Close 57/64" · Normal 29/32" · Loose 15/16" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
1-5/16"
Socket key (across flats)
3/4"
Close 57/64" · Normal 29/32" · Loose 15/16" — 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
0.5362 in²
22119 lbf
323 lb·ft · 3871 lb·in
SAE Grade 5
0.5362 in²
34185 lbf
499 lb·ft · 5982 lb·in
SAE Grade 8
0.5362 in²
48261 lbf
704 lb·ft · 8446 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 7/8-20 UNEF?
For a 7/8-20 UNEF thread, drill the hole with a 53/64" tap drill (21.03 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 22.225 mm and a pitch of 1.27 mm (20 TPI).
To pass a 7/8-20 UNEF bolt through a part rather than thread it, use a clearance hole: Close 57/64" · Normal 29/32" · Loose 15/16" — 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.