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The cylinder head is fairly easy to understand and service. Normal wear will involve valves, valve seats and guides. Because it is important that the heat from the red-hot exhaust valve is shed as quickly as possible, we must make certain that valve faces and seats are in good condition to encourage adequate heat transfer.
While the engine is running, the exhaust valve is closed approximately two thirds of the time, thus allowing heat to escape from the valve face to the seat and thence to the engine coolant. The remaining heat is transferred to the coolant via the valve stem and its guide.
Grinding-in the valves during decarbonisation is a basic operation, but we must bear in mind a seat that is too narrow will be unable to absorb sufficient heat from the head of the valve resulting in rapid wear of both valve face and seat.
OK, so why not make the valve face and seat as wide as possible? Well, too wide a seat is unable to get rid of the inevitable formation of carbon deposits very well, meaning that after a while the valves cannot seat properly and may result in burned seats and valve faces.
It’s not only the valve face that wears; rocker arms and valve-stem tips also suffer. Carbon, acids and moisture, by-products of combustion, also take their toll in this area.
Valve timing may become retarded as a result of wear in the valve train and too much valve lash and we must not forget that retarded ignition timing will raise combustion temperature, as will pre-ignition and detonation which may be caused by a build-up of carbon in the combustion chamber.
Valve stems need looking at too. They wear at the top of the guides and at the bottom. Too much valve lash may bring high-velocity valve seating as can too much valve-spring pressure, meaning rapid valve and seat wear.
The answer here is simple.
Use the correct springs and ensure that valve clearances are spot on. In severe cases, a badly seated valve may allow the exceedingly hot gases to bypass it and do a good imitation of a welding torch on it! It has been known for valves to crack or even break up when this has been allowed to happen.
We had a case recently of high oil consumption, caused by excessive wear in the guides and valve stems. You will probably find hardened and cracked stem seals when wear in the guides or stems area is detected. Naturally, these will be renewed automatically when doing a head job.
RATHER USE NEW VALVE SPRINGS
The vacuum formed on the intake stroke may draw oil past the worn guide and valve stems if the seals are worn or missing.
If you are tempted to re-use the old valve springs, first compare them against new ones. If there is hardly any difference in height, you may go ahead, but I would always opt for new ones. For what they cost, is it worth the risk not to?
Remember that a valve spring that is tilted will bring uneven seating, meaning that perfect closure is impossible. Valve springs are precision-made components and are ground flat at each end and any tilting is bad news. - Star Motoring