He just ovalized the round liner. I attached a pic. you can see- the 6” round (outlet of the stove) to oval adapter essentially transitions to a ovalized 8” pipe. if you were to just ovalize 6” pipe you’d create a restriction . you need to upsize it. With my fireplace the its not just the damper- the throat of the fireplace is actually narrow. It’s a true Rumsford with the curved cement leading up to the narrow throat. I didn’t want to start chipping away concrete.Thanks. Can you let me know if your installer used an oval liner or did he just ovalize the 8" round liner to fit thru the damper ?