Here is a little gizzie that will save you half a lifetime of messing around:
It has the tap drill size, body size and a TAPPED hole, for national coarse AND fine threads.
With this plate, I screw the bolt through, cut it off with a hacksaw, and either file or grind it flush with the back of the plate. When you unscrew the bolt from the plate, the threads get cleaned up automatically.
Great reference when you want to know what drill to use for tapping a certain thread. Not sure what drill you have in your hand? run it through the hole, and you'll know if it's the wrong size.
It's almost like having a brain.
Acer