(G) Head Skimming

Morning.

I am intreeged as to what head skimming is.

Could someone explain to me:-

What the head is? (is it head gasket?)

What will head skimming do?

And how would I go about it?

Thankyou.

I'll be bk on this afternoon to see the verdict.



Aj x

The cylinder head is the top part of the engine. It sits on top of the engine block. Where the two join there is a gakset. This is the head gasket. The head and block both need flat, straight, clean surfaces. If an engine has over heated the head can twist or warp (very very slightly, we are talking fractions of a millimetre). Or corrosion can wear away some of the face of the cylinder head. If either of these happen then you need to remove the head, clean it up and then cut a very small amount off the face of the clyinder head. This is called skimming, as the tool only just skims the surface being cut.

But you can use skimming to aid performance. If you skim a certain (calculated) amount off the head, when you re-fit the head the space that the air / petrol mix is compressed into (the combustion chamber) is smaller, and this means the compression ratio is higher, and as a general rule a higher CR gives slightly more power. But you must work it out, and you must not go too high.

So, there you are, two reasons for skimming. Cost for a light skim is about £30 ish.

Ohh thankyou. was a very detailed explanation. and now know what it is mite think bout it for future prehaps. .


Thankyou!


Aj x

Good stuff always wanted to know that!

has your head gasket blown? you dont just take head off for the sake of it and get it skimmed

heh.. urm no..not that i know of was just wandering what it was. because its seems more people are getting it done..heh.


Aj x

you dont have to worry about it, it only needs doing if your head has to come off after a blown gasket, rare on your engine

hah. cheers for that. (cute bunnie)


Aj x

ive got loads of them http://www.fiatforum.com/gallery/sho...ry.php?cat=855

What about cam bearings, if the head warps, surely these are going to move abit, but still no one does out about them

my last 899cc head gasket went . but then i crash so no need to fix .

Is actually not that hard at all, to get the head off .

Ming

Tis true, unless you want/need to increase the compression ratio, in which case head skimming is one way to do it.

One of engineering's great mysteries, but I suspect cam to bearing tolerances are less critical than mating face tolerances. After all, one is a gas seal, the other a floating fluid tolerance.

Two reasons, one, checking the cam bearings for 'in-line-ness' is very hard to do, compaired with decking and mic'ing up a head. And then line boring is the only remedy, and then you wreck the hardened bearing faces and then you need oversized bearings (specialy made I would guess).

Secondly, think about the axis the cam is in relative to the head, if a head is REALLY badly warped, the main twisting action will be diagonal corner to corner, lets say 1mm. This distortion will be along the line of the cam, so the head is rotated about this axis, so therefore the cam will still be relativly straight. Very hard to explain. Try again. Get a show box (the head) stick a knitting needle through it (the cam) twist the box. Has the needle bent? No. And this is how heads tend to distort, one corner lifts (well from my experience) you don't get side to side bending as the hardened steel cam tends to re-inforce the softer alloy head. Make sense?

if it aint broke, dont try to fix it!!