Don't overlook this small but critical part! This is a genuine, brand new BMW radiator pressure cap, part number 17117639020 (also 17-11-7-639-020). It's an official OEM part, not an aftermarket copy. A faulty radiator cap can cause your cooling system to not pressurize properly, leading to overheating, or it can cause coolant loss. This direct replacement ensures your BMW's cooling system maintains the correct pressure for efficient operation. It replaces the older part number 17-11-7-521-071. Includes a 1-year manufacturer warranty. Please note: The listing specifies that only the part shown as #9 in the reference diagram is included (the cap itself). An easy, affordable piece of preventative maintenance.