Dimensions, drill sizes and wrench sizes for 1-3/8-16 UN.
Designation
1-3/8-16 UN
Major (outside) diameter
34.925 mm · 1.375"
Pitch
1.588 mm · 16 TPI
Pitch diameter
33.894 mm · 1.3344"
Minor diameter (external)
32.977 mm · 1.2983"
Minor diameter (internal)
33.207 mm · 1.3073"
Recommended tap drill
1" · 25.4 mm · ≈462% 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)
2-1/16"
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 1-3/8-16 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.3563 in²
33568 lbf
769 lb·ft · 9231 lb·in
SAE Grade 5
1.3563 in²
75274 lbf
1725 lb·ft · 20700 lb·in
SAE Grade 8
1.3563 in²
122066 lbf
2797 lb·ft · 33568 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 1-3/8-16 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 1-3/8-16 UN thread, drill the hole with a 1" tap drill (25.4 mm) before tapping. That produces roughly 462% 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 1.588 mm (16 TPI).
To pass a 1-3/8-16 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 1-3/8-16 UN thread?
Drill 1" (25.4 mm) before tapping 1-3/8-16 UN. That leaves roughly 462% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-3/8-16 UN a coarse or fine thread?
1-3/8-16 UN is in the UN constant-pitch series: it runs 16 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 1-3/8-16 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 1-3/8-16 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 1-3/8-16 UN bolt?
Estimated tightening torque for 1-3/8-16 UN runs from 769 lb·ft (SAE Grade 2) to 2797 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.