Replacement pressure cap for the coolant expansion tank - the cap's spring and seal weaken with age and heat cycling, which lets the cooling system lose pressure and run hotter than it should. Cheap insurance against overheating; BMW recommends renewing it whenever the cooling system is opened up for other service.

Fitment: broad BMW coverage across E30, E31, E32, E34, E36, E38, E39, E46, E52, E53, E65, E66, E83 chassis plus Z3/Z4, spanning 1.8L-6.0L engines - confirm against your specific chassis/engine with the vehicle selector.