Dimensions, drill sizes and wrench sizes for 1-1/4-20 UN.
Designation
1-1/4-20 UN
Major (outside) diameter
31.75 mm · 1.25"
Pitch
1.27 mm · 20 TPI
Pitch diameter
30.925 mm · 1.2175"
Minor diameter (external)
30.192 mm · 1.1887"
Minor diameter (internal)
30.375 mm · 1.1959"
Recommended tap drill
1" · 25.4 mm · ≈385% 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)
1-7/8"
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 1-1/4-20 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.1334 in²
28052 lbf
584 lb·ft · 7013 lb·in
SAE Grade 5
1.1334 in²
62904 lbf
1310 lb·ft · 15726 lb·in
SAE Grade 8
1.1334 in²
102006 lbf
2125 lb·ft · 25501 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 1-1/4-20 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 1-1/4-20 UN thread, drill the hole with a 1" tap drill (25.4 mm) before tapping. That produces roughly 385% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 31.75 mm and a pitch of 1.27 mm (20 TPI).
To pass a 1-1/4-20 UN 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 1-1/4-20 UN thread?
Drill 1" (25.4 mm) before tapping 1-1/4-20 UN. That leaves roughly 385% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-1/4-20 UN a coarse or fine thread?
1-1/4-20 UN is in the UN constant-pitch series: it runs 20 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 1-1/4-20 UN 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 1-1/4-20 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 1-1/4-20 UN bolt?
Estimated tightening torque for 1-1/4-20 UN runs from 584 lb·ft (SAE Grade 2) to 2125 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.