I'm getting ready to purchase Cometic head gaskets for my F/I build. The block is a 1-piece seal SBC (non-lt1) with a 4.030 bore. I've always ran the Fel-Pro 1003 gasket (both N/A and Boosted). This has a bore dia. of 4.166. Cometic offers their gaskets in 4.060, 4.100, 4.125, 4.165 dia.'s. Each of these are different cc's, affecting the final compression ratio (even though its not much between them.) Is there any advantage or disadvantage to running any certain dia. gasket? What woud you guys run?

Thanks

 

What final compression are you shooting for? I typically use the headgasket as a last measure to get the compression where I want it to be. I assume you already have your cylinder heads and pistons.

What cylinder heads are you using and what is the combustion chamber size?
What pistons are you using and what type of relief do they have (dishcc)?
What crankshaft are you running and what is it's stroke?

Cometic gaskets: Standard MLS
Bore Dia Material Thickness P.N.
4.060 MLS / C.O.T. .040" C5877-040
4.100 MLS / C.O.T. .040" C5878-040
4.125 MLS / C.O.T. .040" C5879-040
4.165 MLS / C.O.T. .040" C5880-040
4.200 MLS / C.O.T. .040" C5881-040

All of these run a compressed thickness of .040", but you can custom order these up to .100 thick to drop the compression even further. I personally like SCE copper gaskets over the MLS. I am not trying to convince you to change your mind as much as to make you aware they offer a larger variety of thicknesses off the shelf.

What GM 1-piece rear main block do you have? Is it a corporate or GMPP block? Does it have blind tapped head bolt holes?

 

Kendol,

I was shooting for 9.5 to 1. This 383 motor WAS 11.1 to 1 with AFR 64cc heads, -5cc pistons ,0 deck height and .041 thick (the Fel-Pro 1003) headgaskets, the block is a stock 92 GM block, worked over for the stroker.

I've since swapped to 75cc AFR Eliminators and am looking at the .051 thick Cometics. This saves me from buying a new rotating assy (balanced 4340 Eagle Crank 3.75 stroke, 4340 h-beam Eagle rods, and Forged Wiseco pistons).

The .051 Cometics give me the following compression ratios:

4.06 bore 10.82 cc gasket = 9.63 C.R.

4.1 bore 11.03 cc gasket = 9.61 C.R.

4.125 bore 11.17 cc gasket = 9.59 C.R.

4.166 bore 11.39 cc gasket = 9.57 C.R.

Like i said, the difference in C.R. is small, I just didn't know if there was a advantage or disadvantage to any of the bore sizes considering my bore in the block is 4.030.

This is the info i got from Cometic:

Lead time is 3-5 business days for a small quantity. The gasket is available
in .051" ($90.47), .056"($118.45), .060"($118.45) and .066" ($118.45).

My local dealer (Star Performance) carries the Cometics for cheaper than this, as you can see going thicker than the .051 gasket, the cost goes up considerably.

I was aware that they offered the gaskets in other thicknesses. I just didn't want to go too far outside of the .041 quench.

 

92Z--

My suggestion would always be to go with the gasket that has the same bore size or .030 bigger than the block and vary thickness to achieve the compression you are after. This will minimize the quench area in the chamber, reduce the chances of a hotspot on the head mating surface or block mating surface, and limit fuel puddling crevices that could alter combustion efficiency.

 

Just an FYI, I called Cometic and they recommended that i use the 4.060 bore gasket with a 4.030 bore. Jegs has these in stock to ship today also! Thanks for the replies.