LannyL81

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.

Tim81

Lanny, When I was building my engine I was told that head gaskets with the larger bore size were made to fit a large variety of builds in other words a company could make a head gasket size 4.166 and it could be used for builds that have a bore of .010 to .166 over, its cheaper for the manufactures to make them this way. If you price head gaskets you'll notice that the 4.030, 4.060 size gaskets are much more expensive. Tim

LannyL81

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??

63mako

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.

AZDoug

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

69 Chevy

Could you please expand on this statement?

jackson

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.

63mako

Jackson beat me to it. I bought a set of Cometic 4.060 bore gaskets for a 4.030 bore engine and could not use them and they are some of the best you can get.

LannyL81

Well aware that all compression calculations require gasket volume....although the gasket volume is small...it does make a difference.

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

69 Chevy

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?