It’s about a 9 inch by 9 inch hole. 8 inch chimney and pipe is 8 inch into 7. Stove piping is about .040 inch thick. Tension of the wall ring is and height of the stove pipe holds the 8 to 7 inch reducer in place. Has a 1 to 2 inches of some type concrete but not a crock. This concrete rings appears to sit in some of material that looks to be a pale white material not sure if it’s wood but kind of looks like it’s not. House built in 1980. Thanks.