EXPANSION TANK CAP O-RING; S6. 28.5 x 3.8mm. A circular disk covered in rubber or plastic with a hole in the center where the edge is sometimes tensioned by a spring that prevents fluid from leaking at the Engine Coolant Expansion Tank Cap. A circular disk covered in rubber or plastic with a hole in the center where the edge is sometimes tensioned by a spring that prevents fluid from leaking at the Engine Coolant Expansion Tank Hose. A component which provides a sealing surface at the Engine Coolant Overflow Reservoir Cap. Engine Coolant Reservoir Seal. O-Ring. Part serviced with reservoir cap. Seal used on the Radiator. Gasket. Overflow Hose. Pipe O - Ring. washer. Radiator Air. Included with: Cap, Cover, Reservoir cap.