This O-ring seals the turbocharger coupling on the host tractor, preventing pressurized air and oil from escaping at the joint between the turbocharger outlet and the intake piping. It sits in the groove at the coupling flange; a compromised seal here allows boost pressure to bleed off, reducing engine power and causing a lean air-fuel mixture. The ring is molded to the original cross-section and groove geometry specified by the manufacturer, so it seats correctly without additional sealant or shimming. On reinstallation, clean the mating surfaces and verify the ring is not twisted in its groove before tightening the coupling bolts.