What bore size cometic gasket?
 

400 +o.030”

What bore size head gasket is used with iron heads??

I believe the gasket should be as big as the combustion chamber…or the bore…whichever is LARGER.

A part number is greatly appreciated!

🤙🏼

Why shovel out big bucks for that brand gasket when a Fel-Pro 1016 works perfect.

With a Pontiac you can’t have a gasket with a dead on bore size to match the cylinder anyway due to the width of the chamber with the 67 and up heads, but even if you could having 4.150” bore gasket only gets you back like 1.7 CCs and that little amount will not make you any extra power.

However having a notch / intake valve chamfer in the
top of the bore and loosing 1.5 CCs is worth more power then the drop in compression.

Reason is to tighten up the quench distance, short block is together already.
I have 66cc 48 heads and I’m really up there ~11.0+ CR.
So still needing help with the correct gasket bore.

I’m thinking 4.300?

Oh well those details are quite important then!

What Can are you running?

The cylinder pressure you will have are as big a part as the overall CCs above the piston top.

I assume those 48 casting are 100% redone top to bottom?
The open chamber heads are a tad more prone to pinging and knock with a given level of compression than the earlier chambers and a greater deck clearance only adds some to that chance of problems.

Depending on your cam pick I am not sure that even running a 4.300” wide x .065” tall gasket will drop you down to a safe pump gas friendly compression.

I want to say that if you’re not locked into having to run those 66 Cc 48 castings that you sell them and pick up a set of # 62 castings .

Once again your cam pick is crucial here!
Some folks with top notch tuning skills can get I iron head 10.5 to 1 motor to run fine on todays 94 octane, but your locked into having 94 in your tank or cutting it with race fuel.

Cometic offers head gaskets by part number listed in their catalog. They also offer "round" head gaskets that match the bore. Gaskets that are "round" don't have cut outs for valve notches but are made to match a specific bore. They're standard head gaskets (catalog part numbers) will be similar to the factory gaskets and match the valve notches or combustion chamber and be slightly larger than the specified bore size. This should be explained by the place one purchases their gaskets from so they are sure to sell you the correct part.

The heads are fully reconditioned, including hardened ex seats.
60919 crower, 6.6cc reliefs, slugs ��o.015” in, hence the o.027 gaskets.

Off the top of my head I don’t see where even a quite thick head gasket is going to drop you down below 10.5 compression.
Sorry I don’t have time right now to run the exact numbers for you.

I think he’s looking to tighten the quench with a thinner gasket, not reduce the compression.

Play with the numbers

http://www.wallaceracing.com/cr_test2.php

Yes, you’re with my logic here! This will be a good test of my tuning skills.

measure the widest part of combustion chamber, bore is 4.150 look for deck chamfers around bore whatever your largest number is add a cushion factor for misalignments and that would be your round number

when i mocked up my 433 i sat the heads on my block put a bolt in them turned it over on eng stand and looked down the cylinder much to my suprize and pleasure the edges of chamber were at the edge of the bores front to rear (Edelbrocks)

Static CR 10.7
DCR. 7.30
4.10 rear gears with an effective first gear ratio of
10.2:1
will be using Rhoads VMax lifters.
Project has been sitting a few years too long.

Thank You for all the advise! Still unsure what is correct gasket.

Here’s what I get when I run your stated numbers.

1) Since you posted a head gasket thickness of .027” I will assume a round bore in the head gasket that is 4.150”.

2) I added in 1.5 CCs for top compression ring volume due to bore clearance.

3) I did NOT add in any CC volume for intake valve chamfer in the block because you did not post that your block had such.

Here are the numbers I got using what you posted.

Note these are in CID.

Bore = 50.72

Head chamber ( 66 CCs) = 4.02

Valve notch volume ( 6.6 CCs ) =.4027

Top ring volume ( 1.5 CCs) .0915

Deck clearance ( .015”) = .25833

Head gasket volume ( .027” round bore) = .4649

True compression = 10.68 to 1, so not over 11 to 1 as you originally posted.

True compression when substituting a Fel Pro 1016 gasket that has a non round bore and checks out at 9.4 CCs.

True compression = 10.48 to 1.

Like I first posted, for your combo I don’t see a drop of advantage in going with the special head gasket even putting the very slight difference in compression aside and trying to look for any possible reduction in ping or knock happening due to it being a round bore.

Op states quinch is the interest, you quench off of piston top, no mention of piston to deck spec,

and we keep going in circles talking about static compression

figure your desired quench distance from piston for gasket thickness and the hole size needs to be slightly larger than the widest player, chamfers scallops, chamber, its pretty easy really and its at OP fingertips

and if you dont wish to figure it out get a standard FelPro gasket and run with it

What I wanted to get across to the OP in regards to post 3 ( over 11 to 1 ) and before running his numbers is this.

If you have even just a 11 to 1 comp ratio running an ideal quench clearance and shape is none too likely to keep you out of trouble with an iron head and pump fuel.

The OP should without fail take the time to polish off any sharp edges around the plug hole and unused plug threads that sick out there also.

Also lightly polish off the eye brow lip on the intake valve side of the chamber, like by .010”.
The same chamber lip on the exh valve side can be well rounded over with no concern for loss of wet flow fuel sheering.
As in this picture here the lip of the chamber to the deck should be broken also.

I use a 4.200 gasket on my 400ci with a 4.160 bore and my 389 with a 4.095 bore with no issues

Top ring volume was not used in my calculation of compression ratio, adding it makes me feel better…sort of. 😀

I’m going to look into getting it zero decked vs cost of gaskets and my time.

Anything simple I can do to these head castings to increase flow?

Definitely going to relax all sharp edges in the chambers.

Thank You for All the input!

Please list what you have for porting tools and what intake you’re running .

Are you running headers?

Unless your first name is Merlin I doubt tuning skills will matter much at close to 11:1 -- especially when Rhoads lifters are added into the mix.

If it were mine and I was set on using those heads I would run the Cometic gaskets, but would go with .120 thickness and say to hell with worrying about quench. We concentrate on the bottom side of the danger area with quench and want to stay under .050" total to avoid detonation, but the far side is anything over .080" the danger also goes away. Short track guys in my area have gotten away from the D-shaped dish and found more power in an open dish. I'll give you that at off-idle power might be down, but there is probably enough swirl in the chamber at mid to high RPM to make loss of quench a moot point. I would take the bet that a thick gasket and standard timing is going to make more power throughout the powerband that an engine that has to be band-aided with massively retarded timing.