Are the Duo-Bloc & Type 21 ball valves interchangeable?
They are not interchangeable in sizes 1/2″-2″. Dimensionally they are different as well as the “union nuts” are of a different thread and will not screw on one another. Sizes 2 1/2″-4″ are dimensionally identical and also share the same union nuts,therefore they are interchangeable in those sizes only.