Head gasket sizing

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K.O. SWINGER

Meeting in the alley since 1976
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My question is this when determining what size of head gasket to run in relation to the bore size should the gasket be the same exact size as the bore or is it optimum to be a little larger? Keeping compression as high as possible is also a consideration.
 
Yes if possible, you can get common overbore sizes from a manufacturer like Cometic. Most common SBM head gaskets are 4.125 bore.
 
Very little option for the Correct size now. It seems that all gaskets available are the 360 size unless you go Custom Cometic or something.
A 360 Gasket on a 273 is about a Quarter Inch too big!
 
Definitely don't want to try and get a gasket the same as the bore. If it overhangs the bore or chamber at all you'll end up with problems. A little bigger is typically better.
 
I always go .010 over. If you are building the motor install heads and trace the bore onto the cylinder head then add .010 to the diameter.
 
Most common head gasket i have ever used is the fel pro 1008 039. Its a super common performance head gasket for Mopar small blocks. Its 4.180 bore. Never had any issues with them running any 340 or 360 bore, all of which are lots smaller. Also have used the Mr Gasket 028 for issues where a bit more compression is desirable. Its 4.140 bore.
Cometic has the ability to get more specific, but recently can be hard to find in a timely fashion.
 
Have you used those .028s at say 190psi cranking pressure, and how long did they survive?

had them on my 360 that ran 11.20’s at 3300+ for the last few years before swapping to my stroker. Cranking pressure wasn't that high, but that's a steel shim gasket, its capable. Rated for up to 12 to 1 compression.
 
Oh steel, I thought it was a sandwich,
I once used .028 sandwiches but I caught them migrating into the valley in less than a year at 185psi. I don't recall the Brand but seem to remember the color orange associated with them; that was year 2000 or so.
 
Those aren't like the original embossed steel shim. They are a graphite/elastomer construction. I used them on my 273 build and worked fine. This is what the originals looked like. Note the strange shaped bore.

273 head gasket.jpg


stock head gasket on 920.jpg


273 head gasket.jpg3.jpg
 
I bought a set of ported Sidewinders for my 340 from a well known mopar SB guy a few years back, and I also ordered a set of head gaskets. I told him that my bore was 4.080" and he set me up with a set of the Cometic head gaskets that have a 4.080" bore.
I recently pulled the heads and the gaskets for an engine rebuild and this thread made me go out and measure the bores on the 4.080" gaskets for my own reference. After a quick cleaning I took a few measurements of 4.082" to 4.086". I ran these gaskets for about three years and they still look to be in good shape around the bore that I measured. I was surprised when he spec'd me out the same size gasket as my bores, but it appears to have worked out just fine.

7002A2C9-79B9-46EB-8364-7ED1B3BB402D.jpeg
 
My question is this when determining what size of head gasket to run in relation to the bore size should the gasket be the same exact size as the bore or is it optimum to be a little larger? Keeping compression as high as possible is also a consideration.
I think there was this idea that you didn't want the fire ring right in the firing line.
A lot of people get MLS or cometic type gaskets and when ordering pretty much get them at the bore size their engine is and when that motor comes apart they see that the gasket is all charred and crispy , burnt off edges...burning away.

I'd stay a minimum of .120 bigger than the bore. That's only .060 all around .
 
Another thing to be considered is the top of the cylinders chamfer.
Older shops used to put a decent-sized chamfer on it it ...kind of assists in ring installation ..
but newer shops tend to barely chamfer the bore. You could have a 4.070 and the chamfer be 4.100
 
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