7/16-20 Tap Drill Size

Unified fine (UNF) · 20 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 7/16-20 tap?

25/64"fractional drill

0.3906inch

9.92mm

72%thread

No 25/64" drill? Use 9.9 mm (74%).

To scale section of the tapped hole

Drawn from the 7/16-20 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 7/16-20
Drill	Inch	mm	Thread	2B
9.8 mm	0.3858	9.80	80%	✓
W	0.3860	9.80	79%	✓
9.9 mm	0.3898	9.90	74%	✓
25/64"Use	0.3906	9.92	72%	✓
10 mm	0.3937	10.00	67%	✓
X	0.3970	10.08	62%	✗
10.1 mm	0.3976	10.10	61%	✗
10.2 mm	0.4016	10.20	55%	✗

By material Aluminum, brass, mild steel: 25/64" (72%) · Stainless, tool steel: 10 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 (0.383 in to 0.395 in).

7/16-20 UNF ASME B1.1, class 2B

Major diameter	0.4375 in / 11.11 mm
Threads per inch	20 TPI (pitch 0.0500 in)
Pitch diameter (basic)	0.4050 in
Minor diameter 2B	0.3834–0.3949 in
Thread-forming tap drill	Z 0.4130 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
+0.250 inextra drill depth below the thread for a plug tap (5 pitches), +0.100 in with a bottoming tap

Shop notes

7/16-20 is one of the few fine threads where the standard drill is a plain fraction: 25/64 gives about 72% thread and is easy to find. A 9.9 mm drill is a near twin at about 74%, and a W pushes it to about 79% if you want more engagement in soft material.

The common 10 mm drill is the interesting one. It gives about 67% and still sits inside the 2B minor diameter limits, which makes it a sensible choice in steel. It is also the last safe step: the X, at 0.3970 in against 0.3937 in, is outside the limits at about 62%.

Hydraulics is where this thread lives. 7/16-20 is the -4 size for SAE straight-thread O-ring ports and for 37 degree flare (JIC) fittings, so a lot of the 7/16-20 holes people tap are ports. A port is more than a thread (it needs a spot face and the right counterbore for the O-ring), so for a real hydraulic port a combination port tool is the right tool, not a drill and a tap.

Do not confuse it with 7/16-14, which is drilled with a U (0.3680 in). That drill is under the 0.3834 in minimum minor diameter for 7/16-20 and will make the fine tap work far too hard.