Which Head Gasket?

[richard btype]

It is believed that the cylinder head on my 1962 3.8 FHC has never previously been skimmed. However I had cause to remove the head (another story) and found that the corrosion of the cylinder head waterway necessitated some welding.

The head was then skimmed by a staggering 18 thou - I'm sure some machine shops routinely go over the top - however I am concerned that with lack of 5 star 100 octane (remember this?) that by fitting a standard tin gasket 20 odd quids worth the increased compression would be detrimental.

SNG can supply a composite gasket 1.8mm for 50 odd quid and a race shop can supply a composite 1.3mm for 170 odd quid.

Which should I order?

Richard

[timhum]

18 thou is only 0.25 mm so fitting one of those two gaskets will cause quite a significant lowering of compression ratio.
Tim

[christopher storey]

18 thou is actually about 0.45 mm . The ordinary gasket would be at least 1mm thick uncompressed so the effect of fitting either of the thicker gaskets will be minimal . In any event, with modern unleaded fuels it is far better with a hemi head to err on the side of lower rather than higher compression, because, lacking squish, hemi heads are said to be more prone to detonation . On a 3.8, the piston area for 1 cylinder is about 59.45 sq.cm , so a 1mm increase in the effective height of the combustion space increases the combustion volume by just under 6cc. On a 9:1 engine the total standard combustion volume is 630/8cc = 78.75cc . Each increase of 1mm in height by e.g. fitting a thicker than standard gasket only lowers the CR by about 0.6 of a ratio . To put it another way , machining 18 thou off the head would only raise the CR to about 9.3:1 on a 9:1 car if a standard gasket were used