Jim,
I had the same barrier with my installation. It was a 6" flexible liner though a 5" opening for the damper. You have a much bigger size difference, so I don't know if this is practical, but here is what I did.
1. I pulled the liner down the chimney to the back of the damper, reached up and attempted to squeeze it into an oval. Between my weakness and the angle I was working from, I couldn't budge the shape at all.
2. Cut up some 1x2" lumber into 2 8" pieces. I used 2 8" threaded rods on either end and made sort of a "clamp" to squeeze the liner between the two 1x2"s. One end had a double nut with a lock washer in between and a large fender washer between that and the wood. The other end I chiseled out a nut shaped hole, and threaded the rod through.
Code:
.
X X <---double nut
___ ___ <--- fender washer
---------------------------- <---- 1x2 wood
X X <-----threaded rod
X X
X X
X X
X X
X X
----------------------------- <1x2 with nut buried in the wood so it won't turn.
3. Went to HD and got that Skill power ratchet. I use the dang thing all the time now, it is a little rechargeable 1/4" drive ratchet with a motor in it.
4. Reached up in the damper and put my clamp contraption on the liner. I used the power ratchet to alternately turn the double nutted end of the threaded rods and slowly squeezed the liner into an oval. That rachet had pleanty of power to squeeze the liner. I suppose you could do it with a standard rachet or nut driver, but it would take a lot longer.
5. This crushed a couple inches at a time, so I had to loosen the clamp, pull down some liner through the damper and repeat the ovalizing.
3. Finally, I was able to use the same clamp to squeeze the end of the liner back into a circle and connect it to the flue collar coupling.
I hope this helps, I can take a picture of my contraption if need be.
