skimming the head

[Steve in Cairns]

Just wondering if it is worth skimming the head to increase the compression as the head is in the shop having some work done. I have three questions and asking for your advice.


1. Is it worth it?
2. How much do I have taken off?
3. How can I be sure that this hasn't been done before?


Thanks
Steve

[chazza]

Just wondering if it is worth skimming the head to increase the compression as the head is in the shop having some work done. I have three questions and asking for your advice.


1. Is it worth it?
2. How much do I have taken off?
3. How can I be sure that this hasn't been done before?


Thanks
Steve

1. It is worth doing if the head is not flat, so that you can be sure that there will be no head gasket leaks. Get them to check the top of the block as well.

2. The absolute minimum. If yours is the low compression head then the compression ratio cannot be raised to a significant amount anyway, without destroying it. If you want a higher compression ratio the, 8:1 head was used on the S3 and has an 8 stamped on the square boss near the centre of the manifolds.

3. Unless someone has an original drawing of the head or a brand new one to measure, I don't know. Too much metal removed apparently allows the coolant passages to get too close to the face.

Make sure that they crack test it before they do any work to it - a friend of mine had 5 heads fail the crack-test recently, including one of mine!

If you want more oomph out of this engine, I think the best way may be to use a supercharger, as the low compression ratio is ideal,

Cheers Charlie

[JDNSW]

The above is not quite accurate. Early heads, which cannot support significant shaving were 7:1. Later heads are identified by a square boss on top between the rockerre cover and the carburetter. This should be stamped with the original compression ratio, either 7 or 8.

Unfortunately, there is no easy way of being certain whether it has been skimmed since manufacture, but either will stand increasing up to about 9:1 or even higher, although above 8 the engine may become intolerant of everyday fuel.

The only certain way of determining the present ratio is to measure the current combustion chamber capacity by filling it with a measured quantity of liquid (upside down on bench), including any space in the cylinder above piston, and taking the ratio of this the the cylinder displacement. This will also allow calculation of the amount to skim to increase the ratio by the desired amount.

John