Complex sentence: The hydraulic lifter consists of a hollow expanding piston situated between the camshaft and valve, either operated by a rocker mechanism or, in the case of double overhead camshafts, directly by the camshaft.
Split into simpler sentences: The hydraulic lifter consists of a hollow expanding piston situated between the camshaft and valve. It is operated either by a rocker mechanism, or in the case of dual overhead camshafts, directly by the camshaft.