Cylinder Head

Cylinder heads for 4 stroke engines are of a complex design. They have to house the inlet and exhaust valves, the fuel injector, the air start valve, relief valve and indicator cock. The passages for the inlet air and exhaust gas are incorporated, as are the cooling water passages and spaces.

Normally manufactured from spheroidal graphite or nodular cast iron which is easy to cast. Although not as strong as cast steel, which is difficult to cast into complex shapes due to its poor fluidity, it maintains a reasonable strength under load. Adequate cooling is essential to prevent thermal fatigue due to uneven expansion throughout the casting, and bore cooling has been introduced along with cooling spaces to ensure effective cooling of the "flame plate" (the underside of the cylinder head which forms the top of the combustion chamber).

Cracking of cylinder heads can occur due to poor cooling causing thermal fatigue. Poor cooling can be the result of scale build up within the cooling spaces due to inadequate water treatment. Overloading of the unit causing high peak pressures is also a cause as is incorrect tightening down of the cylinder head. Cracking normally occurs between the valve pockets and/or cooling water spaces. Cracked cylinder heads can be repaired by specialised welding; but this must be done under the guidance and with authorisation from the classification societies.