Oil seals and gaskets Oil has a habit of creeping through small openings into areas where it is not wanted and, to contain it, the engine has a number of oil seals and gaskets.

 A gasket is simply a piece of soft, oil-resistant material that is sandwiched between metal joints to prevent oil seeping out. The gasket may be made of cork, a composition material, or, in some instances, may be a silicone-based compound that is squeezed from a tube.

Virtually all engines have a cork or composition gasket between the valve cover and the cylinder head. If oil leaks from this joint, it is possible that the gasket has been dam- aged during fitting, or is not properly located. Many engine oil leaks from the valve cover are caused by the fixing screws or bolts being over- tightened, distorting the cover by bending it into the soft gasket material. Straightening the valve cover and fitting a new gasket will make it oil-tight.

Where metal surfaces have to make close contact and yet provide an oil seal, some car manufacturers specify a thin layer of mastic sealant on the mating surfaces. During tightening, any excess sealant is squeezed out, leaving a very thin film behind to discourage oil seepage. Oil seals are necessary where oil must be prevented from entering or leaving a moving component.

The stems of valves at their upper end work in an atmosphere where oil is being continually dripped or squirted on to the camshaft or rockers that operate the valves. It is important that excess oil cannot dribble down the valve stems into the combustion chambers because burning it will dramatically increase oil consumption.

To prevent this happening valve stems have oil seals. Depending on the engine, the seal may be in the form of a tight-fitting 0-ring, an inverted mushroom shape threaded on the stem, or a spring-loaded seal clipped to the top of the valve guide and spring-loaded to hold its sealing lip against the sliding valve stem.

All perform the same function—they deflect, or wipe, most of the oil from the valve stem to prevent it being burned in the combustion chamber. A small film of oil does get through, and this lubricates the valve guides.

s particularly important that oil does not reach the clutch linings and cause clutch slip, so the rear of the crankshaft passes through some form of oil seal, where it emerges from the crankcase behind the flywheel.

Engines with road draught crankcase ventilation systems quite often use a scroll.

This is a spiral groove machined on the crankshaft surface that works in a close-fitting housing and ‘winds’ the oil back into the engine as the shaft rotates. Frequently, a disc fitted to the crankshaft, on the engine side of the scroll, is used as an oil thrower, to spin off excessive amounts of oil and prevent it reaching the scroll.

With the increasingly widespread use of closed circuit crankcase brea-thing systems, which can cause a small build-up of pressure in the crankcase, neoprene lip seals are more usually fitted to seal both ends of the crankshaft. The lip seal is moulded into a metal casing that is pressed into a register.

At the rear of the crankcase, a recess is pro- vided for the seal, and the soft lip around its inner circumference rubs against a machined face on the shaft. The lip faces towards the oil, and a circular coil spring around the lip acts like a rubber band and presses it against the rotating shaft.

 Lip seals rely on making contact with a perfectly smooth moving surface in order to pro- vide an oil-tight seal—any damage to the rotating shaft surface will cause an oil leak.