Coefficient of linear expansion for Al is 1/0.0000208 per deg C Cast Fe is 1/0.0000108 per deg C piston crown gets to 500 deg C; so 500 x 0.0000208 =0.0104 or 1% piston diameter 76mm... 1% is 0.76mm or 30 thou!!!!! This is 30 times 1 'Loren'....(8oD) Miser Math[s] is Good Math[s]