Originally Posted by Stu @ M Developments

Brad,
Its a misconception that the head becomes "Too thin, or weak" and also that its about the altered "Compression Ratio"

The issue is the area around the valve seat becomes dangerously thin after 138.68mm. As a last resort the head can be diagonally skimmed if the damage is around the exhaust side, but a new casting is of course the proffessionals prefferred choice.