by 
The Curious Bag
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All engines have cylinder heads, which comprise about 20 percent of the total mass of the engine. These crucial components serve several important functions, which include housing the valve-train, controlling air/exhaust flow into and out of an engine, and focusing combustion pressure on the piston head.
Capping the Cylinder
Perhaps the most obvious function of cylinder heads is to provide a roof for the combustion chamber. Without a roof, the explosion of air and fuel inside your engine would simply shoot out of the cylinder instead of pushing the piston down.
Flow Control
Cylinder heads have intake and exhaust runners cast into them. The runner carry fresh air into the combustion chamber and used exhaust gases out.
The Valve-Train
The intake and exhaust runners terminate at the valves, which open a certain distance for a certain amount of time, to coordinate airflow into and out of the engine.
Camshaft Housing
The camshafts (which control the movement of the valves) on overhead cam engines are also housed in the cylinder head. There may be one or two per engine bank.
Cooling
Heads have cast-in cooling channels, which carry engine coolant over the incredibly hot combustion chamber roofs. Without these cooling channels, your engine would rapidly self-destruct.
Source:
Resource:
How to Port Cylinder Heads (Video)
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