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 1-1/32" · Loose 1-1/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table.
Hex head (across flats)
1-1/2"
Socket key (across flats)
3/4"
Close 1" · Normal 1-1/32" · Loose 1-1/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table.
1-20 UNEF in numbers
1-20 UNEF is 1.0000 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 24.575 mm and the bolt's minor diameter 23.842 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 0.7107 in², 17% more than the coarser 1-8 UNC at the same diameter. The nearest metric size, M24 × 3.0, is 1.40 mm smaller across the crests and its pitch is 1.730 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 25.4 mm and P = 1.27 mm. A smaller hole leaves more thread; the recommended 61/64" drill above gives about 72%.
Tap-drill hole diameters for 1-20 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
24.41
0.9610
no standard drill within 1/64 in
65%
24.33
0.9578
61/64" (0.9531 in) gives 72%
70%
24.25
0.9545
61/64" (0.9531 in) gives 72%
75%
24.16
0.9513
61/64" (0.9531 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 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 1-1/32" · Loose 1-1/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table..
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 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 1-1/32" · Loose 1-1/16" - nominal + 1/64" / 1/32" / 1/16" (common shop practice), given as the sums themselves because they are larger than the 1" end of the standard drill table. 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.