Cylinder heads

[nigelr]

Does anyone have any information about the acceptable tolerance for the differing thickness of heads?
A cylinder head expert nearby has said that if there is any difference between the amount that has been skimmed off the heads it will result in crankshaft failure. I think my RH head has reached the end of its life, as it has 3 studs stuck in it & has been skimmed several times in the past, so I acquired another. The new head is however about 1mm thicker than the LH head, but when I measured the old RH head it in turn was about 1mm thinner than the LH - if you follow! The heads have been on (with the 1mm dif) for 20000 miles without any crankshaft problems, but I know the Stag used to have problems.

[Andy Rundell]

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I'm guessing that, while ideally, you would want to have a similar amount skimmed from each head to maintain consistency of Compression Ratios, the reality is that most people will be driving around with a pair of heads with different levels of skim.

The important thing is that you have enough meat left to avoid unwanted valve/piston interaction , avoid HGF, and be able to get a decent inlet manifold seal on both sides.

I doubt a crank failure would be related to a slight imbalance of CR, that sounds like an over-active imagination to me

........Andy

[DJT]

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If there is still enough meat to use a standard gasket on the thicker head, use a thick gasket on the thinner head. Problem solved.

Payen do both thicknesses of gasket and both are available from www.ldpart.co.uk (and others).

Dave

[V Mad]

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Yes I think Daves advice is good, use the thick gasket on the 'thinner' head.

As a matter of interest, where do you measure the thickness of the head from?

Re crankshaft failure, the crank is designed to take the peak loads imposed by the firing of each cylinder, so a little variation of load from one cylinder to the next will in my opinion not make any difference in normal use. If you are racing, and have a highly tuned engine running at full revs most of the time, when the crank is close to its limit, then it becomes important.

[nigelr]

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Thanks fellas. I was a bit sceptical about the info I had been given considering my heads had done 20000 miles with a 1mm variation - but you never know.
I took 1 of the caps off the camshaft & then measured from the top of the camshaft bearing journal to the face of the head.
Does anybody know what the difference in thickness between the thick & standard gaskets is?

[elvimto]

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The normal gasket is 0.060 thou and the thick gasket is 0.080 thou The gasket will crush about 0.014 thou when you torque the head. I believe the thickness of a new head is 4.435 measured from the bottom of the cut out or valley to the face of the head at the rear.

4.435 std head thickness plus std gasket 0.060 minus crush 0.014 = 4.481 std height the numbers that I used. I am using a thick gasket on a head that is4.405 as long as you know that there is enough not to hit the valves you should be fine. Unless you plan running an engine at top revs racing etc you should be ok. I havea mix set of Mk1 and Mk2heads that I intend to use for spares that I will have to cc the combustion chambers to match because the compression ratio is different.

Hope this helps

cheers Stuart

[nigelr]

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Thanks Stuart, that is really useful info. I don't have a problem with valve/piston clearance at the moment, but I was concerned about it after another skim. I had planned to put the head on the block without a gasket, some plasticene on the crown of the piston, & turn the engine over. That would give me the clearance between piston & valve, but without knowing the crushed gasket dimension it was not a great deal of use. So thanks.
Nigel