I have an old Franklin fireplace with bi-fold doors and 3 legs. The cast iron collar that accepts black stove pipe is an odd size. It sits on top of the stove, tapers upward from oval to round, and contains the damper. The round top is tapered, and uncrimped 8" pipe fits OVER it. But isn't the pipe supposed to fit INSIDE the collar, preventing drips? The inside diameter is 7 1/2", and no adapter I've yet found fits that spec. If I install the pipe outside the collar and seal it with furnace cement, will that work? Thanks for your comments. Tim