on a 2wd cossie head, what was the original thickness and what is minimum useable on a turbo'd car ( have been told 138mm ). i have also been told that sometimes the heads are skimmed at a slight angle to keep the compression down if this is true where is the best place to measure from?

cheers

 

start out at just over 139mm total height (4x4 certainly are 139.1/139.2), measured top face to bottom face (measure in a few places). Quoted minimum is 138.68, practically you can get away with a tad less..
It's not really compression that's the issue, it's the edge of the inlet valve seat inserts getting close to the machined face.

 

Pic shows the potential problem area:

 

cheers mate