VIton Group Head Gasket/Seal 71x56.3x9.2mm Conical

Compared to most other elastomers, Viton is better able to withstand high temperature, while simultaneously retaining its good mechanical properties. Oil and chemical resistance

Ideally, they should be replaced every 6 months, as rubber becomes brittle with the constant cycle of heating and cooling. Once the rubber becomes brittle, the group head may start leaking around the portafilter, the portafilter will generally start traveling past perpendicular and the seal becomes increasingly hard to remove.