One of the advances is with the development of four valves for each cylinder. Having the four valves gives a larger opening for the gasses to passes to pass when entering and leaving the combustion chamber. Due to the number of openings required for the valves, four-stroke cylinder heads are of a complex shape and for this reason spheroidal graphite cast iron is a suitable material for the manufacturer, since it is relatively easy to cast. A modern cylinder head for an engine operating on heavy fuel is shown in figure 2.47. Such a cylinder head should have: