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
1-13/64" · 30.56 mm · ≈72% thread
Clearance hole
Close 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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)
1-7/8"
Close 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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.
1-1/4-20 UN in numbers
1-1/4-20 UN is 1.2500 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 30.925 mm and the bolt's minor diameter 30.192 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 1.1334 in², 17% more than the coarser 1-1/4-7 UNC at the same diameter. The nearest metric size, M33 × 3.5, is 1.25 mm larger across the crests and its pitch is 2.230 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 31.75 mm and P = 1.27 mm. A smaller hole leaves more thread; the recommended 1-13/64" drill above gives about 72%.
Tap-drill hole diameters for 1-1/4-20 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
30.76
1.2110
65%
30.68
1.2078
70%
30.60
1.2045
75%
30.51
1.2013
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 1-13/64" tap drill (30.56 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 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 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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 1-1/4-20 UN thread?
Drill 1-13/64" (30.56 mm) before tapping 1-1/4-20 UN. That leaves roughly 72% 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 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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 1-1/4-20 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.