A top every engine block sits a mechanical assembly that serves as traffic cop letting fresh air and fuel enter the engine cylinders and exhaust gases exit. This device is the cylinder head, and it opens/closes to let these gases come and go as needed to keep the engine running.
The cylinder head consists of a casting, typically made from aluminum or iron that bolts to the top of the engine block, closing off the tops of the cylinders in which the pistons move. The head can be for a single cylinder or for multiple cylinders. Each head is attached to the block using studs, bolts, or a combination of the two. Between the two is a high-temp gasket used to seal the piston bores and water passages. On a water cooled engine, water passages incorporated in the head are used to route coolant to/from the block and/or intake manifold.
Intake ports or passages on the side of the head mate to the intake manifold. This is where the incoming fuel mixture enters the head. When an intake valve in the head is opened, the incoming fuel charge flows from the intake port through the valve opening in the combustion chamber. The intake valve closes so cylinder compression and firing can take place. Once the spark condition has occurred, an exhaust valve opens, allowing the spent gases to exit the head into the exhaust manifold.
For many decades heads used only two valves per cylinder: one intake and one exhaust. With the push for higher performance and efficiency, designers found that using two smaller valves to the one larger valve increases flow into and out of the head. As such, heads have been designed and are in production with two, three, four and five valves per cylinder.
Each valve is shaped like a thin steel rod with a tapered circular disc at its end. The rod portion slides up and down in a tubular guide in the head as the valve opens and closes. The circular disc sits on a valve seat when closed to seal the intake or exhaust opening and then lifts off the seat as needed to open the valve.
A rocker arm is often used to push on the end of the valve stem to open it. The rocker gets mechanical input from an elliptic lobe on a rotating camshaft so that the lobe pushes and causes the valve to open and close as the engine rotates. A spring on each valve rod keeps the valve closed whenever the rocker is not pushing to open it.
At 6,000 rpm, an engine is asking each valve to open and close 50 times per second. That's a lot of fast mechanical action. Proper, constant lubrication of the valve train is needed to keep the contact surfaces cool and retard wear. If too light a valve spring is used then the valve may not return to its close position quick enough as engine speed increases. This is referred to as "valve float" and prevents complete cylinder sealing for the combustion process. Use of higher rate springs help prevent this problem at high engine speeds, but using overly stiff springs can increase valve train wear.
As a cylinder head ages, there are a number of wear mechanisms that may need to be addressed during a rebuild. The valve face and its mating seat may be worn or pitted causing the sealing juncture to be poor. The valve guide that the valve stem slides in can become loose. As the rocker pushes on the valve stem, there is a slight scuffing motion that loads the stem sideways. Over time, this can cause the valve guide to wear. A worn guide (along with a drying valve stem seal) can start to allow oil from the top of the head to seep down the guide into the valve area. When the engine is started after sitting a while, puffs of blue-white exhaust smoke may signify this aging.