20 to 25 mm. According to the BMW official website, the pipe of the BMW x6 heater water valve is 20 to 25 mm. BMW defines the X6 as a SAC, which means an all-around coupe. Compared with the X5, which is positioned to focus on highway driving performance, the X6 has evolved more thoroughly in terms of highway performance, integrating the sports capabilities of a sports car with the multi-functionality of an SUV in terms of appearance design and power control.