ZM-IR-602 Round Cast Iron Grille 6" Hole Size Without Damper; 8" Exterior Diameter
Made to fit into a 6" round hole or duct, these grates have an exterior diameter of approximately 1" greater than their boot size. The flange measures 3/16", and is pictured in the side view photo. You may need to modify for different mounting applications.
None better anywhere at any price.