O-Ring – Fuel Injector – Diesel – sealing ring in the fuel injector and pipe assembly on diesel engines.

The ring seals the injector joint against fuel under pressure, preventing external leakage and stopping air being drawn into the fuel circuit, which would show up as hard starting or a rough idle. It is a diesel fuel-circuit seal and is not used on petrol engines.

O-rings harden and take a permanent set in service. Fit new rings whenever an injector or its pipework is disturbed; do not refit used rings.