The camshaft operated the valves via hydraulic bucket tappets; valve clearance adjustment is automatic. The camshafts act on four valves per cylinder via bucket tappets. Overhead camshaft engines use fingers or bucket tappets, upon which the cam lobes contact. Additionally, the camshaft, tappets and valve springs were also changed. Camshafts act directly on bucket-type tappets. Properly optimizing the system avoids undue stresses to the camshaft lobes and tappets. Compared with its M10 predecessor, the M40 uses a belt-driven camshaft, and hydraulic tappets. The engine was switched from solid-lifters to hydraulic tappets. The overhead valves are actuated through very short tappets and rocker arms. SOHC crossflow designs with a breakerless ignition system and hydraulic tappets. For example, shims are inserted into or under bucket tappets to control valve clearances. In this engine the valves were operated by bucket tappets. Compared to the M21 the M51 now has tappets and a hydraulic valve lash adjustment. Hydraulic tappets require more complex and more expensive cylinder head design. In 1987 it used a two-barrel Holley carburetor and hydraulic tappets.