Rocker arm

A rocker arm is a pivoting lever that forms one of the final links in the chain of components responsible for opening an engine's valves. Positioned in the cylinder head, it rocks back and forth on a fixed pivot, with one end driven by the engine's valve-actuating mechanism and the other end pressing down on the stem of a valve. Its purpose is to translate the motion supplied by the camshaft into the precise opening and closing of the intake and exhaust valves that lets an engine breathe.

The way the rocker arm receives its motion depends on the engine's layout. In an overhead-valve engine, where the camshaft sits low in the block, a pushrod travels up from a cam follower and lifts one end of the rocker, which pivots and pushes the valve open at its other end. In an overhead-camshaft engine, the camshaft is mounted directly above the valves in the head, and its lobes act on the rocker arm without any pushrod in between, sometimes through a roller to reduce friction. In both cases the rocker is the component that ultimately bears down on the valve stem and, as the cam lobe rotates away, lets the valve spring close the valve again.

One of the rocker arm's most useful properties is its ability to act as a mechanical lever and so multiply motion. Because the pivot divides the arm into two sides, the distances from the pivot to each end can be made unequal, giving a leverage ratio. A common arrangement produces a ratio greater than one to one, so that a given amount of cam lift at the input is amplified into more valve lift at the output. This lets engine designers achieve the desired valve opening with a smaller, gentler cam profile, and tuners sometimes fit higher-ratio rockers to increase lift and improve breathing.

The rocker arm operates within a system that must accommodate small dimensional changes and wear. The clearance between the rocker and the valve stem, the valve lash, has to be maintained correctly: too little and the valve may not close fully, too much and the valvetrain becomes noisy and the valve opens less than intended. Many engines manage this automatically using hydraulic valve adjusters, often built into the rockers or their pivots, which use pressurised oil to take up clearance silently and remove the need for periodic manual adjustment. Others retain solid arrangements that must be checked and shimmed at service intervals.

Materials and design have evolved considerably. Traditional rockers were stamped from steel or cast iron, while modern engines may use forged steel, aluminium or roller-tipped designs that cut friction and wear at the contact points. Whatever the construction, the rocker arm remains a small but pivotal part working in concert with the camshaft, the pushrods where fitted, the valve springs and the valves themselves, quietly performing thousands of strokes a minute to keep the engine charging and exhausting cleanly.