This thermostat ring sits between the engine block and the thermostat housing, forming the seal that prevents coolant bypass around the thermostat. It maintains the correct pressure differential across the thermostat, ensuring the engine reaches and holds its operating temperature. A degraded or missing ring allows coolant to flow past the thermostat continuously, causing the engine to run below target temperature, which reduces combustion efficiency and increases fuel consumption. The ring is molded to match the original seal groove geometry so it seats without distortion and does not require additional sealant. On reinstallation, clean the mating surfaces and verify the ring is fully seated before tightening the housing bolts.