Product review (3 stars): The round flange on the back of the vent hood ...
The round flange on the back of the vent hood has a 6" inside diameter.  This makes the outside diamater about 6 1/16".  The pipe I was fitting this onto has a 6" outside diameter and slightly small inside.  This made installation rather difficult as the pipe did not fit well over the flange.  I made a couple of small modifications to the hood flange and made it work.  From the outside no one could ever tell.  If the outside diameter had been 6" this would have been a piece of cake to install.