No, it is not recommended to mount an expansion vessel upside down. Normally, the expansion vessel is mounted so that the connection points downwards. This allows air that accumulates in the system to collect in the upper part of the expansion vessel and escape through air vents or other mechanisms. If the expansion vessel is turned upside down, air could accumulate in the system and affect the proper operation of the water cooling system. There is also a risk of leakage or malfunction if the expansion vessel is not installed according to the manufacturer's instructions.