Dimensions, drill sizes and wrench sizes for 5/16-24 UNF.
Designation
5/16-24 UNF
Major (outside) diameter
7.938 mm · 0.3125"
Pitch
1.058 mm · 24 TPI
Pitch diameter
7.25 mm · 0.2854"
Minor diameter (external)
6.639 mm · 0.2614"
Minor diameter (internal)
6.792 mm · 0.2674"
Recommended tap drill
I · 6.91 mm · ≈75% thread
Clearance hole
Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
1/2"
Socket key (across flats)
1/4"
Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
5/16-24 UNF in numbers
5/16-24 UNF is 0.3125 in across the crests with 24 threads per inch, a 1.058 mm pitch. The basic pitch diameter is 7.25 mm and the bolt's minor diameter 6.639 mm, so the thread is 0.649 mm deep on the bolt. Its tensile stress area is 0.0581 in², 11% more than the coarser 5/16-18 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 0.2806 and 0.2843 in, and a class 2B nut between 0.2854 and 0.2902 in, so even the largest bolt clears the smallest nut by 0.0011 in. The nearest metric size, M8 × 1.25, is 0.06 mm larger across the crests and its pitch is 0.192 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 7.938 mm and P = 1.058 mm. A smaller hole leaves more thread; the recommended I drill above gives about 75%.
Tap-drill hole diameters for 5/16-24 UNF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
7.11
0.2801
K (0.2810 in) gives 58%
65%
7.04
0.2773
J (0.2770 in) gives 66%
70%
6.98
0.2746
J (0.2770 in) gives 66%
75%
6.91
0.2719
I (0.2720 in) gives 75%
Class 2A and 2B limits
The commercial fit for 5/16-24 UNF: class 2A for the bolt, class 2B for the nut or tapped hole. Inches.
Class 2A external and class 2B internal limits of size for 5/16-24 UNF.
Dimension
Min
Max
2A major diameter
0.3042
0.3114
2A pitch diameter
0.2806
0.2843
2B pitch diameter
0.2854
0.2902
Matches the ASME B1.1 class 2A/2B table for this size, as printed in the data sheets of American Fastener (Cap Screws & Bolts - External Threads Class 2A) and American Fastener (Nuts - Internal Threads - Class 2B). Basic pitch diameter E = D − 0.649519 P = 0.2854; the allowance between basic and the 2A limits is 0.0011. The B1.1 formula, class 2A pitch tolerance T = 0.0015 D^(1/3) + 0.0015 √D + 0.015 P^(2/3) with the allowance 0.3 T and the 2B tolerance 1.3 T, gives the same figures for this size. Minor-diameter limits are not given here.
Tightening torque estimate
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Proof-load preload and estimated tightening torque for 5/16-24 UNF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0581 in²
2400 lbf
13 lb·ft · 150 lb·in
SAE Grade 5
0.0581 in²
3675 lbf
19 lb·ft · 230 lb·in
SAE Grade 8
0.0581 in²
5213 lbf
27 lb·ft · 326 lb·in
Strengths are SAE J429 for this diameter band. J429 steps down with size: Grade 2 falls to 33 ksi proof above 3/4 in and Grade 5 to 74 ksi above 1 in, so these figures are not the small-size values. The Grade 2 row applies to bolts up to 6 in long; a longer Grade 2 bolt of the same diameter drops to 33 ksi proof.
These figures assume a nut factor of K = 0.20 (dry / as-received steel). K captures thread and head friction and can swing torque by 30% or more - a lubricated joint needs noticeably less. Change the lubrication assumption in the bolt-torque calculator.
Put 5/16-24 UNF on a job sheet
The bolt schedule collects the fasteners for one job onto a single sheet - tap drill, wrench size, grade and tightening torque on every line, taken straight from pages like this one. Free schedules hold 3 lines and are saved in this browser only.
For a 5/16-24 UNF thread, drill the hole with a I tap drill (6.91 mm) before tapping. That produces roughly 75% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 7.938 mm and a pitch of 1.058 mm (24 TPI).
To pass a 5/16-24 UNF bolt through a part rather than thread it, use a clearance hole: Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice)..
All values come from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables. See the full reference.
Frequently asked questions
What drill size do I need to tap a 5/16-24 UNF thread?
Drill I (6.91 mm) before tapping 5/16-24 UNF. That leaves roughly 75% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 5/16-24 UNF a coarse or fine thread?
5/16-24 UNF is a Unified fine (UNF) thread at 24 threads per inch. Fine pitches have a larger stress area than coarse at the same diameter and suit thin walls, fine adjustment and vibration-sensitive joints.
What size clearance hole does a 5/16-24 UNF bolt need?
Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 5/16-24 UNF bolt pass through the part instead of threading into it.
How much torque should I use on a 5/16-24 UNF bolt?
Estimated tightening torque for 5/16-24 UNF runs from 13 lb·ft (SAE Grade 2) to 27 lb·ft (SAE Grade 8), targeting 75% of proof load with a dry nut factor of K = 0.20. Lubricated threads need noticeably less torque.
Nearest standard metric sizes by major diameter. These are substitutes by size only - the thread forms are not interchangeable, so never run one into the other.
Each grade is a row on the torque chart, worked out at four nut factors from the diameter and clamp load, with the figures the published charts print beside it.