you will need a 25/64 in drill bit and a 29/64 in drill bit. First you will want to drill through original bolt hole with the 25/64 bit. Next you drill into the bottom of rack 2/3 of the way through or the depth of your new T-nut with the 29/64 bit. To insure proper depth wrap drill bit with tape at desired depth. Next us the 29/64 drill bit to make the holes on your tunnel the right size to accept the new T-nuts. Then reinstall racks.