Close 17/64" · Normal 9/32" · Loose 5/16" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
7/16"
Socket key (across flats)
3/16"
Close 17/64" · Normal 9/32" · Loose 5/16" — 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
0.0318 in²
1313 lbf
5.5 lb·ft · 66 lb·in
SAE Grade 5
0.0318 in²
2029 lbf
8.5 lb·ft · 101 lb·in
SAE Grade 8
0.0318 in²
2864 lbf
12 lb·ft · 143 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 1/4-20 UNC?
For a 1/4-20 UNC thread, drill the hole with a #7 tap drill (5.11 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 6.35 mm and a pitch of 1.27 mm (20 TPI).
To pass a 1/4-20 UNC bolt through a part rather than thread it, use a clearance hole: Close 17/64" · Normal 9/32" · Loose 5/16" — 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.