In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins.
Intake Over Exhaust (IOE) engines combined elements of the sidevalve and overhead valve designs.
Used extensively in American motorcycles in the early 1900s, the IOE engine remained in production in limited numbers until the 1990s.
In an overhead valve (OHV) or overhead camshaft (OHC) engine, the cylinder head contains several airflow passages called ports; intake ports deliver the fuel+air intake charge from the intake manifold to the combustion chamber, and exhaust ports route combustion waste gases out the combustion chamber to the exhaust manifold.
A single camshaft located in the engine block uses pushrods and rocker arms to actuate all the valves.
[2] DOHC designs allow optimal crossflow positioning of valves to provide higher-RPM operation.