Close 49/64" · Normal 25/32" · Loose 13/16" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
1-1/8"
Socket key (across flats)
5/8"
Close 49/64" · Normal 25/32" · Loose 13/16" — 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
0.3513 in²
14492 lbf
181 lb·ft · 2174 lb·in
SAE Grade 5
0.3513 in²
22396 lbf
280 lb·ft · 3359 lb·in
SAE Grade 8
0.3513 in²
31618 lbf
395 lb·ft · 4743 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 3/4-12 UN?
For a 3/4-12 UN thread, drill the hole with a 43/64" tap drill (17.07 mm) before tapping. That produces roughly 72% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 19.05 mm and a pitch of 2.117 mm (12 TPI).
To pass a 3/4-12 UN bolt through a part rather than thread it, use a clearance hole: Close 49/64" · Normal 25/32" · Loose 13/16" — 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.