Dimensions, drill sizes and wrench sizes for 3/4-20 UNEF.
Designation
3/4-20 UNEF
Major (outside) diameter
19.05 mm · 0.75"
Pitch
1.27 mm · 20 TPI
Pitch diameter
18.225 mm · 0.7175"
Minor diameter (external)
17.492 mm · 0.6887"
Minor diameter (internal)
17.675 mm · 0.6959"
Recommended tap drill
45/64" · 17.86 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).
3/4-20 UNEF in numbers
3/4-20 UNEF is 0.7500 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 18.225 mm and the bolt's minor diameter 17.492 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 0.3863 in², 15% 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 1.230 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 = 1.27 mm. A smaller hole leaves more thread; the recommended 45/64" drill above gives about 72%.
Tap-drill hole diameters for 3/4-20 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
18.06
0.7110
no standard drill within 1/64 in
65%
17.98
0.7078
45/64" (0.7031 in) gives 72%
70%
17.90
0.7045
45/64" (0.7031 in) gives 72%
75%
17.81
0.7013
45/64" (0.7031 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 3/4-20 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.3863 in²
15933 lbf
199 lb·ft · 2390 lb·in
SAE Grade 5
0.3863 in²
24624 lbf
308 lb·ft · 3694 lb·in
SAE Grade 8
0.3863 in²
34763 lbf
435 lb·ft · 5215 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 3/4-20 UNEF 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 3/4-20 UNEF thread, drill the hole with a 45/64" tap drill (17.86 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 1.27 mm (20 TPI).
To pass a 3/4-20 UNEF 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 3/4-20 UNEF thread?
Drill 45/64" (17.86 mm) before tapping 3/4-20 UNEF. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 3/4-20 UNEF a coarse or fine thread?
3/4-20 UNEF is a Unified extra-fine (UNEF) thread at 20 threads per inch, used for thin-wall parts, short thread engagement and fine adjustment.
What size clearance hole does a 3/4-20 UNEF 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 3/4-20 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 3/4-20 UNEF bolt?
Estimated tightening torque for 3/4-20 UNEF runs from 199 lb·ft (SAE Grade 2) to 435 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.