threadsizes

2-56 Tap Drill Size

Unified coarse (UNC) · 56 threads per inch · ASME B1.1, class 2B

What size drill bit for a 2-56 tap?

#50number drill
0.0700inch
1.78mm
69%thread
No #50 drill? Use 1.75 mm (74%).

To scale section of the tapped hole

Section of a #2-56 tapped hole to scale: #50 (0.0700) drill, thread length 1.5 times the diameter. ⌀0.0860 P 0.0179 (56 TPI) DRILL #50 Thread 1.5 × D +5 P drill
Drawn from the #2-56 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 #2-56
DrillInchmmThread2B
1.7 mm 0.0669 1.70 82% ✓
#51 0.0670 1.70 82% ✓
1.75 mm 0.0689 1.75 74% ✓
#50Use 0.0700 1.78 69% ✓
1.8 mm 0.0709 1.80 65% ✓
1.85 mm 0.0728 1.85 57% ✓
#49 0.0730 1.85 56% ✓

By material Aluminum, brass, mild steel: #50 (69%) · Stainless, tool steel: 1.8 mm (65%)

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 (0.067 in to 0.074 in).

2-56 UNC ASME B1.1, class 2B

Major diameter0.0860 in / 2.18 mm
Threads per inch56 TPI (pitch 0.0179 in)
Pitch diameter (basic)0.0744 in
Minor diameter 2B0.0667–0.0737 in
Thread-forming tap drill5/64" 0.0781 in

Hole depth rule of thumb

  • 0.086 inthread length for full strength in steel (1 × D)
  • 0.129 inin cast iron or brass (1.5 × D), up to 0.172 in in aluminum
  • +0.089 inextra drill depth below the thread for a plug tap (5 pitches), +0.036 in with a bottoming tap

Shop notes

The #50 drill (0.0700 in) is the usual call for #2-56 and it gives only about 69% thread. That lower figure is deliberate. In a hole this small the screw is rarely what fails; the tap is, and every extra point of engagement is extra torque on a very thin tool.

A 1.75 mm metric drill (74%) is a perfectly good substitute if it is the one in the drawer. Be more careful with the #51. It is only 0.003 in smaller than the #50, yet it pushes engagement to about 82%, and at 56 threads per inch that much extra material is enough to snap a tap in steel. In brass or soft aluminum you can get away with it. In anything tough, stay with the #50 or open up to 1.8 mm (65%).

The 5/64 that sits right there in the fractional index is not a cutting tap drill for this size. At 0.0781 in it is bigger than the 0.0737 in maximum minor diameter; it is the drill for a thread-forming tap, which pushes the metal into shape instead of cutting it.

Do not let a metric kit decide the hole either. An M2 screw is 0.079 in across against 0.086 in for a 2-56, with a different pitch, and the M2 tap drill (1.6 mm) is below the 2-56 minimum minor diameter.

Questions

What size is a #50 drill bit?

A #50 drill is 0.0700 in (1.78 mm), between 1/16 (0.0625 in) and 5/64 (0.0781 in). It is the standard tap drill for #2-56.