Drill with 23/32" drill bit and tap with a 1/2 NPT tap. If you are buying a tap, buy a skip tooth pipe tap. They work better than the standard hardware store pipe taps. Use lots of lube......gear oil works good when tapping cast iron.
I usually do this on a Bridgeport mill....... but it can be done on a big drill press or even freehand if you are careful.