I quote:-
To ensure efficient compression and expansion of the working fluid, the rotor has to be sealed across the apices between the rotor faces and around the sides; and while the system of side seals presents few if any difficulties, the seals at the tip of the tri-pointed rotor (known as the 'apex seals') present some of the biggest issues.
But as far as seal materials go, the choice these days is very much down to the housing material and specific application. On Norton race motorcycles fitted with rotor housings made from high-strength, high-silicon aluminium alloy (LM13), a two-piece plain cast-iron apex seal is used. This runs directly against a traditional nickel-silicon carbide surface with few if any reported issues. In applications where cast-iron housings are used - perhaps for reasons of cost or durability and where the extra weight is not much of a handicap - the material of choice will most likely be a form of heat-treated acicular cast-iron for normal road-type motoring. It's a higher strength, more shock-resistant material than other cast irons but one that shouldn't be used at temperatures higher than about 300 C.
However, for the serious racer the only, an albeit somewhat expensive option is to use a fibre-reinforced ceramic apex seal of a type used in the 1991 Le Mans-winning Mazda.
