1-3/8-6 UNC tap drill size

1-7/32 drill — 1.2188 in / 30.96 mm (72% thread)

Major Ø 1.3750 in · 6 threads per inch · coarse series. The chart pick targets 75% thread; nearby standard drills:

Drill	Decimal (in)	mm	Thread %
1-15/64	1.2344	31.35	65%
1-7/32	1.2188	30.96	72%
1-13/64	1.2031	30.56	79%
1-3/16	1.1875	30.16	87%

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, fine series: 1-3/8-12 UNF