This hydraulic pump O-ring seals the interface between the pump housing and its mounting face, preventing pressurized fluid from leaking externally. It sits in a machined groove on the pump body, where its cross-section compresses against the mating surface to maintain a leak-free joint. A degraded or hardened O-ring allows oil seepage, which reduces system pressure and can lead to cavitation in the pump inlet. The manufacturer machines the seal groove to a specific depth and width, and this O-ring matches that geometry so the seal compresses to the correct height without over-squeezing. On installation, clean the groove thoroughly and lubricate the O-ring with clean hydraulic fluid to prevent rolling during assembly.