My cooler doesn't have a spigot hole so I need to drill one. I have all the parts for a 1/2" bulkhead. I bought a 1/2" drill bit but this is clearly too small and I can't find the O.D. of the 1/2" brass nipple. It is a Watts A-836 1/2" x 1 1/2" brass nipple. Anyone know what size drill bit or hole cutter I need?