I have recently had my cylinder head on my 2wd sapph skimmed and it measure's to be 138.50. I now its under the reccomended limit, But i heard about a saver shim for the cosworth that i can get hold of. what im wanting to know is are they a good under all that pressure and do they work properly with no hassle. Does anyone reccomend them or use them and has had any problems. cheers

 

The "problem" is that the exhaust port seats fall out when running lots of boost big bhp on heads that have been skimmed too much.
Personally I wouldn't worry at your head thickness unless you're going for 500+ and tracking it etc ?
The seats can be stitched in if necessary.
tabetha

 

but what im asking is is it worth getting a saver shim to put the compression right and to be on the save side from any piston to valve contact.

 

I've never know it be needed yet tbh from a head skim, don't worry about CR, the amount really is insignificant in relation to the sept volume of the piston, head/volume, won't cause a problem.
tabetha