This o-ring seals a connection point inside the gearbox housing, preventing lubricant leakage between stationary and rotating components. It sits in a machined groove on a shaft or housing face, maintaining a static seal under normal operating pressures and temperatures. A degraded or compressed o-ring allows oil to seep past the seal face, leading to a visible drip or a gradual drop in gearbox oil level. The manufacturer specifies the cross-section and durometer to match the original seal groove geometry, so the o-ring seats correctly without being pinched or rolled during installation.