This O-ring seals the piston-to-rod interface inside a hydraulic cylinder, preventing fluid bypass under pressure. It sits in the groove machined into the cylinder end cap or piston face. A degraded or nicked O-ring causes internal leakage, which reduces cylinder holding force and can produce drift in the implement or header it supports. The manufacturer specifies the cross-section and durometer to match the original seal groove geometry, so the ring compresses correctly without extrusion. On reinstallation, inspect the groove for burrs and lubricate the O-ring before assembly.