Face To Face
In this and future issues we will look at one of the most critical seals in a race engine, the seal between the top of the cylinder bore and the cylinder head.
There are of course many ways of achieving this seal, but perhaps the first question we should ask ourselves is whether or not we need to have a joint which needs sealing in the first place.
Race Engine Technology magazine is soon to feature an in depth review of a historic racing engine which did away with this joint by casting the cylinder liners integrally with the cylinder head. This engine was the Hart 415T four-cylinder Formula One turbo engine of the early 1980s.
Without pre-empting the magazine article, the Hart was produced in this way because the cylinder pressures were so high that it was deemed the most feasible solution despite the inherent difficulties which this solution presents.
If though we assume that we have a separate cylinder head, what choices do we have? Well, we can still ask if we need a gasket. Will two parallel surfaces with a sufficiently good surface finish seal against combustion gasses if the clamping pressure between them is high enough?
The answer of course depends on various factors such as the medium to be sealed, clamping pressure, component stiffness, surface profile (whilst hot and clamped) and surface finish. Certainly it is not unheard of for manufacturers to eliminate conventional gasket technology, a classic example of this would be brass sealing rings. These are typically 8 mm deep and 3 mm in radial thickness and locate into a recess at the top of the cylinder liner. Sealing is then achieved purely by the flat face at the top of the sealing ring pressing against the fire face.
Whilst brass is an obvious material choice due to its ability to conduct heat away from the combustion chamber, it is heavy and difficult to achieve a perfect surface finish with. There is no act of parliament which says sealing rings have to be made from brass; steel is one perfectly viable alternative, and is easier to grind and polish to a low Ra value.
For sealing ring technology to operate well requires a high standard of machining and finishing of both the cylinder head and the sealing ring, and it also requires that the engine is not abused. Overheating the cylinder head for example could warp the fire face to the extent that the seal will no longer operate.
Sealing rings are almost universally used in conjunction with O-rings which seal the oil and water passages.
In future issues we will look at developments in head gasket technology, and examine another innovative sealing method from the recent past.
Written by Tom Sharp.