The cylinder head is the metal casing that covers the engine. The cylinder head seals the main cylinders of the engine and prevents oil and other substances from getting inside. The also contains a number of valves which are vital to the functionality of the engine. Internally the cylinder head has passages known as ports for the air / fuel mixture which travel to the inlet valves from the intake manifold, for exhaust gases to travel from the exhaust valves over to the exhaust manifold and for the antifreeze to cool the engine and the head. A damaged or cracked cylinder head can cause serious damage to an engine overall so it must always be good functioning condition.