This o-ring seals the gearbox housing at the shaft passage or cover joint, preventing lubricant leakage and keeping contaminants out of the gear train. It sits in a machined groove on the gearbox casing or cover face. A hardened or flattened o-ring allows oil seepage, which can lead to low lubricant level and accelerated gear wear. The manufacturer machines the seal to the original cross-section and hardness specification, which maintains the correct compression in the groove for a leak-free joint. Replace the o-ring whenever the gearbox cover or shaft housing is opened; clean the groove thoroughly before installation.