threadsizes

7/16-14 Tap Drill Size

Unified coarse (UNC) · 14 threads per inch · ASME B1.1, class 2B

For a 7/16 tap: 7/16-14 coarse takes a U drill, 7/16-20 fine takes a 25/64" drill.

What size drill bit for a 7/16-14 tap?

Uletter drill
0.3680inch
9.35mm
75%thread
No U drill? Use 3/8" (67%) or 9.3 mm (77%).

To scale section of the tapped hole

Section of a 7/16-14 tapped hole to scale: U (0.3680) drill, thread length 1.5 times the diameter. ⌀0.4375 P 0.0714 (14 TPI) DRILL U Thread 1.5 × D +5 P drill
Drawn from the 7/16-14 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 7/16-14
DrillInchmmThread2B
9.2 mm 0.3622 9.20 81% ✓
9.3 mm 0.3661 9.30 77% ✓
UUse 0.3680 9.35 75% ✓
9.4 mm 0.3701 9.40 73% ✓
9.5 mm 0.3740 9.50 68% ✓
3/8" 0.3750 9.53 67% ✓
V 0.3770 9.58 65% ✗
9.6 mm 0.3780 9.60 64% ✗
9.7 mm 0.3819 9.70 60% ✗

By material Aluminum, brass, mild steel: U (75%) · Stainless, tool steel: 3/8" (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 (0.360 in to 0.376 in).

7/16-14 UNC ASME B1.1, class 2B

Major diameter0.4375 in / 11.11 mm
Threads per inch14 TPI (pitch 0.0714 in)
Pitch diameter (basic)0.3911 in
Minor diameter 2B0.3602–0.3760 in
Thread-forming tap drill13/32" 0.4063 in

Hole depth rule of thumb

  • 0.438 inthread length for full strength in steel (1 × D)
  • 0.656 inin cast iron or brass (1.5 × D), up to 0.875 in in aluminum
  • +0.357 inextra drill depth below the thread for a plug tap (5 pitches), +0.143 in with a bottoming tap

Shop notes

The chart answer for 7/16-14 is a letter U (0.3680 in, about 75% thread), but letter drills come as a separate set that plenty of people do not own. That makes the 3/8 drill the practical question, and here it is a good answer. At 0.3750 in it cuts about 67% thread and stays inside the 2B minor diameter limits. In steel the lighter thread is welcome, since a 7/16 tap is real work by hand. If you have metric drills, 9.3 mm (77%) or 9.4 mm (73%) lands closer to the U.

The fractional to avoid is 23/64, the size just under 3/8. It gives about 84% thread and falls below the minimum minor diameter, which means a tight, stiff hole and a fair chance of breaking the tap in anything harder than aluminum. On the other side, letter V (65%) is already outside the limits, so 3/8 is about as big as you should go.

Check the pitch before drilling: 7/16-14 and 7/16-20 share the diameter, and the fine one takes a 25/64.

Questions

What size is a letter U drill bit?

A letter U drill is 0.3680 in (9.35 mm), between 23/64 (0.3594 in) and 3/8 (0.3750 in). It is the standard tap drill for 7/16-14.