I have a 1930 Sumbeam Cabinet Heater (Fox Furnace) and I want to upgrade my stove pipe to double wall. I have a huge problem ! My flue coming out of the stove measures 6 5/16" inside diameter with an outside diameter of 6 3/4" which angles up to 7 5/16". My grandfather installed the stove and used 7" pipe in the install. The stove flue he said was male. When this stove was made people were smarter than today with all their airtight stoves and knew how to dry wood back in the day. This is a coal stove I said and he just looked at me like I was one of them. Now I would like to put in double wall pipe so I can take the heat shield down from the ceiling. but trying to find out to start it with the existing flue is driving me crazy. Do I have to go "back in the day" or can I bring my stove pipe into the 20th century?