Rocker arm

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On an engine a rocker arm is a lever that transmits the motion of the camshaft to open and close the valves.

The rocker arm pivots on either a roller shaft or a ball pivot. One end is raised and lowered by the rotating lobes of the camshaft (either directly or via a lifter (tappet) and pushrod, while the other end acts on the valve stem. When the camshaft lobe raises the outside of the arm, the inside presses down on the valve stem, opening the valve. When the outside of the arm is lowered by the camshaft, the inside rises, allowing the valve spring to close the valve.

The effective leverage of the arm (and thus the force it can exert on the valve stem) is determined by the rocker arm ratio, the ratio of the distance from the rocker arm's center of rotation to the tip divided by the distance from the center of rotation to the point acted on by the camshaft or pushrod.

For car engines the rocker arms are generally steel stampings, providing a reasonable balance of cost economy, strength, and weight. Because the rocker arms are part of the reciprocating weight of the engine, excessive mass limits the engine's ability to reach high operating speeds.

Truck engines (mostly Diesel) use stronger and stiffer rocker arms made of cast iron (usually ductile), or forged carbon steel.