Close 41/64" · Normal 21/32" · Loose 11/16" — nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
15/16"
Socket key (across flats)
1/2"
Close 41/64" · Normal 21/32" · Loose 11/16" — 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
0.256 in²
10558 lbf
110 lb·ft · 1320 lb·in
SAE Grade 5
0.256 in²
16317 lbf
170 lb·ft · 2040 lb·in
SAE Grade 8
0.256 in²
23036 lbf
240 lb·ft · 2880 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 5/8-18 UNF?
For a 5/8-18 UNF thread, drill the hole with a 37/64" tap drill (14.68 mm) before tapping. That produces roughly 65% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 15.875 mm and a pitch of 1.411 mm (18 TPI).
To pass a 5/8-18 UNF bolt through a part rather than thread it, use a clearance hole: Close 41/64" · Normal 21/32" · Loose 11/16" — 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.