threadsizes

6-32 Tap Drill Size

Unified coarse (UNC) · 32 threads per inch · ASME B1.1, class 2B

What size drill bit for a 6-32 tap?

#36number drill
0.1065inch
2.71mm
78%thread
No #36 drill? Use 7/64" (71%) or 2.75 mm (73%).

To scale section of the tapped hole

Section of a #6-32 tapped hole to scale: #36 (0.1065) drill, thread length 1.5 times the diameter. ⌀0.1380 P 0.0313 (32 TPI) DRILL #36 Thread 1.5 × D +5 P drill
Drawn from the #6-32 basic profile. Hatched area is the part; the thread runs 1.5 × D and the drill goes 5 pitches deeper for a plug tap.

Drill options % of thread

Drill options for #6-32
DrillInchmmThread2B
2.7 mm 0.1063 2.70 78% ✓
#36Use 0.1065 2.71 78% ✓
2.75 mm 0.1083 2.75 73% ✓
7/64" 0.1094 2.78 71% ✓
#35 0.1100 2.79 69% ✓
2.8 mm 0.1102 2.80 68% ✓
#34 0.1110 2.82 67% ✓
2.85 mm 0.1122 2.85 64% ✓
#33 0.1130 2.87 62% ✓

By material Aluminum, brass, mild steel: #36 (78%) · Stainless, tool steel: #34 (67%)

75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.104 in to 0.114 in).

6-32 UNC ASME B1.1, class 2B

Major diameter0.1380 in / 3.51 mm
Threads per inch32 TPI (pitch 0.0313 in)
Pitch diameter (basic)0.1177 in
Minor diameter 2B0.1042–0.1139 in
Thread-forming tap drill1/8" 0.1250 in

Clearance hole ASME B18.2.8

Close#230.154 in
Normal#180.170 in
Loose#130.185 in

For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.

Hole depth rule of thumb

  • 0.138 inthread length for full strength in steel (1 × D)
  • 0.207 inin cast iron or brass (1.5 × D), up to 0.276 in in aluminum
  • +0.156 inextra drill depth below the thread for a plug tap (5 pitches), +0.063 in with a bottoming tap

Shop notes

A #36 drill (0.1065 in) gives #6-32 about 78% thread, a little more than most charts aim for. That makes the substitute the interesting part of this size. The 7/64 is the drill nearly everyone owns, and here it is a good choice rather than a compromise: 70.5% thread, well inside the minor diameter limits, and a tap that turns more easily. For a 6-32 in steel or stainless it is arguably the better drill. A 2.75 mm metric drill (73%) sits between the two.

The #37 is the one to skip. At 84% it falls below the 0.1042 in minimum minor diameter, so the tap is asked to cut more than the standard allows.

At 32 threads per inch the window is wide. Every drill from 2.65 mm to #33 lands inside the limits, which makes 6-32 far more forgiving of a near-miss drill than 4-40 or 2-56.

In the US, 6-32 is the screw that mounts wall switches and receptacles to their electrical boxes and holds most cover plates on. It is also the common screw for 3.5 in hard drives and many desktop PC case parts, so a stripped hole in a case or a box usually wants this tap.

Questions

What size is a #36 drill bit?

A #36 drill is 0.1065 in (2.71 mm), between 3/32 (0.0938 in) and 7/64 (0.1094 in). It is the standard tap drill for #6-32.