Is there a sleeve, or something, to pass 6" pipe through 8" clay thimble?

[DeadTreeBurner]

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?

 

[begreen]

Try 1" stove rope gasket.

 

[DeadTreeBurner]

Try 1" stove rope gasket.

Thanks, that was gonna be my last ditch effort if I couldn't find anything else. I wanted to make sure I wasn't missing anything that might have been preferred.
This whole process of looking for venting etc. to move to a pellet stove with existing masonry chimney has been frustrating. I decided on Duravent pellet vent pro, but they don't make it obvious as to what to use to transition properly from 3" PVP to a 6" liner. They have an adapter that seems like it might be intended to transition to a liner tee. Looks like it should work, I'll just need high temp silicone to ensure a tight connection.