This injector seal ring sits between the injector body and the cylinder head in the host engine. It prevents combustion gases from escaping past the injector and stops engine oil or coolant from entering the combustion chamber. A failed seal causes a loss of compression, visible as a sooty deposit around the injector base, and can lead to rough idle or hard starting. The ring is manufactured to the same cross-section and material specification as the original engine assembly, so it seats correctly in the injector bore without additional shimming or sealant. On replacement, clean the injector bore thoroughly and lubricate the ring before installation.