I helpeed a buddy do his, we just removed the entire small inner ring with a router. cant remember the exact measurement. Hope this helps ya
__________________
2007 Megacab 2500, 6.7L, 6speed man tran. 3" leveling kit, S&B Cold Air Intake 5" straight pipe, 6" silverline tip. Nitto 295/70/17 meats. AVIC D-3, pioneer speakers, 10.5" screen and dvd in the back.