This is a special story.
The biggest hole saw you can find on the shelves of hardware stores across Canada is 6 inches. All the companies stop at 6.
But you can't put the new 6" ventilation ducts through a 6 inch hole, you need a little more. So I personally pushed Starrett Tools to see if they couldn't find a 160mm hole saw in Europe for us.
As you can see, it measures 6-5/16 inches diameter!
You will probably have to special order it, but Starrett now stocks them in Canada, for about $80 retail.