Rocker arms (featured in all OHV (overhead valve) engines transmit lifter movement (via pushrods) to the valves. Because the rocker arm is essentially a lever, the length of the rocker (pushrod contact point to pivot) increases valve lift relative to cam lobe lift. In other words, the rocker’s length provides a ratio-multiplier that “magnifies” the lift of the cam lobe to create effective valve lift.
ROCKER ARM RATIO: Ratio is increased by shortening the distance between the rocker’s fulcrum point to the pushrod cup, increasing the leverage effect of the rocker arm fulcrum point to the valve.
Determining gross lift (valve lift) involves considering both the camshaft lobe lift and the rocker arm ratio. The ratio of the rocker arm (length of fulcrum) increases the effective valve lift beyond just the camshaft’s lobe lift.