This O-ring seals the joint between two stationary components in the engine cooling circuit, preventing coolant or oil leakage at the connection point. It sits in a machined groove on the mating face of the housing or fitting, where its cross-section compresses to form a static seal. A degraded or hardened O-ring allows fluid to weep past the seal, leading to gradual coolant loss and potential overheating of the engine. The manufacturer machines this component to the original groove dimensions so that the seal compresses to the correct squeeze ratio for the joint.