Thimble Question

[sawjo]

I have a free standing Dutchwest woodstove in my basement family room that has its own chimney, The chimney is 8"X8" terra-cota lined and is outside. I have been venting the stove with a 6" to 7" step right out of the stove because the thimble is 7" masonary. This year I am going with 6" stove pipe all the way from the stove thru the thimble and I will get a new thimble trim ring. Any suggestions for the air space as the 6" stove pipe goes into the 7" thimble? I was thinking of wrapping the "thru thimble" section with stove gasket rope to make up the difference. I woud rather not cement something in to facilitate easy changes for new stove pipe. Any ideas?