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/ Categories: Archive, engine-structure

"Rings of Fire"

seals-gasketsWhat better way to introduce the subject of fire rings than to refer to the song by the late Johnny Cash? But apart from the imagery it produces, sadly it would seem to give little further insight into the subject of sealing technology. However, the joint between the combustion chamber and the cylinder cannot be disregarded in the same off-hand manner. Differing materials and new methods of construction have all enabled the traditional cylinder head gasket to progress over the years, but when combustion pressures get higher and / or the relative expansion becomes greater, the solutions offered to give a reliable seal, need to be more robust.

Throughout history experiments have always taken place questioning why a purpose-designed seal for this part of the engine should ever be needed. Surely the narrow rim of a hard cast iron or steel cylinder liner, abutting the soft aluminium surface of the fire face on a cylinder head should be sufficient to seal the gap? And indeed sometimes this can be the case, but in doing so the damage caused visible only after dismantling can make the cylinder head unserviceable again. Way back, the 4-cylinder 2.5-litre Formula One Vanwall engine, instead of using a normal gasket, solved the problem using a separate sealing component around each cylinder bore. Designed and made by the Coopers Mechanical Joint Ltd in England, Coopers Rings as they were inevitably called, consisted of a pack of narrow steel rings, some flat and some corrugated, each 0.010 in. (0.25 mm) or 0.015 in (0.38 mm) thick, encased within a spun and folded outer casing. Made from stainless steel and Nimonic material, these were inserted into the groove formed at the top of the liner when it was located into the cylinder block. Rectangular in shape, these compressed from their manufactured size of around 0.126 in (3.2 mm) to 0.092-0.094 in (2.34 - 2.39 mm) when the cylinder head was finally fully 'torqued' into position. At the combustion pressures of the time, and apart from when the liners were bottom mounted rather than hung from a spigot at the top of the block, this design was reported to work with 100% reliability. So it is therefore little wonder that seals of this type were used and would continue to be used in many engines of the period. So in 1963 while Johnny Cash was 'ringing' out the first few bars of his song, the Coopers joint was winning all before it.

seals-gaskets-coopers-joint

Nevertheless, the complexity of its manufacture and inevitable extra cost, eventually forced the business to close and made way for the cheaper and some say, technically superior, metal 'O' ring technology. Originally developed by the Wills family in Bridgewater, England and now known irrespective of manufacturer the world over as Wills Rings, these were, and still are essentially coated or uncoated thin tubes formed into circular rings. Made from copper, mild or stainless steel, and pressurized from within with nitrogen, stainless steel versions can withstand up to 800 deg C. For the most arduous of applications, Inconel is considered the best choice. Designed to conform into an oval, or more correctly, 'race track' shape in service, the circular cross-section makes it much more tolerant to the amount of crush necessary to ensure a positive seal around its whole circumference.

Today however, in many engines both historic and contemporary, the preferred material for any fire ring has to be solid copper beryllium, but this is one, I think, we'll leave for another time.

Fig. 1 - Cross-section of the Cooper's Mechanical Joint

Written by John Coxon

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