Fly cutting 964 heads vs compression ratio
 

Hi List,

I'm in the middle of two arguments. On the one hand I'm being advised that I should have my heads fly cut approx. 0.1mm. On the other I'm advised that this should never be done as it changes the compression ratio and the cylinder and heads should be lapped together. (whatever that entails).

I've attached a photo of one of the heads.

I'd be interested to have your input, please.

TIA

John

PS. I've posted on Rennlist as well.http://forums.pelicanparts.com/uploa...1522228128.jpg

FWIW, flycutting the head surface by .1mm will not significantly change the compression ratio. Anybody telling you differently has his mechanical knowledge from books with no practical experience. If anything you might find that your heads need to be cut a bit more to have the cutting tool touch every part of the surface due to slight warpage. Lapping after cutting the mating surface is not a bad idea and will not harm anything.
BTW, cutting the head surface by .1mm will increase your compression ratio about .05 points.

I should add that this is a 1992 gasketed engine

Thanks Ollies930.

Looks like your heads when media blasted, the sealing surfaces where blasted too, leaving a rough finish.

It should be smooth in those areas. Not sure how you'd lap the sealing ring groove.

Sorry, brainfart at 3:30am this morning. I was thinking turbo compression, not normally aspirated. Compression increase will be more like 0.1 point increase instead of 0.05(not that it really matters).

I hope I am not hijacking - sorry if I am.

Ollies930, is that the Ollies that does the machine work for cylinder heads?

Does the compression ratio change when the fly cut is done for the early 964 heads and the cylinders are machined for the ring?

Thanks, I can start another thread, however it is a very similar question.

Sorry, different Ollie. But to answer your question, anytime you machine the sealing surfaces, you will increase the compression ratio unless you put appropriately thicker base gaskets under the cylinders. However, the compression ratio change is minimal when surfacing heads. Not worth mentioning, unless you intentionally cut a substantial amount off.

John, which shop is saying machine and which is saying lap? I went through a similar thing.
I'm in Aus too.. feel free to PM.
Mike

seal surface
 

Just a cleanup cut will take care of that surface, you will not lap that surface in the picture, with the heads apart and cleaned it around a 200.00 to 300.00 job and done
Mike Bruns