Which of the following components is usually found in a valve train assembly?

In a valve train assembly, the push rod plays a critical role in the operation of an internal combustion engine. It serves as a mechanical link between the camshaft and the valves, transmitting the motion generated by the camshaft to the rocker arms, which then open and close the engine's intake and exhaust valves. This process is vital for proper engine operation, as it regulates the flow of air and fuel into the combustion chamber and the expulsion of exhaust gases.

The other components listed, such as the crankshaft, connecting rod, and flywheel, are essential to the engine's overall function but do not belong to the valve train assembly. The crankshaft converts linear motion from the pistons into rotational motion, while the connecting rods link the pistons to the crankshaft. The flywheel, on the other hand, helps maintain engine momentum and smoothens the operation of the engine. Therefore, the push rod is uniquely associated with the valve train assembly, making it the correct choice.