Close 1" · Normal 1" · Loose 1" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
2-1/4"
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
1.6542 in²
68237 lbf
1706 lb·ft · 20471 lb·in
SAE Grade 5
1.6542 in²
105457 lbf
2636 lb·ft · 31637 lb·in
SAE Grade 8
1.6542 in²
148881 lbf
3722 lb·ft · 44664 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 1-1/2-20 UN?
For a 1-1/2-20 UN thread, drill the hole with a 1" tap drill (25.4 mm) before tapping. That produces roughly 770% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 38.1 mm and a pitch of 1.27 mm (20 TPI).
To pass a 1-1/2-20 UN 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.