How much can you skim off heads?

[jpyke]

I've just taken my heads to an engine rebuilder for a light skim but I know the engine has beeen rebuilt once before and the heads skimmed (by an unknown amount). They need another light skim but I don't know how much I can get away with or how to measure this. I've seen two directives but are either absolutely definitive?

1. Lay a straight edge across the head, if the valves protrude below the head you're in trouble (one or two do already before the skim - but only by a whisker).

2. Theres a measurement from the cam bearing to the head face. I assume this is with the heads on a plane surface and the measurement done at 90 degrees to the surface. Trouble is with the faces being a bit lumpy (bits of old gasket etc - hence a skim needed) how can you hope to get an accurate measurement?

I was planning to use the thick payen gaskets as a matter of course as I've read that they seal better due to the red rings on them and the thicker material coping better with the continous expansion/contraction, and that if the heads have been skimmed they help to correct the compression ratio and valve timing.

The other point to consider is that the block has been bored to +040 and the new pistons have a slight dish in the middle. Its a MK1 car - would these be MK1 pistons - I can't remember what the differences between MK1 and MK2 were on this count.

Anyway, can anyone help? Aww thanks!

[jpyke]

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My comment re the dished pistons was me musing about whether I'd have more/less valve/piston clearance...

[PaulW]

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The flat pistons will give 8.8:1H/C 9.2:1. Worked on some heads recently which had been skimmed considerably, rear water housing was proud of head face as were valves. We calculated CR by measuring all various volumes and was surprised that the CR was still within reasonable limits. Would be more concerned about valve to piston clearance and getting inlet manifold to bolt up correctly. Engine was running ok until gasket failure due to corroded waterway

PaulW

[Tdm]

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I have actually had my Heads skimmed twice but I could not now have them skimmed again. The 1st time they were skimmed (after a HG failure) only a minimum amount was taken off & the work was done at a Head Skimming specialist in Mochdre, North Wales. The 2nd time was more recently in Tenerife after another HG failure. They did an excellent job (that was all they did - skim heads) and afterwards I fitted thick Payen gaskets from Paddocks. Touch wood everything is now O.K. and I hope no more Gasket failures. My Oil pressure is good, engine temperature is good, the car runs good, sounds good, and also looks good, and I believe in the old maximum - if it aint bust - do not touch it.

[DF961]

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I had the same concerns when I had a Dolomite 1850 head skimmed. I decided the safest way to determine if there was clearance between the valves and the pistons was to put a layer of plasticine about three mm thick (Not important) and then fit the head and bolt it down. It meant setting up the cam chain etc to enable me to turn the engine over gently by hand feelingfor the first signs of resistance of which happily there were not any in this case. I then stripped the head off and checked the impressions in the plasticine which showed that there was clearance between valves and piston crowns.

I wasted a head gasket doing this because I did not re use the one in the test but it was worth the expense for the peace of mind gained from the exercise. The engine cost a lot to rebuild as I'm sure yours is and although I considered taking measurements I can get them wrong, this was fool proof; proof of the pudding etc.

Bear in mind if you go down the route of measuring, eg checking the positionof the top of the piston in the bore versus the protusion of the valves into it -less the thickness of the head gasket,and thisresults inmarginal clearance, there may be expansion of both these components in a running engine which you are not allowing for due to heat. A couple of millimeters of plasticine at the bottom of the valve impression confirms you will have enough.

Just an idea.Regards Dick

[Ian Durrant]

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I think Dicks advice is spot on. The valves being above the head face is to my mind only a really rough guide. For a start this is slightly a function of the depth to which the valve seats are cut.

I am certain that with the current flat top pistons, compression ratio is only likely to be a problem if you are at maximum overbore *and* maximum skim of the heads. Of course if you are getting towards the upper limits, running on 97 octane is probably a good idea.

However, much more important on the stag engine is piston to valve clearance. What i'd really like to know is what the minimum (cold) piston to valve clearance is!

[Klaus Schlueter]

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jpyke wrote:

2. Theres a measurement from the cam bearing to the head face. I assume this is with the heads on a plane surface and the measurement done at 90 degrees to the surface. Trouble is with the faces being a bit lumpy (bits of old gasket etc - hence a skim needed) how can you hope to get an accurate measurement?

Hi jpyke

Ifound this in the techn. reprints page 28 (by John Clayton). Hope it helps.

Regards Klaus

[jpyke]

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Hi Klaus, that is exactly the information I was looking for, thanks very much indeed!

James

[DF961]

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Hi again James. Appreciate Klaus has given you the answer you were looking for. I'm just trying to upload a picture of the plasticine excercise to see if I can master uploading photo's - now lets see what happens!

[DF961]

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Well that seemed to work okay! Here are a couple morejust to show that a Dolomite 1850 engine is very much half a Stag engine. Just think a V8 1850 would be 3.7 litres!Regards Dick

[Dieter H. Marschall]

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Hello,

Is this the same head as in the TR7?

Kind regards, Dieter.

[jpyke]

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Yeah, and imagine having 2 sprint heads to go on it. 3.7 litres and 32 valves, now that would be a powerful motor. (not so fun grinding the values in though!)