The piston/plunger o-ring is considered a wear item. From the factory, the FiftyShot™ piston is equipped with a silicone o-ring. Silicone provides moderate resistance to chemicals but will degrade rather quickly from cleaning. We manufacture a much more durable O-ring made of Viton. Viton has excellent resistance to acids, fuels, mineral and vegetable oils and greases, aliphatic, aromatic and chlorinated hydrocarbons, non-flammable hydraulic fluids (HFD) and many organic solvents and chemicals. Viton also has excellent resistance to aging and ozone. Low gas permeability, low compression set.