1/2-20 Tap Drill Size and Correct Drill Bit

Use a 29/64 drill bit

1/2-20 · 0.4531 in / 11.51 mm · ~75% thread

1/2-20 is the UNF fine version of the 1/2 inch thread, common in automotive and precision assemblies. A 29/64 inch bit gives the correct fine-thread tap drill for a strong 75% thread.

At a glance

Tap drill	29/64
Inches	0.4531 in
Millimetres	11.51 mm
Major diameter	0.5000 in
Pitch	0.0500 in
Threads per inch	20

Why this size

1/2-20 tap drill: drillØ = major − (pitch × 0.75) = 0.5000 − (0.0500 × 0.75) = 0.4625 in → nearest standard bit = 29/64 (0.4531 in)

Compare with 1/2-13 coarse, which uses a 27/64 in bit. The 1/2-20 fine thread gets the tighter 29/64 drill. For a clearance hole a 33/64 in bit is the close fit.