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Head needs skimming ! is this OK?

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My machine shop just called and said the head needs skimming, 2/100 of a mm, is this OK ?

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Ask why. Deep grooves or scratches?

 

If they say its warped I would say dont get it skimmed.

 

Buddsy

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I'd be amazed if they can machine to an accuracy of 2/100's of a mm.  

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And I'd be amazed if they could measure it, too. I think a confirmation of the measurement might be in order first.

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I've been trying to call him again to comfirm the measurment, but he did say it was due to deep scratches, failed a presure test on glass !!

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2/100 of a mm is 0.02mm just under a thou in old money. 

 

Pressure test on glass? Did you have a head gasket failure before? If not Id say the gasket should take up any scratches. I would advise you to avoid a skim if possible or else you could run into cam binding issues. 

 

Buddsy

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I usually say "Do Not skim the head unless absolutely necessary!"

 

That's just based on my experience of 2 different machinists who messed up my head, causing me to have to fix it myself...

 

Generally the 910 head is pretty robust, unless it is warped or has a groove, it shouldn't be machined.

 

If it needs machining, then it is very important to ensure that the head surface is still parallel with the cam tower mating surface!!!

The cam tower surfaces may then need to be machined relative to the head surface after it is machined.

 

The skimming limit for the 910 head is around .015".  The later gasket is about .018" thicker than the original, so the compression won't change much, and the timing won't be retarded. 

 

Machining the head, and then the cam towers, would change the valve clearances, so you'd need to remeasure and get new shims.

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They skimmed off 0.14mm, nice job. also polished in the valves. I found one of the buckets to be cracked, never seen this before, managed to find one with the same thickness from the other motor. Considering the valves have been out and polished into the seats, there was not a lot of difference when I checked the clearance, only need to order 2 shims :-)

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my exhaust cam followers had started to crack.  I replaced them all.

 

IMG_2587.jpg

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Interesting about the cracks on your cam followers. A couple of years ago I was doing a spirited run when the engine made an awful noise and I pulled over immediately. When I investigated a couple of the cam followers had broken up and the bits had damaged the overhead cam but the real damage was the tops of the valves were hitting the cam directly and were unusable. A lengthy engine rebuild then followed.

 

Most people said they had never heard of these followers cracking but talking to the experts, it seems that until the S4 they were all made of cast iron and vulnerable to cracking and breaking up in dramatic style. I was told that whenever taking the head off it was always advisable to change them for the later steel ones as a matter of course as the cracks were not always visible to the eye.

 

I found the best place to buy them was Burton Power and I managed to negotiate a discount by phoning them and ordering all 16.

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