Genuine OEM Volkswagen Coolant Expansion Tank Cap, part number 3D0121321B. This cap is designed to seal the coolant expansion tank, maintaining the correct pressure within the cooling system and preventing coolant evaporation or leakage. It is a critical component for regulating cooling system pressure. The cap is made of durable blue plastic and measures approximately 80mm in diameter. Using a genuine OEM cap ensures the correct pressure relief valve calibration and material resistance to high temperatures and corrosive coolants, vital for engine cooling efficiency and safety.