Filling void between 6" SS durablack pipe and 8" clay thimble

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Jan 29, 2021
VA, east central
I have a masonry chimney w/ clay 8" thimble and am putting a Harman p68 in to take over the job of a Buck Model 81. Initially I'm just running the vent into the chimney. I bought a 6" x 12" section of SS durablack pipe to go through the thimble. I'll be connecting my 3" durablack PVP pipe to this using the M&G DuraVent 3PVP-X6 adapter for the time being. In the next month or two, I'll be running a 6" SS liner down the chimney. I want a 6" liner because my effective vent length is too long if I used a 4" liner AND I want to be able to swap the buck stove back in if ever needed, I'm not getting rid of that.

I've looked for a sleeve or similar product to allow a tight seal between the 6" pipe and the thimble but haven't found anything. What is the acceptable way of filling that space? Is there a high temp insulation that I can pack into it and maybe use trim rings on the back and front within the thimble to support the pipe?