threadsizes

1/2-13 Tap Drill Size

Unified coarse (UNC) · 13 threads per inch · ASME B1.1, class 2B

For a 1/2 tap: 1/2-13 coarse takes a 27/64" drill, 1/2-20 fine takes a 29/64" drill.

What size drill bit for a 1/2-13 tap?

27/64"fractional drill
0.4219inch
10.72mm
78%thread
No 27/64" drill? Use 10.8 mm (75%).

To scale section of the tapped hole

Section of a 1/2-13 tapped hole to scale: 27/64" (0.4219) drill, thread length 1.5 times the diameter. ⌀0.5000 P 0.0769 (13 TPI) DRILL 27/64" Thread 1.5 × D +5 P drill
Drawn from the 1/2-13 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 1/2-13
DrillInchmmThread2B
10.6 mm 0.4173 10.60 83% ✓
10.7 mm 0.4213 10.70 79% ✓
27/64"Use 0.4219 10.72 78% ✓
10.8 mm 0.4252 10.80 75% ✓
10.9 mm 0.4291 10.90 71% ✓
11 mm 0.4331 11.00 67% ✓
11.1 mm 0.4370 11.10 63% ✗
7/16" 0.4375 11.11 63% ✗
11.2 mm 0.4409 11.20 59% ✗

By material Aluminum, brass, mild steel: 27/64" (78%) · Stainless, tool steel: 11 mm (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 (0.417 in to 0.434 in).

1/2-13 UNC ASME B1.1, class 2B

Major diameter0.5000 in / 12.70 mm
Threads per inch13 TPI (pitch 0.0769 in)
Pitch diameter (basic)0.4500 in
Minor diameter 2B0.4167–0.4336 in
Thread-forming tap drill15/32" 0.4688 in

Clearance hole ASME B18.2.8

Close17/32"0.531 in
Normal9/16"0.562 in
Loose39/64"0.609 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

  • 0.500 inthread length for full strength in steel (1 × D)
  • 0.750 inin cast iron or brass (1.5 × D), up to 1.000 in in aluminum
  • +0.385 inextra drill depth below the thread for a plug tap (5 pitches), +0.154 in with a bottoming tap

Shop notes

27/64 is the standard drill for 1/2-13: 0.4219 in for about 78% thread. Smaller drill sets that step in 32nds skip it, which is why this thread sees more substitutions than most.

The good news is that 1/2-13 has room. Every metric drill from 10.6 mm (83%) to 11 mm (67%) is inside the 2B minor diameter limits. A 10.8 mm (75%) is the closest match to the usual target, and 11 mm is the kinder choice for a hand tap in steel.

The bad substitute is 7/16, because it is the fractional size people already have. It is only one 64th bigger than 27/64, yet it drops the thread to about 62% and outside the limits. A bolt will still go in and feel normal, which is exactly why the mistake goes unnoticed until the thread is loaded.

Letter drills stop at Z, so from this size up every choice is fractional or metric. And check the pitch first: 1/2-20 looks the same on the shelf and uses a 29/64.