I originally bought 3/4 inch nipples but couldn't find a step bit that would go that large. Somewhere back in this thread other's have suggested other tools that would cut nice clean round holes, but I don't remember what they were.... sorry!
You will be able to get close to the outside diameter of the pipe nipple by just putting in one a tape measure or ruler.
If you can't figure out a tool for the job, don't hesitate to go with the 1/2 nipples. I usually have 1 open and the one with the ball valve about half open to keep 220-230. If I want to do some chickens a bit higher I open a third. Rarely have I had to open all 4. I have done it a couple of times just to see what kind of temps I could get and 300-325 was a piece of cake.
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Weber 22 inch w/mods
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