Close 1" · Normal 1" · Loose 1" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Close 1" · Normal 1" · Loose 1" — 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
3.9988 in²
164952 lbf
6873 lb·ft · 82476 lb·in
SAE Grade 5
3.9988 in²
254925 lbf
10622 lb·ft · 127463 lb·in
SAE Grade 8
3.9988 in²
359895 lbf
14996 lb·ft · 179947 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 2-1/2-4 UNC?
For a 2-1/2-4 UNC thread, drill the hole with a 1" tap drill (25.4 mm) before tapping. That produces roughly 462% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 63.5 mm and a pitch of 6.35 mm (4 TPI).
To pass a 2-1/2-4 UNC bolt through a part rather than thread it, use a clearance hole: Close 1" · Normal 1" · Loose 1" — 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.