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These represent approximately 6" gate valves in O scale. An actual 6" pipe (this is a nominal dimension) is 6-5/8" O.D. Since the same gate valve with a stud to fit the I.D. of Evergreen tubing (4b) means that the Evergreen tubing is 5/8" small .. that's .. "Good Nuff" for me in order to use the readily available tubing.