The Medium Speed 4 Stroke Trunk Piston
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.