Large exhaust valves are provided with detachable seats made form molybdenum steel. The main parts of the valve casing is of cast iron and water cooled, there being no particular strength requirement for this part. It is the seat area which is subject to high temperatures and wear, hence the use of better

Rocker operation of valves presents certain problems:

1. A tappet clearance must be allowed to suit thermal expansion of the valve stem. Insufficient clearance may result in the valve not fully closing whilst excessive clearance can effect timing and the period the valve is open. Hammering will also take place at the tappet face. (The use of self adjusting hydraulic tappets went some way to alleviating these problems)
2. A side thrust is imparted to the valve stem when the valve opens. Thus causing wear at the stem bushing and subsequent leakage
   
3. Wear takes place at the pushrod and valve contact face, additionally the rocker bearings require constant lubrication.

 

To avoid these problems hydraulic valve actuation is used there is no tappet and no tappet clearance to set. Thermal expansion is accounted for by allowing the oil to escape at a relief valve on the pump unit. Oil loss is made up at the pump unit from the cam lube oil supply system. The opening face is always axial. Note: The hydraulic pipe must be sheathed to avoid the risk of fire in the event of pipe failure.

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