So my pipe goes out the cement foundation downstairs. The thimble is 8 inches. I noticed the clay looking thimble is cracked and also chipping away. My neighbor suggested looking for what he calls a "donut" because to him that is what it looks like. It is an 8 to 6 reduction that sits inside the thimble and allows your 6 inch pipe to go INSIDE the so called donut. And you can stick your 6 inch stove pipe into this donut as far as you want. What is this called and where can I find? Thanks.