Understanding the Overhead Valve System

An overhead valve (OHV) system is a common type of valve train in internal combustion engines. In this design, the intake and exhaust valves are located in the cylinder head, above the combustion chamber. This design contrasts with older flathead engines where valves were in the engine block.

Cylinder Head's Role in OHV Engines

The cylinder head is a crucial part of the engine. It sits on top of the engine block, sealing the combustion chamber. In an overhead valve system, the cylinder head houses the valves themselves. Because the valves are in the head, the mechanism that operates them is also typically located on or supported by the cylinder head.

Components Supported by the Cylinder Head

In an overhead valve system, the motion to open and close the valves originates from the camshaft, which is usually located in the engine block. This motion is transferred upwards through pushrods to rocker arms. The rocker arms pivot and press down on the valves, opening them.

The rocker arms need a pivot point. This pivot point is provided by the rocker shaft assembly. The rocker shaft assembly consists of a shaft (or shafts) that the rocker arms pivot on, often held in place by supports or pedestals.

Since the rocker arms directly interact with the valves in the cylinder head, and receive motion from pushrods coming up towards the head, the logical place to support the rocker arms' pivot point (the rocker shaft assembly) is on the cylinder head itself. The cylinder head provides a stable base for the rocker shaft, ensuring precise valve operation.

Why Other Options are Not Supported by the Cylinder Head

• Crankshaft: The crankshaft is located at the bottom of the engine block. It converts the linear motion of the pistons into rotational motion. It is supported by main bearings within the engine block, not the cylinder head.
• Oil pan: The oil pan is located at the very bottom of the engine block, acting as a reservoir for engine oil. It is bolted to the underside of the engine block, not the cylinder head.
• Oil pump: The oil pump circulates oil throughout the engine for lubrication and cooling. Its location varies depending on the engine design, but it is typically driven by the crankshaft or camshaft and mounted on the engine block or within the oil pan area, not on the cylinder head.

Summary: Cylinder Head Support

Based on the design of an overhead valve system, where the valves are in the cylinder head and operated by rocker arms, the cylinder head is designed to provide the necessary support for the components directly involved in valve operation at that level. The rocker shaft assembly serves as the pivot point for the rocker arms and is therefore supported by the cylinder head.

Revision Table: Overhead Valve System Components

Let's quickly review the relationship between the cylinder head and other engine components in an OHV system:

• The cylinder head contains the valves.
• The rocker arms operate the valves.
• The rocker arms pivot on the rocker shaft assembly.
• Therefore, the cylinder head must support the rocker shaft assembly.

Additional Information: Valve Train Components

The valve train is the system that controls the opening and closing of the intake and exhaust valves. In a typical OHV system, it includes:

• Camshaft: Usually in the engine block, it has lobes that determine valve timing and lift.
• Lifters (or Tappets): Ride on the camshaft lobes and transfer motion upwards.
• Pushrods: Long rods that transfer motion from the lifters to the rocker arms.
• Rocker Arms: Pivot on the rocker shaft assembly to push on the valve stems.
• Valves: (Intake and Exhaust) Located in the cylinder head, they open and close ports to control gas flow.
• Valve Springs: Close the valves after the rocker arm pressure is released.
• Rocker Shaft Assembly: Provides the pivot point for the rocker arms, mounted on the cylinder head.

Understanding how these components interact helps clarify why the rocker shaft assembly is supported by the cylinder head in an overhead valve system.