The spring must be retained under normal operating temperatures. They must not crack under high temperature and pressure ranges. Rings are generally of spherical graphite cast iron because of the strength and limited self lubricating properties. With modern long stroke engines the rings do considerably more rubbing than equivalent sections of the liner and so the rubbing faces are usually made slightly harder. This is achieved by a case hardening process (usually Nitriding) some rings are contoured on the rubbing face in order to promote faster running in. Copper or carbon coatings are sometimes provided for the same purpose. When running in cylinder l.o. is increased to provide an additional flow to carry away metallic particles and a straight mineral oil without antiwear properties is used.
The ring axial depth must be sufficient to provide a good seal against the liner but it must not be so great so that an oil wedge does not form. The ring actually distorts in the groove to form the wedge but if they are too deep they cannot do so. Thin rings will distort easily and scrape the oil from the surface. Radial depth must be sufficient to allow adequate support for the ring in the groove when the ring is on max. normal wear for its self and the liner.
Rings must be free in their grooves and the correct clearance is required. Excessive clearance can allow rings to twist while insufficient clearance can cause jamming and prevent the gas pressure from acting behind the rings. Also the rings may tend to twist excessively. Radial clearance must be sufficient between groove and ring back to allow a gas cushion to build up. The butt clearance must be sufficient to allow for thermal expansion. If insufficient the rings may seize and if excessive can lead to excessive blowpast
Grooves are sometimes coated with chromium to restrict deposit build up. For reconditioning the bottom face of the groove is generally provided with a replaceable steel wear ring.
As the rings maintain the gas seal there is a desire to position the top or firing ring as close to the piston crown as possible. However ,since the crown is highly stressed, thermally, this results in distortion of that zone. There is thus a desire to position the ring a long distance away from the crown. A compromise position is decided upon in each engine design.
In order to minimise wear, a film of lubricating oil must be maintained between the moving parts i.e. the rings and liner, and rings and groove. Also the lubricating oil must spread over the liner surface by the rings, this helps to combat acidic products of combustion.
Skirts fitted to pistons on some designs perform the function of sealing the exhaust ports at T.D.C. these extended skirts have bronze rubbing rings inset to provide a bearing surface during the running in period.