threadsizes

4-40 Tap Drill Size

Unified coarse (UNC) · 40 threads per inch · ASME B1.1, class 2B

What size drill bit for a 4-40 tap?

#43number drill
0.0890inch
2.26mm
71%thread
No #43 drill? Use 2.25 mm (72%).

To scale section of the tapped hole

Section of a #4-40 tapped hole to scale: #43 (0.0890) drill, thread length 1.5 times the diameter. ⌀0.1120 P 0.0250 (40 TPI) DRILL #43 Thread 1.5 × D +5 P drill
Drawn from the #4-40 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 #4-40
DrillInchmmThread2B
2.15 mm 0.0847 2.15 84% ✗
#44 0.0860 2.18 80% ✓
2.2 mm 0.0866 2.20 78% ✓
2.25 mm 0.0886 2.25 72% ✓
#43Use 0.0890 2.26 71% ✓
2.3 mm 0.0906 2.30 66% ✓
2.35 mm 0.0925 2.35 60% ✓
#42 0.0935 2.38 57% ✓
3/32" 0.0938 2.38 56% ✓

By material Aluminum, brass, mild steel: #43 (71%) · Stainless, tool steel: 2.3 mm (66%)

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 (0.085 in to 0.094 in).

4-40 UNC ASME B1.1, class 2B

Major diameter0.1120 in / 2.85 mm
Threads per inch40 TPI (pitch 0.0250 in)
Pitch diameter (basic)0.0958 in
Minor diameter 2B0.0849–0.0939 in
Thread-forming tap drill#38 0.1015 in

Hole depth rule of thumb

  • 0.112 inthread length for full strength in steel (1 × D)
  • 0.168 inin cast iron or brass (1.5 × D), up to 0.224 in in aluminum
  • +0.125 inextra drill depth below the thread for a plug tap (5 pitches), +0.050 in with a bottoming tap

Shop notes

#43 is the standard drill for #4-40 and it leaves about 71% thread. Like most small screw sizes, the chart aims under 75% on purpose: a #4 tap is thin enough that tapping torque, not thread strength, decides whether the job goes well.

If the #43 is missing, a 2.25 mm drill (72%) is the closest stand-in, close enough that you will not feel the difference. The #44 (80%) and 2.2 mm (78%) are both inside the minor diameter limits and suit brass or aluminum where you want every bit of engagement, but in steel they make the tap work harder for little gain. The 3/32 found in every fractional set is on the other edge: 56% thread, still inside the limits, and a loose fit you would only choose in hard material.

4-40 is the thread on the jackscrews and standoffs of D-sub connectors, so electronics work runs into it constantly.

4-40 or M3?

An M3 screw is 0.118 in across, a 4-40 is 0.112 in, and the pitches are 0.5 mm against 0.635 mm. They look alike in a parts bin and do not swap. The M3 tap drill (2.5 mm, 0.0984 in) is also larger than the 0.0939 in maximum minor diameter of a 4-40, so a hole drilled for M3 is too big to tap 4-40 properly.

Questions

What size is a #43 drill bit?

A #43 drill is 0.0890 in (2.26 mm), between 5/64 (0.0781 in) and 3/32 (0.0938 in). It is the standard tap drill for #4-40.