I have defiant encore (not sure exact age, but mid 80s). I had to replace the oval-round adapter as it had rusted and I can't fit the new one into an also new chimney thimble. It's not even close. I measured the oval-round adapter (which is vermont castings) and it looks to be more than 8" in diameter. Is this normal? Do I need to get a special VC brand chimney thimble or something?