Wood Stove to Pellet Stove

[DB79]

Hi there. I'm new to pellet stoves. I am converting from wood stove to pellet stove. I have an internal chimney, in the middle of the house. I am attempting to connect my 3" piping to the 8" wood stove opening. I have it capped off right now and I measure the male part of the cap and it measure 7 15/16". I have tried a couple different parts to got from the the chimney opening to the 3" pipe. Nothing is working. Does any one have a suggestion?

Most recently I bought this: https://www.northerntool.com/produc...teel-model-3pvl-x8-4840576#hotbar-description but it turns out the 8" piece is closer to 9".

Thanks in advance.