Thimble reducer

So it's almost that time of year again and I'm working on replacing my thimble adapter and stove pipe to double wall 22 gauge and putting in the clean out Tee.
It turns out my thimble is 8 1/2 in.

So my two questions are:

Is this an odd size? I thought they are usually 8"?
What is the best way to reduce down to 8 or 7"? I've found reducers but not for 8 1/2" only for 8.?

Regards,
Valter

Valter, not realy sure what you mean--but--this is my old wood burning beast in the back of the house--6" black pipe going into 8" clay thimble--I've got fiberglass insulation stuffed around 6" pipe in thimble & then a beauty ring for cosmetics??? hope this helps???

Fred,
Fiberglass I can do thanks for the tip.

Regards,
Valter