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Your Rocker cover & cylinder head worn out? » Read more about common malfunctions • Repair • Replacement manual
Top with a complicated inner life: the cylinder head
The cylinder head is situated above the cylinders on top of the cylinder block. It consists of several moving parts and must be able to withstand high pressure. Therefore, its construction is complex. High pressures can cause damage which can only be repaired by replacing the cylinder head.
Position and function of the cylinder head
The cylinder head sits on top of the cylinder shafts. It includes the following components:
- valve springs
- camshaft bearings
- rocker levers
- hydraulic tappets
- valve stem gaskets
- timing belt disc or timing chain pinion
Bringing together all those components together in such a narrow space requires a great deal of precision. Consequently, the installation and disassembly of the cylinder head is a demanding task. In addition to accommodating all these components, the cylinder head's main task is primarily to withstand the high compression and explosion forces from the combustion chambers. This requires a downright massive construction of this component.
Cylinder head defects
Consequence of engine overheating is usually a burnt cylinder head gasket which does not necessarily mean a damaged cylinder head, although its repair requires re-measuring the component. Often the cylinder head is slightly warped and once removed, it no longer fits on the cylinder block and must be levelled. Replacement is not necessary. A serious defect are cracks in the inter-valvular crosspieces. This defect is not visible from the outside, but becomes apparent through several symptoms:
- constant overpressure in the engine with bursting vent hoses
- increased oil consumption
- irregular engine run
- overpressure in the coolant circulation
This happens when the individual cylinders are standing too close. Cracked cylinder heads were a typical problem of VR6 engines and a regular cause of annoyance for the drivers of these sporty vehicles. A cylinder head is a die-cast component. Therefore it cannot be welded and must be replaced in case of crack or rupture.
Cylinder head replacement
Replacing a cylinder head requires its removal and complete disassembly. The components of the old head can, save from the gaskets, bearings and hydraulic tappets, be used again. Measuring the camshaft, valves and valve springs is nevertheless recommended. If signs of wear are observed, the occasion warrants replacement. This applies to the timing chain and tensioner or the timing belt with its tensioner roll and water pump as well.
Valve stem gaskets and the bearing cups of the camshaft are always replaced after removal.
Cylinder heads are usually levelled at construction. Nevertheless they should be profoundly measured prior to installation just to make sure. The appropriate tools are a cylinder head temperature gauge and a feeler gauge. Ultimately, the cylinder had must always be installed with new bolts. Cylinder head bolts are used only once. Once tightened with the correct torque, they expand. After disassembly and re-installation they can no longer attain the specified torque and break off. This can cause a total loss of the engine.
Cylinder head costs
A cylinder head is a very expensive component. Depending on type of car, prices ranging from 500 – 1500 EUR (± £450 – £1,340) are no exception. The components should be supplied "barebone", i.e. without any additional component. Its complete installation requires several special tools, such as:
- valve-spring compressor
- torque wrench
The valve-spring compressor is necessary for disassembly and installation of the valves. As a rule, a valve costs only 15 – 25 EUR (± £13 - £22) and therefore installing new valves along with the new head should be considered. The valves – whether used or new – should be ground in the valve seats. This requires grinding paste and the necessary experience. Removal and installation of the camshaft requires working with the torque wrench. It is essential to loosen the bolts in the right order and correctly tighten them on the new head. All in all the replacement of a complete cylinder head is a rather demanding task. Therefore it might be useful to test this repair on a used cylinder head first before working on a new part. This gives a better guarantee for a successful repair.