A method of selectively matching at least one diameter of a valve spool or piston of a spool valve to at least one diameter of a valve bore in an automatic transmission valve body reduces hydraulic fluid leakage in the valve body. The method includes the steps of fabricating a spool valve having at least one spool or piston, measuring a diameter of the at least one spool or piston and identifying the spool valve as belonging to certain size class, fabricating a valve body having a plurality of machined bores, measuring the diameter of at least one portion of a bore and, based on this measurement, installing a spool valve of the appropriate size class in the bore of the valve body.