Head gasket size

[fishman]

I was reading a post today I ran across about a fellow asking how much bigger should his head gasket then the blocks bore. Some answers were +.010 to +.025, what I got out of it was very mixed. I just tore apart my sbc little m dart block
That has a 4.165 bore and the head gasket I’m running is 4.166 bore , I raced this motor for six years and after tearing it down I found no issues so I’m wondering what u guys opinion is, I’m putting motor back together soon and have no reason why not should I put same gaskets in

[MadBill]

Especially with a shim gasket, you don't want any bore overhang, as the thin edge could become a preignition source. On the other hand, if the gasket ID is more than say bore + 0.040", the resulting increased crevice volume can become a surprisingly substantial source of lost power, as the mixture compressed into it can't burn until too late in the cycle to do any good. (Math available on demand.)

Bottom line: you're perfect as is.

[BOOT]

Especially with a shim gasket, you don't want any bore overhang, as the thin edge could become a preignition source. On the other hand, if the gasket ID is more than say bore + 0.040", the resulting increased crevice volume can become a surprisingly substantial source of lost power, as the mixture compressed into it can't burn until too late in the cycle to do any good. (Math available on demand.)

Bottom line: you're perfect as is.

Interesting how much of a loss percentage wise roughly?

[mag2555]

As posted above if you are not running a steel shim type gasket then do not waste any more time thinking about it and just slap back on the same gaskets you have been using.

While you have the heads off and in the same vein as the possible issue with shim gaskets you should lay a gasket on each head and confirm that there is no sharp lip of the meeting of the deck and chamber sticking out passed the fire ring of the gasket.
Any sharp lip should be sanded/ rolled over with the concideration of not undermining the fire ring.

Also any sharp chamber wall edeges around the plug hole and and unused threads of the plug sticking out into the chamber should get the same roller treatment!

[MadBill]

..
Interesting how much of a loss percentage wise roughly?

The only data I've seen relates to top ring land volume, where several test results I've seen have shown gains of as much as 3% by reducing the top land height from 0.312" to ~ 0.200", but the premise is the same: approximately atmospheric pressure mixture is compressed into the crevice volume at peak cylinder combustion pressure, where it cannot burn due to the quenching effect of the adjacent cool metal. (akin to a marine carb flame arrestor) When it does emerge late in the stroke, it contributes virtually nothing to the cylinder pressure.

Here's some plausible (to me anyway) math for a 4.0" bore, 800 psi peak cylinder pressure 500 HP 350" (43.75"/cyl.) engine with a 0.040" thick by 4.10" bore gasket:
"Excess" gasket volume = 4.0 x π x 0.050 x 0.040 = 0.025 in³
"Peak firing compression ratio" = 800/14.7 = 54.4:1, so 54.4 x 0.025"³ = 1.37"³ inducted mixture "lost" to quench, thus 1.37/43.75 = 0.031, 3.1% x 500 = 15.7 HP.

A typical "Old School" ring land volume could be ~ 0.12"³, so a reduction in land height from 0.312" to 0.160" could theoretically gain 0.06"³, an unrealistic ~7% or 35 HP in this example. I suspect the less than half real world gain is accounted for by low volatility oil accumulating in the crevice during the compression stroke and so reducing the potential volume change. (Either that or I've muffed the calculations... )

[ptuomov]

Is there any fuel in those crevices, or just air?

I’m noticing that the new turbo BMWs have gone back to tall ring lands. I wonder if direct injection etc. somehow keeps any fuel out of ring land and gasket crevices? You’d think that emissions and fuel consumption requirements would otherwise conflict with the big ringnlsnds.

[rebelyell]

If your cylinder has a chamfer/lead-in (many do), the gasket should be barely larger than top of chamfer; as before, to prevent any overhang

[MadBill]

Is there any fuel in those crevices, or just air? ...

Good question. For CV to affect power there would pretty much have to be combustible mixture in it. In a port injected or carbureted engine I don't see that there would be any mechanism to separate out fuel from the mixture, so I'd say yes. With DI, possibly not.

[rfoll]

Many of the gaskets I have seen do not actually center on the bore. I have seen significant overhang with a 4.160" bore gasket on a 4.155 bore.

[ProPower engines]

Many of the gaskets I have seen do not actually center on the bore. I have seen significant overhang with a 4.160" bore gasket on a 4.155 bore.

A few reasons come to mind there.

1 is the bore spacing is off in relation to the head/gasket locating dowels where the bores were off set to save an OE block to use a particular bore size.
2 The head gaskets were made wrong. Not uncommon to see some gasket brands of the composition type be off slightly. Even MLS gaskets can be off a bit but a 4.155 bore block using a 4.160 bore head gasket would be safe

Putting the heads on the blocks with the chamber circumference dye marked so the bore to chamber can be scribed to check the head dowel location in the head may also show they were drilled off set depending on the type of head.
It can be corrected with off set dowels