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).
Tightening torque estimate
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0581 in²
2395 lbf
12 lb·ft · 150 lb·in
SAE Grade 5
0.0581 in²
3702 lbf
19 lb·ft · 231 lb·in
SAE Grade 8
0.0581 in²
5226 lbf
27 lb·ft · 327 lb·in
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.
What tap drill do I need for 5/16-24 UNF?
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 are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables — not hand-entered. See the full reference.