ZM-Q601  Round 6" Hole Size Brass Grille With Damper; 7 3/8" 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 damper is made from finely cast iron.  None better anywhere at any price.

You may experience some resistance when opening and closing the cast iron louvers for the damper.  The friction helps to keep the louvers in a set position to your liking, and withstand normal increases in air pressure caused by higher fan speeds from heating and cooling units.