We recently purchased a Fisher Grandpa Bear stove with a rear pipe. Our problem is we have been unable to get and 8 to 6 adapter to fit. We have tried an 8 inch to fit over the outside if the collar but is just a shade to small. An 8 inch with the crinkled edges (sorry, don't know the technical term) is to big to fit in. We are getting very frustrated and now have $35 in unusable parts. Any advice? The collar is 8 inches and we have a 6 in chimney liner.