To maintain low surface temperatures in the combustion space and at the valve seat bore cooling is employed. The bore cooling passages are shown in figure 2.47. Four valves are usually employed on four-stroke engines. This configuration allows the designer to maximize the Cross Sectional Area (CSA) of the inlet and exhaust ports and so improve the flow through the cylinder. This arrangement results in more complicated valve actuation since the two exhaust and two inlet valves must each be operated by one push rod. Examples of the various ways that the valves are actuated can be seen in figure 2.48. All of the designs shown control the valves together,