Recently bought a used Buck model 81 non-Cat that is about 13 years old. Great fit as an insert for our well-hearthed fireplace but despite a 13x13 chimney opening (that we need to line), the damper only has about 5" of opening and that is if you go in at the 45 degree angle that the door slants on. It is built in to the chimney (or so it would appear, no movement and no visible attaching hardware when I get in there with my headlamp and mess around with it.) I have heard some options ranging from sawzall (cut the whole thing out)to cutting torch to an offset grinder. I like the idea of the grinder to just get a small half-circle cut in on the non-door part so I can fit a 6" flex through it, but I defer to the experts here for comment and suggestion. Thank you in advance from someone who has been lurking and benefitting from the info here for a while.