threadsizes

5/16-18 Tap Drill Size

Unified coarse (UNC) · 18 threads per inch · ASME B1.1, class 2B

For a 5/16 tap: 5/16-18 coarse takes an F drill, 5/16-24 fine takes an I drill.

What size drill bit for a 5/16-18 tap?

Fletter drill
0.2570inch
6.53mm
77%thread
No F drill? Use 6.6 mm (73%).

To scale section of the tapped hole

Section of a 5/16-18 tapped hole to scale: F (0.2570) drill, thread length 1.5 times the diameter. ⌀0.3125 P 0.0556 (18 TPI) DRILL F Thread 1.5 × D +5 P drill
Drawn from the 5/16-18 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 5/16-18
DrillInchmmThread2B
6.4 mm 0.2520 6.40 84% ✗
6.5 mm 0.2559 6.50 78% ✓
FUse 0.2570 6.53 77% ✓
6.6 mm 0.2598 6.60 73% ✓
G 0.2610 6.63 71% ✓
6.7 mm 0.2638 6.70 68% ✓
17/64" 0.2656 6.75 65% ✗
H 0.2660 6.76 64% ✗
6.8 mm 0.2677 6.80 62% ✗

By material Aluminum, brass, mild steel: F (77%) · Stainless, tool steel: G (71%)

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 (0.252 in to 0.265 in).

5/16-18 UNC ASME B1.1, class 2B

Major diameter0.3125 in / 7.94 mm
Threads per inch18 TPI (pitch 0.0556 in)
Pitch diameter (basic)0.2764 in
Minor diameter 2B0.2524–0.2650 in
Thread-forming tap drillL 0.2900 in

Clearance hole ASME B18.2.8

Close21/64"0.328 in
Normal11/32"0.344 in
Loose23/64"0.359 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

  • 0.313 inthread length for full strength in steel (1 × D)
  • 0.469 inin cast iron or brass (1.5 × D), up to 0.625 in in aluminum
  • +0.278 inextra drill depth below the thread for a plug tap (5 pitches), +0.111 in with a bottoming tap

Shop notes

Letter F is the standard 5/16-18 tap drill: 0.2570 in, about 77% thread, comfortably inside the 2B minor diameter limits. If your letter set has gaps, a G (71%) or a 6.6 mm (73%) does the same job, and either one is a little easier on the tap in steel.

The drill people reach for when the F is missing is 17/64, because it sits in every fractional index. Here it is a poor choice. At 0.2656 in it cuts about 65% thread and lands just over the maximum minor diameter, so the hole is out of spec before the tap touches it.

5/16-18 or M8?

A 5/16 bolt is 0.3125 in and an M8 is 0.315 in, so they pass for each other until you look at the pitch: 1.41 mm against 1.25 mm. Neither will run into the other's hole for more than a turn or two. The drills cross over as well, and that is the part that catches people out. 17/64 is a good drill for M8 x 1.25 (77%), but in a 5/16-18 hole it gives the thin 65% thread described above. The 6.8 mm M8 drill is worse here: 62%, and outside the limits.

Questions

What size is a letter F drill bit?

A letter F drill is 0.2570 in (6.53 mm), between 1/4 (0.2500 in) and 17/64 (0.2656 in). It is the standard tap drill for 5/16-18.