The piston will actually be 65mm if that is was it is supposed to be. You can't accurately measure a bore with a pair of calipers as the measuring edges wind up bridging inside bore. You need a bore gauge, ID micrometer, or a set of telescoping bore tools to accurately measure a bore. In any case the bore should be larger than OD of the piston by an amount that is dicatated by the engine design, bore type (nikasil, chrome, iron, etc.), ring type, and piston composition. :geek: