This hydraulic cylinder O-ring seals the piston-to-rod interface inside the cylinder barrel, preventing pressurized oil from leaking past the piston during extension and retraction strokes. It sits in the groove machined into the piston or rod gland and maintains a static or dynamic seal against the cylinder wall. A worn or extruded O-ring causes internal bypass, reducing cylinder force and allowing drift under load. On reinstallation, ensure the groove is clean and the O-ring is lubricated with hydraulic fluid to prevent rolling or pinching during assembly.