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MG MGB Technical - Cylinder head gasket volume

I am in the process of rebuilding my MGB engine and calculating compression ratios. What I am trying to find out is the volume or thickness of a standard compressed cylinder head gasket.
The worked example in Peter Burgess book refers to a 4.5cc gasket volume. This value seems very low to me. Is this an example value or the actual value of a standard compressed gasket that could be used in the calculation?
RH Davidson

I calculated mine to be 0.85mm UNcompressed. With a bore +30 thou, I calculated the volume to be 4.3cc.

It's difficult to say how thick the gasket would be in the compressed state but, bearing in mind there is some steel around the bore, then it isn't going to be much.

Even if the gasket compressed to half it's free thickness, the difference is only going to be 2cc which has a negligible effect on CR. For my engine, that would make three tenths of a CR difference.

N
Neil22

RH,

The last standard gasket I had measured was .055". Book states standard compressed at .023" and it will change your compression ratio. I think Peter B uses this measurement for his book.

You also need to measure your deck height. Book states .040" but on standard pistons we came up with anywhere from .030" to .034". This will also change CR.

Ray
Ray 1977mgb

Just re-read my post and it's not clear. I mean that there isn't going to be much compression, hence the compressed thickness isn't going to be all that different from the uncompressed thickness.

My assumption on compression (ie that it would go to half thickness) is borne out by Ray's figures.

Deck height is not such an issue. WIth 40thou above the piston and the bore at +30thou, I reckoned the volume above the piston (not including the dish) to be 1.54cc (!). Dropping 10thou on the deck would reduce this by a quarter, ie to 1.66cc.

The bits and pieces add up, but if you're going to this accuracy then you've got to measure the stroke, and have a pretty accurate means of measuring chamber volume. Using a sheet of perspex, plugs fitted, and a pipette I reckon on being plus or minus a cc anyway, maybe more.

N
Neil22

Found it. After what you guys confirmed, I rechecked the thickness of the gasket I removed from the engine and found it to be 2.2mm (=11.6cc). Out with the magnifying glass and sure enough, the PO had neatly glued two gaskets together and put them on. I also checked for blow-by, and sure enough, after carefully separating them, I found blow-by between the gaskets, between 2 & 3 cylinders. This also confirmed previous experiments on mini engines that two gaskets don’t work.
In conclusion,
Uncompressed standard head gasket thickness = 0.04" (=1mm)
Diameter of sample gasket = 82mm
Volume = 5.28cc (close enough)

Standard head gasket thickness under compression = 0.033" (0.85mm)
Volume = 4.49cc

Thanks for your help. You can see my concern; 11.6cc is a long way from 4.49cc in a compression ratio calculation.

Richard.
RH Davidson

Just measured an old used copper sided gasket I have and it is.022" thick---- cc'd it and it went just on 3.1 cc There must be a few different thicknesses about
Cheers Willy
WilliamRevit

Gaskets flow over time and with heat/cool cycles, hence get thinner. That's why you have to retorque, regardless of the propaganda. The copper composite ones are tough and won't blow catastrophically even when they are quite loose and even leaking noticeably. So you tighten it down and it keeps getting thinner.

FRM
FR Millmore

This thread was discussed between 25/11/2007 and 21/12/2009

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