You can figure out any tapping hole size as follows:
-(tap drills for 75% thread)-
Take the O.D. (major diameter) of the thread and subtract 1 divided by
the pitch from it.
For example: 1/2 -20----- 0.500 - (1/20) =
0.500 - 0.050 = 0.450. Use 29/64 bit (0.453).

For example: 1/2 -13 -------- 0.500 - (1/13) =
0.500 - 0.077 = 0.423. Use 27/64 (0.422) for a good fit or 7/16 (0.4375) for
a looser fit.

Of course tables are available for this. I still use my 35 year old
Craftsman "How to run a Lathe" book for this. I have bookmarks in three
places: For the threading tables, For the decimal equivalent tables and
For the tables on diameters of numbered and lettered drills.

But I think it's good practice to work the brain now and then. Besides,
I am not always in the same place where the tables are.

Pete Stanaitis
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Ignoramus9727 wrote:

If I want to tap a hole for 1/2"-20 threading, what hole size should I
use? thanks

i