This arrangement denotes an engine with a single cam per head . The cam actuates rocker arms that press down on the valves, opening them.
Springs return the valves to their closed position.
These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms. On single and double overhead cam engines, the cams are driven by the crankshaft, via either a belt or chain called the timing belt or timing chain . These belts and chains need to be replaced or adjusted at regular intervals.