Usually on a normal cylinder head say Vauxhall as an example, a good whack can be skimmed off on the lathe, as they have inication marks on the cylinder head to show how far it can be skimmed before any piston to valve contact, if the full amount was taken off it would raise the ratio a good bit and would give a noticable difference to performance, When i removed my starlet head from my spare engine, the top of the piston inserted into the head ever so slightly as the piston sat a few millimetres above the block, I didnt want to risk skimming the starlet head as there seemed to me, to be more chance of internal contact, plus two of the valves were close to the face of the head. If the head could be skimmed alot it would make a difference but it looks like there is only enough to compensate for head warpage from head gasket failure Gary