This bolt-down billet adapter features 62mm threads to accept the 2.5" Monza Flip-Up Gas Cap (not included). The adapter itself has an outside diameter of 3.7", and the inside hole is 2.2". Hardware and lock nuts are included to fit the countersunk holes in the adapter ring perfectly. Also included is a gasket to go between the adapter ring and your gas tank. This unique kit is 'bolt-down' and no welding is required, though it is a custom kit that requires you to cut and drill into your gas tank. Also, you need a flat spot on your gas tank that is 3.7" diameter, minimum, to accept this adpater plate properly.