threadsizes

5/16-24 Tap Drill Size

Unified fine (UNF) · 24 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 5/16-24 tap?

Iletter drill
0.2720inch
6.91mm
75%thread
No I drill? Use 6.9 mm (76%).

To scale section of the tapped hole

Section of a 5/16-24 tapped hole to scale: I (0.2720) drill, thread length 1.5 times the diameter. ⌀0.3125 P 0.0417 (24 TPI) DRILL I Thread 1.5 × D +5 P drill
Drawn from the 5/16-24 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 5/16-24
DrillInchmmThread2B
6.8 mm 0.2677 6.80 83% ✓
6.9 mm 0.2717 6.90 76% ✓
IUse 0.2720 6.91 75% ✓
7 mm 0.2756 7.00 68% ✓
J 0.2770 7.04 66% ✓
7.1 mm 0.2795 7.10 61% ✗
K 0.2810 7.14 58% ✗
9/32" 0.2813 7.14 58% ✗

By material Aluminum, brass, mild steel: I (75%) · Stainless, tool steel: J (66%)

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 (0.267 in to 0.277 in).

5/16-24 UNF ASME B1.1, class 2B

Major diameter0.3125 in / 7.94 mm
Threads per inch24 TPI (pitch 0.0417 in)
Pitch diameter (basic)0.2854 in
Minor diameter 2B0.2674–0.2771 in
Thread-forming tap drillM 0.2950 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
  • +0.208 inextra drill depth below the thread for a plug tap (5 pitches), +0.083 in with a bottoming tap

Shop notes

Letter I (0.2720 in, about 75%) is the drill, and there is no fractional size that does the job. That is the first trap: the nearest fraction people reach for is 9/32, and at 0.2812 in it cuts only about 58% thread and leaves the hole above the 2B maximum minor diameter.

If there is no letter set, 6.9 mm matches the I almost exactly at about 76%. The 6.8 mm that sits next to every M8 x 1.25 tap also works but runs tight at about 83%, which is fine in aluminum or brass and a lot of torque in steel. For hard material the better move is a J (66%), still inside the limits.

The M8 x 1 look-alike

This is the inch thread most easily confused with a metric one. 5/16-24 is 7.94 mm across with a 1.058 mm pitch; M8 x 1 is 8 mm with a 1 mm pitch. The difference is small enough that one can start in the other and turn a little before it tightens, and then it feels like a dirty thread rather than the wrong one. If a 5/16 fine bolt gets stiff partway in, check the pitch before adding force.

Questions

What size is a letter I drill bit?

A letter I drill is 0.2720 in (6.91 mm), between 17/64 (0.2656 in) and 9/32 (0.2813 in). It is the standard tap drill for 5/16-24.