This O-ring seals the interface between the hydraulic cylinder rod and its housing, preventing fluid leakage under operating pressure. It sits in the gland groove of the cylinder assembly, where its cross-section compresses to maintain a static seal against the rod surface. A degraded or hardened O-ring allows external leakage, which reduces system pressure and can cause erratic cylinder movement. The manufacturer's original compound matches the durometer and chemical resistance required by the hydraulic fluid in the parent machine. On reinstall, ensure the groove is clean and free of burrs before fitting the new ring.