Head Gasket Volume
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
Head Gasket Volume
Why is there such a range of head gasket bore sizes?
Why so large? Rebuilds usually have 30 or 60 overbore.....so head gasket bores would be 4.040 or 4.070...but sizes are 4.125, 4.166, 4.200....and so on.....why?
I know this has an effect on compression ratio...but this is small.
And everyone talks "quench"...which is a distance....would not the quench volume....thickness and bore be a better parameter?
Just trying to understand all the parameters of engines.
Why so large? Rebuilds usually have 30 or 60 overbore.....so head gasket bores would be 4.040 or 4.070...but sizes are 4.125, 4.166, 4.200....and so on.....why?
I know this has an effect on compression ratio...but this is small.
And everyone talks "quench"...which is a distance....would not the quench volume....thickness and bore be a better parameter?
Just trying to understand all the parameters of engines.
Burning Brakes
Joined: Aug 2006
Posts: 1,058
Likes: 27
From: Eastern North Carolina
Why is there such a range of head gasket bore sizes?
Why so large? Rebuilds usually have 30 or 60 overbore.....so head gasket bores would be 4.040 or 4.070...but sizes are 4.125, 4.166, 4.200....and so on.....why?
I know this has an effect on compression ratio...but this is small.
And everyone talks "quench"...which is a distance....would not the quench volume....thickness and bore be a better parameter?
Just trying to understand all the parameters of engines.
Why so large? Rebuilds usually have 30 or 60 overbore.....so head gasket bores would be 4.040 or 4.070...but sizes are 4.125, 4.166, 4.200....and so on.....why?
I know this has an effect on compression ratio...but this is small.
And everyone talks "quench"...which is a distance....would not the quench volume....thickness and bore be a better parameter?
Just trying to understand all the parameters of engines.
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
Hi Tim,
So it comes down to money.....nice.....I had hope that there was some technical reason.
You are sure correct about prices though....wow.
I just wonder if it is better to get a closer gasket to block bore...cause to me having the fuel/air mix sitting on top of the block does nothing.
Good grief...what block has a 4.166 bore....is that a big block or something??
So it comes down to money.....nice.....I had hope that there was some technical reason.
You are sure correct about prices though....wow.
I just wonder if it is better to get a closer gasket to block bore...cause to me having the fuel/air mix sitting on top of the block does nothing.
Good grief...what block has a 4.166 bore....is that a big block or something??
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
A 4.00 bore gasket will not work on a 4.00 bore block. GM tolerances are not that close. There are varying thicknesses as well as bore. A 265 has a bore of 3.750. A 283 has a bore of 3.875. A 305 has a bore of 3.736. A 350 has a 4" bore. A 400 SBC will have a 4.125 bore stock. They can be bored to 4.165 if they sonic test ok. The aftermarket 400 blocks go to 4.185. The World 454 small block comes with a 4.250 bore. All of these are small blocks and can be bored to varying degrees to rebuild so, as you can see there are a wide variety of bore sizes needed in SBC head gaskets. Thickness can be used to adjust compression. A difference in thickness from .015 to .040 can take you from 9 to 1 to 9.5 to 1 or more or less depending on the stroke and bore. Bore size can be used to further adjust compression to a point. Some racing applications require o rings reciever grooves machined into the block or head to contain the compression on turbo, nitrous, alcohol high compression or blown applications. There are also multiple materials used to make these and custom gasket manufacturers so there are hundreds of possible combinations. So yes there is a technical reason for these options. All of your compression calculators will ask for gasket compressed thicness and gasket bore.
Last edited by 63mako; Apr 20, 2011 at 11:13 PM.
Race Director
Joined: Oct 2005
Posts: 12,470
Likes: 1,548
From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
My small block has a 4.190" bore, thus I have a 4.20" head gasket.
Now, you can also vary thickness...
I personally like the MLS gaskets, which are typically ~.035" thick, or up to perhaps .050" or maybe more.
Doug
Now, you can also vary thickness...
I personally like the MLS gaskets, which are typically ~.035" thick, or up to perhaps .050" or maybe more.
Doug
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
The head gasket's fire ring edge must not extend over the bore's edge or gasket will leak & burn thru. An extreme soon-to-fail example of what not to do: 305 head gasket on a 350; **it happens.
Most bores are chamfered at deck (assists ring lead-in / install into bore & relieves stress)... so a thirty-over bore becomes much larger at deck surface.
Most gaskets are not mfd with high precision ... the location of gasket's bore opening (fire ring) falls within a range & most will not be exactly same; not even within same lot/run.
Block's cylinder bore may not be exactly centered within block's perimeter of headbolt holes; same location varies within a range.
Most bores are chamfered at deck (assists ring lead-in / install into bore & relieves stress)... so a thirty-over bore becomes much larger at deck surface.
Most gaskets are not mfd with high precision ... the location of gasket's bore opening (fire ring) falls within a range & most will not be exactly same; not even within same lot/run.
Block's cylinder bore may not be exactly centered within block's perimeter of headbolt holes; same location varies within a range.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
A 4.00 bore gasket will not work on a 4.00 bore block. GM tolerances are not that close. There are varying thicknesses as well as bore. A 265 has a bore of 3.750. A 283 has a bore of 3.875. A 305 has a bore of 3.736. A 350 has a 4" bore. A 400 SBC will have a 4.125 bore stock. They can be bored to 4.165 if they sonic test ok. The aftermarket 400 blocks go to 4.185. The World 454 small block comes with a 4.250 bore. All of these are small blocks and can be bored to varying degrees to rebuild so, as you can see there are a wide variety of bore sizes needed in SBC head gaskets. Thickness can be used to adjust compression. A difference in thickness from .015 to .040 can take you from 9 to 1 to 9.5 to 1 or more or less depending on the stroke and bore. Bore size can be used to further adjust compression to a point. Some racing applications require o rings reciever grooves machined into the block or head to contain the compression on turbo, nitrous, alcohol high compression or blown applications. There are also multiple materials used to make these and custom gasket manufacturers so there are hundreds of possible combinations. So yes there is a technical reason for these options. All of your compression calculators will ask for gasket compressed thicness and gasket bore.
I need to use a 0.050 or 0.051 compressed gasket due to my negative deck height so I need the added thickness to get a good quench distance.
I still think the smaller the gasket bore the better...but that's my $0.02 worth.
LannyL81
Melting Slicks
Joined: Aug 2008
Posts: 2,200
Likes: 3
From: Lehigh county Pennsylvania
I hate to sort of take this thread off on a tangent, or to belabor the point, but is there a rule of thumb for bore diameter when buying head gaskets? Do I have to order a pair online, thinking they'll be fine, then find out they won't work when actually trying them on the short block?
To put it another way; if I have a 4" bore plus .060" overbore, what size head gaskets do I ask for?
To put it another way; if I have a 4" bore plus .060" overbore, what size head gaskets do I ask for?
