Dimensions, drill sizes and wrench sizes for 1-3/8-20 UN.
Designation
1-3/8-20 UN
Major (outside) diameter
34.925 mm · 1.375"
Pitch
1.27 mm · 20 TPI
Pitch diameter
34.1 mm · 1.3425"
Minor diameter (external)
33.367 mm · 1.3137"
Minor diameter (internal)
33.55 mm · 1.3209"
Recommended tap drill
1-21/64" · 33.73 mm · ≈72% thread
Clearance hole
Close 1-25/64" · Normal 1-13/32" · Loose 1-7/16" - 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)
2-1/16"
Close 1-25/64" · Normal 1-13/32" · Loose 1-7/16" - 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.
1-3/8-20 UN in numbers
1-3/8-20 UN is 1.3750 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 34.1 mm and the bolt's minor diameter 33.367 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 1.3815 in², 20% more than the coarser 1-3/8-6 UNC at the same diameter. The nearest metric size, M36 × 4.0, is 1.08 mm larger across the crests and its pitch is 2.730 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 34.925 mm and P = 1.27 mm. A smaller hole leaves more thread; the recommended 1-21/64" drill above gives about 72%.
Tap-drill hole diameters for 1-3/8-20 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
33.94
1.3360
65%
33.85
1.3328
70%
33.77
1.3295
75%
33.69
1.3263
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 1-3/8-20 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.3815 in²
34193 lbf
784 lb·ft · 9403 lb·in
SAE Grade 5
1.3815 in²
76676 lbf
1757 lb·ft · 21086 lb·in
SAE Grade 8
1.3815 in²
124339 lbf
2849 lb·ft · 34193 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 1-3/8-20 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 1-3/8-20 UN thread, drill the hole with a 1-21/64" tap drill (33.73 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 34.925 mm and a pitch of 1.27 mm (20 TPI).
To pass a 1-3/8-20 UN bolt through a part rather than thread it, use a clearance hole: Close 1-25/64" · Normal 1-13/32" · Loose 1-7/16" - 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 1-3/8-20 UN thread?
Drill 1-21/64" (33.73 mm) before tapping 1-3/8-20 UN. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-3/8-20 UN a coarse or fine thread?
1-3/8-20 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 1-3/8-20 UN bolt need?
Close 1-25/64" · Normal 1-13/32" · Loose 1-7/16" - 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 1-3/8-20 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 1-3/8-20 UN bolt?
Estimated tightening torque for 1-3/8-20 UN runs from 784 lb·ft (SAE Grade 2) to 2849 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.