The most common reasons for using a pushrod that is longer or shorter than stock are: a decked cylinder block or heads, thicker/thinner head gaskets, small base-circle camshafts, aftermarket rocker arms, longer-length valves, valve jobs with varying seat depth, non-stock lifter designs and/or seat height, and converting to an adjustable valvetrain. These variables inevitably alter pushrod length; however, following these easy steps will help prevent improper geo-metry and ensure your engine's longevity.

SOURCE
COMP Cams