9/16-18 UNF tap drill size

33/64 drill — 0.5156 in / 13.10 mm (65% thread)

Major Ø 0.5625 in · 18 threads per inch · fine series. The chart pick targets 75% thread; nearby standard drills:

Drill	Decimal (in)	mm	Thread %
17/32	0.5313	13.49	43%
33/64	0.5156	13.10	65%
1/2	0.5000	12.70	87%
31/64	0.4844	12.30	108%

Cut taps only — roll-form taps need a larger hole. 65–75% suits general work; drop toward 55% in tough material. Verify before you cut.

Same size, coarse series: 9/16-12 UNC