Valve train kinematics — in an overhead-valve or overhead-cam engine, the motion imparted by the rotating cam lobes is transmitted to the valves primarily through which component?

Introduction / Context:
The valve train converts camshaft rotation into precise valve lift and timing so that intake and exhaust events occur when required. Depending on the engine layout, this motion can be transmitted through lifters, pushrods, and rocker arms, or directly by cam followers and finger rockers in overhead-cam designs. Understanding which component delivers motion to the valve helps in diagnostics and design discussions.

Given Data / Assumptions:

• A typical production engine with either overhead valve (cam in block) or overhead camshaft architecture.
• Cam lobe displacement needs leverage and direction change to depress the valve.
• Terminology distinguishes actuators from passive structural elements like stems.

Concept / Approach:

Rocker arms act as levers that translate and often amplify the cam or follower motion to open the valve against spring force. In pushrod engines, the cam lifts a tappet and pushrod which then moves the rocker arm. In many OHC engines, a finger follower or rocker directly contacts the cam lobe and the valve stem. Pistons do not transmit valve motion, and the camshaft pulley merely drives the cam via a belt or chain. Valve stems are parts of the valves themselves and are moved by the rocker, not by the cam directly in most common arrangements that use rockers.

Step-by-Step Solution:

Verification / Alternative check:

Exploded diagrams and service manuals for both OHV and OHC engines label the rocker arm as the element that transmits motion to the valve in the majority of designs that are not direct-acting bucket tappet types.

Why Other Options Are Wrong:

Pistons control cylinder pressure, not valve motion. Camshaft pulley only provides drive timing. Valve stems are the receiving members, not the transmitting lever.

Common Pitfalls:

Confusing direct-acting bucket tappets with rocker systems; even there, the bucket is a follower, not a pulley or piston.

Final Answer:

rocker arms