The cylinder head is one of the most complex casting parts in the car. It must combine inlet and outlet ports for up to five valves, hollow cavities for the motor control unit, the cooling system, cam chains and perhaps single spark ignition coils, drillings for lubrication fuel and sometimes even diesel oil and the upper part of the combustion chamber in one component. Furthermore, it should have a good thermal conductivity, form stability and a low thermal expansion. Especially for the diesel engine it should stand high pressures. If the engine is equipped with wet linings or if it is air-cooled it also provides for the necessary pressure of the cylinder in the crankcase.
The in-line engine and 15° respective 10.6 °-VR engine uses, in principle, only one cylinder head. All the other designs need two cylinder heads. The radial engines may even feature more than two cylinder heads. Utility vehicles and very strong diesel engines may even have one cylinder head per single cylinder. If there is more than one cylinder bank the number of camshafts doubles, too. The cylinder head usually consists of an aluminium alloy, sealing the combustion chamber upwards. Most of the heat develops there during the combustion process. It must be led away by cooling channels that are situated nearby.
Aluminium fulfils all the requirements for the cylinder head, except for the low thermal expansion. The thermal movement compared with the cylinder block must be compensated by the head gasket and the mounting.If, for example, the cooling system breaks down, or there is another reason why the engine becomes overheated, the cylinder head can contort and become leaky. Small contortions, clearly less than 0.5 millimetres, can be compensated by suitable sanding. According to the material decrease a bigger head gasket must be inserted.
Avoiding too much tension in the cylinder head, a certain order of screwing must be applied (and, actually, also while unscrewing them). In principle, the cylinder head should be tightened first in the middle, and then slowly towards the outside. Some manufacturers stress this point by drawing a spiral in which the single bolt heads are arranged in tightening sequence. Of course, the torque and possibly a rotary angle must be exactly considered at the end. Excessive tightening can lead among other things to a spanning of the cylinder block and subsequently to a rattling of the pistons, even if it consists of grey iron.
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