Part # 5125148
Fuel Cap Gasket Inner

Tank cap rubber gasket, R60/7 - R100RT.  The old gas caps used to come with a red bottom gasket, which sealed gasoline. It was never available as a replacement part until about 1985 when BMW made a black sealing ring available. This is the gasket for the actual cap. It replaces the old red gasket that breaks down. This ring goes right over the threads to where the red ring was on the bottom of the cap.