Dimensions, drill sizes and wrench sizes for 1-3/8-18 UNEF.
Designation
1-3/8-18 UNEF
Major (outside) diameter
34.925 mm · 1.375"
Pitch
1.411 mm · 18 TPI
Pitch diameter
34.008 mm · 1.3389"
Minor diameter (external)
33.194 mm · 1.3068"
Minor diameter (internal)
33.397 mm · 1.3149"
Recommended tap drill
1-21/64" · 33.73 mm · ≈65% 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.
1-3/8-18 UNEF in numbers
1-3/8-18 UNEF is 1.3750 in across the crests with 18 threads per inch, a 1.411 mm pitch. The basic pitch diameter is 34.008 mm and the bolt's minor diameter 33.194 mm, so the thread is 0.865 mm deep on the bolt. Its tensile stress area is 1.3703 in², 19% 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 2.589 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 = 1.411 mm. A smaller hole leaves more thread; the recommended 1-21/64" drill above gives about 65%.
Tap-drill hole diameters for 1-3/8-18 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
33.83
1.3317
65%
33.73
1.3281
70%
33.64
1.3245
75%
33.55
1.3209
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 1-3/8-18 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.3703 in²
33915 lbf
777 lb·ft · 9327 lb·in
SAE Grade 5
1.3703 in²
76051 lbf
1743 lb·ft · 20914 lb·in
SAE Grade 8
1.3703 in²
123326 lbf
2826 lb·ft · 33915 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 1-3/8-18 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 1-3/8-18 UNEF thread, drill the hole with a 1-21/64" tap drill (33.73 mm) before tapping. That produces roughly 65% 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 1.411 mm (18 TPI).
To pass a 1-3/8-18 UNEF 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 1-3/8-18 UNEF thread?
Drill 1-21/64" (33.73 mm) before tapping 1-3/8-18 UNEF. That leaves roughly 65% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-3/8-18 UNEF a coarse or fine thread?
1-3/8-18 UNEF is a Unified extra-fine (UNEF) thread at 18 threads per inch, used for thin-wall parts, short thread engagement and fine adjustment.
What size clearance hole does a 1-3/8-18 UNEF 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 1-3/8-18 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 1-3/8-18 UNEF bolt?
Estimated tightening torque for 1-3/8-18 UNEF runs from 777 lb·ft (SAE Grade 2) to 2826 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.