Dimensions, drill sizes and wrench sizes for 3/4-12 UN.
Designation
3/4-12 UN
Major (outside) diameter
19.05 mm · 0.75"
Pitch
2.117 mm · 12 TPI
Pitch diameter
17.675 mm · 0.6959"
Minor diameter (external)
16.453 mm · 0.6478"
Minor diameter (internal)
16.759 mm · 0.6598"
Recommended tap drill
43/64" · 17.07 mm · ≈72% thread
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).
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).
3/4-12 UN in numbers
3/4-12 UN is 0.7500 in across the crests with 12 threads per inch, a 2.117 mm pitch. The basic pitch diameter is 17.675 mm and the bolt's minor diameter 16.453 mm, so the thread is 1.299 mm deep on the bolt. Its tensile stress area is 0.3513 in², 5% more than the coarser 3/4-10 UNC at the same diameter. The nearest metric size, M20 × 2.5, is 0.95 mm larger across the crests and its pitch is 0.383 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 19.05 mm and P = 2.117 mm. A smaller hole leaves more thread; the recommended 43/64" drill above gives about 72%.
Tap-drill hole diameters for 3/4-12 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
17.40
0.6850
11/16" (0.6875 in) gives 58%
65%
17.26
0.6796
43/64" (0.6719 in) gives 72%
70%
17.12
0.6742
43/64" (0.6719 in) gives 72%
75%
16.99
0.6688
43/64" (0.6719 in) gives 72%
Tightening torque estimate
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Proof-load preload and estimated tightening torque for 3/4-12 UN by fastener grade or property class.
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
Strengths are SAE J429 for this diameter band. J429 steps down with size: Grade 2 falls to 33 ksi proof above 3/4 in and Grade 5 to 74 ksi above 1 in, so these figures are not the small-size values. The Grade 2 row applies to bolts up to 6 in long; a longer Grade 2 bolt of the same diameter drops to 33 ksi proof.
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.
Put 3/4-12 UN on a job sheet
The bolt schedule collects the fasteners for one job onto a single sheet - tap drill, wrench size, grade and tightening torque on every line, taken straight from pages like this one. Free schedules hold 3 lines and are saved in this browser only.
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 come from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables. See the full reference.
Frequently asked questions
What drill size do I need to tap a 3/4-12 UN thread?
Drill 43/64" (17.07 mm) before tapping 3/4-12 UN. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 3/4-12 UN a coarse or fine thread?
3/4-12 UN is in the UN constant-pitch series: it runs 12 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 3/4-12 UN bolt need?
Close 49/64" · Normal 25/32" · Loose 13/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 3/4-12 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 3/4-12 UN bolt?
Estimated tightening torque for 3/4-12 UN runs from 181 lb·ft (SAE Grade 2) to 395 lb·ft (SAE Grade 8), targeting 75% of proof load with a dry nut factor of K = 0.20. Lubricated threads need noticeably less torque.
Nearest standard metric sizes by major diameter. These are substitutes by size only - the thread forms are not interchangeable, so never run one into the other.