I am currently trying to install a timberline fireplace insert stove with a flue on the top of the firebox...I currently have a double walled chimmney and the stove pipe has in ID of 9". I went out and bought an increaser 8" ot 9" and can't get it to fit into the opening of the woodstove...I took a measurement with my tape and found out that it was 7.75 " ID how can I attach this pre-existing 9" chimmney to this opening?