2-56 Tap Drill Size and Drill Bit Guide

Use a #50 drill bit

2-56 · 0.0700 in / 1.78 mm · ~75% thread

2-56 is one of the smallest common UNC machine screw threads, found in electronics, instruments and hobby models. A #50 bit is the standard tap drill for a 75% thread.

At a glance

Tap drill	#50
Inches	0.0700 in
Millimetres	1.78 mm
Major diameter	0.0860 in
Pitch	0.0179 in
Threads per inch	56

Why this size

2-56 tap drill: drillØ = major − (pitch × 0.75) = 0.0860 − (0.0179 × 0.75) = 0.0726 in → next standard drill #50 (0.0700 in / 1.78 mm)

At this scale use a sharp bit, light pressure and a tapping guide if possible — tiny taps snap easily. The 2-64 fine variant uses a #51 bit (0.0670 in).