See " nominated head thickness charts" on this site.
These specifications are useful in determining the amount of stock removal from the face of the cylinder head. Some OHC heads are machined on both faces and examination has to determine which face has had excess machined from it.
The use of head shims and cambox / tower shims can restore the head to within useable thickness. As well as compression ratios being raised above safe levels, the rocker arm geometry, cam belt or chain tension can be adversely effected.
If head shims are not available some heads will be unserviceable as severe detonation will occur when the head face is excessively machined.