by Ohio HD; Oct 23, 2023 at 08:21 PM.
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Old Oct 23, 2023 | 10:41 PM
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Originally Posted by Ohio HD
Cometic MLS head gaskets for 103" and 95" bore of 3.875" use a C9721 which is 0.030" thick and has an ID of 3.910". The C9894 is 0.030" thick and has an ID of 3.950". The bore size in the catalog is just that, and not the gasket ID.
OK, thanks Ohio.

So if my piston ends up being a 103" piston + .040" oversize = 3.915.

So the C9894 with the 3.950" ID gasket would be the right fit?
 
Last edited by Adam76; Oct 23, 2023 at 10:44 PM.
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Old Oct 23, 2023 | 10:49 PM
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Yes it would be the closest that they show. Keep in mind OEM 103" HD pistons aren't available in 0.040", only aftermarket. That's why the 3.910" gasket size, to match HD OEM.
 
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Old Oct 23, 2023 | 10:57 PM
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Originally Posted by Ohio HD
Yes it would be the closest that they show. Keep in mind OEM 103" HD pistons aren't available in 0.040", only aftermarket. That's why the 3.910" gasket size, to match HD OEM.
yes, I was looking at KB 103 + 040 pistons (not oem HD)
Cheers
 
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Old Oct 25, 2023 | 04:59 PM
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Originally Posted by Ohio HD
Cometic MLS head gaskets for 103" and 95" bore of 3.875" use a C9721 which is 0.030" thick and has an ID of 3.910". The C9894 is 0.030" thick and has an ID of 3.950". The bore size in the catalog is just that, and not the gasket ID.
I'm still confused.
If my piston is an oversize 103 piston + .040".... is the gasket ID i need is 3.915 (3.875" + .040").

If my bore size ID is now is 3.915" what gasket size do I use? Wouldn't the 3.910 ID gasket be too small?

Should I use the .030" oversize piston instead, which gives me a bore size of 3.905" ?

Sorry for the confusion.
 
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Old Oct 25, 2023 | 05:10 PM
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You would use the 3.950" gasket, just like a 3.937" 107" uses. They don't have to be sized just over the bore size, just larger. That's also why compression calculators have an input for head gasket thickness and inside diameter.
 
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Old Oct 25, 2023 | 05:19 PM
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Originally Posted by Ohio HD
You would use the 3.950" gasket, just like a 3.937" 107" uses. They don't have to be sized just over the bore size, just larger. That's also why compression calculators have an input for head gasket thickness and inside diameter.
OK, thanks 👍
 
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