cvh's will have a 48.8cc combustion chamber (including gasket) zetecs have a 50cc combustion chamber including gasket, using pieXd you can work out how much lower it will be with the 1.5mm off the piston crown, do it all in centemeters 8.45x3.14=26.53c squared x .15= 3.978 or 4cc more with the skim, so your combustion chamber is 52.8cc now on a 500 cc cylilnder, easy!