The O-ring K 27211 seals the interface between the injector nozzle and its holder, preventing high-pressure fuel from bypassing the nozzle seat. Typically specified from a fuel-resistant, heat-stable elastomer, this ring must retain elasticity under pressure pulses and temperature swings to preserve injection precision. Because O-rings are wearable, routine checks for hardening, cuts or compression set — and prompt replacement with the correct material grade — are simple but high-impact actions that keep combustion consistent and avoid fuel leaks.