CYLINDER HEAD ASSEMBLY

The cylinder head assembly is comprised of the cylinder head and the valve mechanism. The valve mechanism is driven by the timing gears. Let's begin by breaking down the components and discussing them in detail. 

1. Cylinder Head

The cylinder head is bolted to the machined surface on the top of the cylinder block. Its function is to enclose the top of the cylinders. Both the top of the cylinder block and the bottom of the cylinder head must be machined smooth to allow for an air-tight seal. The cylinder head fits directly over the cylinders and forms the combustion chamber of each cylinder. In these chambers, the engine burns air and fuel to produce mechanical energy. 

THE CYLINDER HEAD ASSEMBLY

You can also see that several small holes are cut into the bottom of the cylinder head. When the cylinder head is bolted onto the cylinder block, the holes are aligned which allows oil and water to pass from the cylinder block into the cylinder head. The oil is then circulated back through the block while the water leaves the engine through the large holes on the front of the cylinder head.

Image below shows the cylinder head in position to be installed on the cylinder block. A head gasket placed between the cylinder head and cylinder block ensures an airtight seal. Now that you have enclosed the cylinders and formed the combustion chambers, you must have a way to allow the air or air-fuel mixture to enter and exhaust (burned gases) to exit while keeping the combustion chamber closed as the air-fuel mixture burns. All this is done with the intake and exhaust valves. 

ENGINE BLOCK AND CYLINDER HEAD

2. Valves

To have heat energy, you must burn air and fuel. After you have burned the fuel, you must get rid of the burned gases. Ports are provided in the engine for this purpose. In some engines, valves must open and close these ports at a given time to allow raw fuel to enter and burned gases to leave the combustion chamber. While the fuel is being burned, these valves help to seal the combustion chamber to allow the heat energy to move the piston.

The valve is a long-stemmed metal piece with a circular top known as the valve head. The mechanism required to operate the valve consists of a valve guide, a valve spring, a valve spring retainer, valve spring locks, a camshaft, and timing gears or sprockets and chain.

VALVE MECHANISM

On the underside of the valve head is the valve face. This face is machined smooth and when the valve is in the closed position, it seats firmly in the valve seat. The valve seat is a metal ring pressed into the cylinder block (or the cylinder head, depending upon the engine design) around the valve port. The valve face and the valve seat are machined smooth to ensure a pressure-tight fit. It is this portion of the valve mechanism that seals the ports while the fuel is being burned. The valve must not be allowed to wobble while it is opening and closing or it will not seat properly and pressure will be lost. To prevent the valve from wobbling, a long tube called the valve guide is pressed or cast into the cylinder block and/or cylinder head. The valve stem travels up and down inside the valve guides as the valve opens and closes. A valve spring closes the valve. The spring seats against the cylinder block (or cylinder head) and the valve spring retainer. 

The retainer is a washer-like device fitted over the end of the valve stem and held in place by two valve spring locks. Therefore, the valve spring is attached to the end of the valve stem. One end of the spring maintains a constant pressure against the valve stem and the other end maintains pressure against the block (or cylinder head). Any time the valve is not forced open by an outside force, the valve spring will keep it closed.

3. Camshaft

The camshaft provides the outside force to open the valves. It is a long shaft that extends through the length of the cylinder block and it has a series of egg-shaped lobes (cam lobes).  The camshaft, driven by a gear mechanism attached to both it and the crankshaft, rotates once for every two revolutions of the crankshaft on a four-stroke cycle engine.

4. Timing Gears

This brings us to the timing gears (or sprockets). The timing gears are a set of two gears (driving and driven). The driving gear is attached to the front end of the crankshaft. The driven gear is attached to the front end of the camshaft. When the gears are installed on the shafts, they must be installed in a certain position in relation to one another, thereby causing each cam lobe to open its valve at precisely the right time to allow fuel to enter or burned gases to leave the combustion chamber. Aligning the timing marks during gear installation accomplishes this. 

TIMING GEARS

In some cases, because of the distance between the camshaft and the crankshaft, sprockets and a timing chain are used instead of timing gears that mesh directly. The only real difference between the sprockets and the gears is that the crankshaft drives the camshaft using a chain instead of a direct gear drive. 

SPROCKET AND TIMING CHAIN