Dimensions, drill sizes and wrench sizes for 1-20 UNEF.
Designation
1-20 UNEF
Major (outside) diameter
25.4 mm · 1"
Pitch
1.27 mm · 20 TPI
Pitch diameter
24.575 mm · 0.9675"
Minor diameter (external)
23.842 mm · 0.9387"
Minor diameter (internal)
24.025 mm · 0.9459"
Recommended tap drill
61/64" · 24.21 mm · ≈72% thread
Clearance hole
Close 1" · Normal 1" · Loose 1" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
1-1/2"
Socket key (across flats)
3/4"
Close 1" · Normal 1" · Loose 1" - 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.
Proof-load preload and estimated tightening torque for 1-20 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.7107 in²
17591 lbf
293 lb·ft · 3518 lb·in
SAE Grade 5
0.7107 in²
45310 lbf
755 lb·ft · 9062 lb·in
SAE Grade 8
0.7107 in²
63967 lbf
1066 lb·ft · 12793 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.
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 1-20 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 1-20 UNEF thread, drill the hole with a 61/64" tap drill (24.21 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 25.4 mm and a pitch of 1.27 mm (20 TPI).
To pass a 1-20 UNEF bolt through a part rather than thread it, use a clearance hole: Close 1" · Normal 1" · Loose 1" - 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.
Frequently asked questions
What drill size do I need to tap a 1-20 UNEF thread?
Drill 61/64" (24.21 mm) before tapping 1-20 UNEF. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-20 UNEF a coarse or fine thread?
1-20 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 1-20 UNEF bolt need?
Close 1" · Normal 1" · Loose 1" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 1-20 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 1-20 UNEF bolt?
Estimated tightening torque for 1-20 UNEF runs from 293 lb·ft (SAE Grade 2) to 1066 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.