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The cylinder head gasket

seals-gasketsAs many might know to your cost, the cylinder head gasket is high up among those engine components to suffer the greatest stress. Often a weak point in some of the best engine designs, its unenviable task is to provide a robust seal between the cylinder head and corresponding crankcase for the combustion gases, oils and coolant, both between each other and the outside atmosphere. If that were not difficult enough, the component also acts in distributing the dynamic loads between the head and block and as a consequence, has a considerable influence on the forces due to combustion and the corresponding distortions that result.

In the world of engineering however, if something is continually presenting a problem, the first course of action is to get rid of it. Such was the thinking of Brian Hart in the Formula One turbo days when the monobloc Hart 415T engine was designed. Casting the cylinder head and crankcase in one piece certainly eliminated the head gasket but must have presented a much wider range of engineering issues in the process. History doesn't record how successful this exercise was but it certainly wasn't the first time it had been tried nor to my knowledge was it repeated thereafter. Around the same time and in the much less glorious surroundings, experiments were taking place using conventional aluminium wet liner technology to discard the cylinder head gasket in another way. Increasing the liner 'stand-off' above the top deck to greater than that normally used and at the same time increasing the cylinder head clamping loads, created a successful seal at the top of the liner past which combustion gases could not pass. To take care of the coolants and oil, the gap between the fire face of the cylinder head and the top deck of the block was bonded together using anaerobic adhesives with an 'O' ring seal for the oil feed transfer hole. Successful in prototype engines, the idea was never truly likely to catch on in the real world outside the R&D lab but the potential was always seen to exist even if removing the cylinder head thereafter was likely to cause all manner of issues.

We are perhaps all familiar with the older style metal/soft material gasket technology. A simple central supporting plate onto which is either side a thin layer of soft material, the combustion chamber is sealed by a simple metal eyelet, which also protects the soft material from the combustion gases. In newer versions of these the water jacket sealing is assisted by small beads of elastomer carefully positioned around each aperture such that water leakage is now rarely an issue. Coatings can also be deposited over the whole of the gasket surface to prevent sticking and assist sealing at a micro level. While these are popular for many older engine applications, lightweight engine designs demanding lower clamping loads with less distortion have moved towards single piece metal-elastomer gaskets. Elastomer sealing lips fitted into an alloy or a stainless steel carrier layer with reinforcement around the combustion zone, the clamping loads can be much more accurately controlled.

For the best in high performance cylinder head gasket technology however, multi-piece metal gaskets are now the norm. Consisting of a number of beaded metal carriers with elastomer coatings the exact design is bespoke to the particular application. Designed to pay particular attention to the dynamic events around the combustion chamber seal, improving its fatigue life, these do however generally require a much better surface finish on both the cylinder head fire face and crankcase top deck, than previous designs.

Fig. 1 - Modern multi-piece metal gasket.

Written by John Coxon

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