Genuine OEM Hyundai Radiator Filler Neck Assembly, part number 25329J9000. This assembly serves as the primary access point for filling the radiator with coolant and is crucial for maintaining the sealed and pressurized environment of the engine cooling system. It integrates into the cooling hose network. Using a genuine OEM filler neck ensures a leak-proof seal, proper pressure regulation, and compatibility with the cooling system's design, preventing overheating and coolant loss.