Learn What a Rocker Arm does in a Car Engine

Author : Rocker Locks | Published On : 11 May 2021

The rocker arm in an internal combustion engine changes radial movement into linear movement; as a reciprocating lever, this kind of device is properly referred to. Turning it into the up-and-down movement, Opening and closing the valves, takes the spinning motion of the overhead camshaft. As they are typically made of steel, rocker arms falling off have a great deal of strength for their weight, and hence can exert a great deal of leverage.


Same as the outside end, the cam end of the rocker arm moves up and down, so does its opposite side. Cam lobe lifts the outside of the rocker arm when it comes round, which rocks and the inner end pushes down on the valve stem, opening it against its spring. The outside drops as the lobe rotate away from the rocker arm, the inner end lifts and the valve’s spring closes it. In this certain way, with the rotation of the camshaft, the opening and closing of the valve are kept in sync, and thus with the movement of the pistons. Collectively, rocker arm, the cam lobe, follower valve, and valve lifters are known as the valve train.


Rocker arms have ratios and the ratio is a measure of the arm’s leverage, which tells you how powerful the force is that it can transfer to the valve stem. Between its center of rotation and the top of the valve stem, by the length of the arm, the ratio is determined, both between its center of rotation and the point it contacts the cam lobe. The ratio of a typical automotive rocker is 1:1.5, which indicates that it moves the valve one and a half times further than the peak of the cam lobe projects up from the camshaft.

Cam Lobe

The camshaft has bulges on it called lobes and is not a true spherical rod. From one side of the shaft the lobes project outward whereas the back of the shaft opposite the lobe does not have a projection. The side without the projection allows it to fall back as the side with the lobe lifts the outside end of the rocker arm as the shaft rotates. By a device called a cam follower, the cam’s movement is transferred to the rocker arm and it has different types that vary by manufacturer and by vehicle age. Lifted and lowered as the camshaft spins, the part of the rocker arm acted upon by the cam follower is lifted and lowered.


Indicating that insufficient lubrication oil is being delivered to the rocker arm and Lash adjusters, the top end of the engine makes a tapping noise. To keep the camshaft area lubricated most of the oil delivered to the top end of the engine is present but much less is used for the rocker arms. Top-end tapping is typically an early warning that indicates that there is an issue developing with either the mechanical parts or with the lubrication system and there is a strong recommendation for prompt service.