These parts fit into the 1/2in air breather hole on drum shells , creating a nice clean, finshed look. The air vents are 16 mm long (about 5/8 inches) and once you give the back piece 2 good turns to hold it in place, leaves about 13 mm left for the shell thickness (up to about 1/2 inch) - so this will barely fit a 20ply or thick stave/sold shell - in this case you may need to apply some loc-tite. Super easy to install and remove. Our best selling vent gommet.