Dimensions, drill sizes and wrench sizes for 1-3/8-12 UNF.
Designation
1-3/8-12 UNF
Major (outside) diameter
34.925 mm · 1.375"
Pitch
2.117 mm · 12 TPI
Pitch diameter
33.55 mm · 1.3209"
Minor diameter (external)
32.328 mm · 1.2728"
Minor diameter (internal)
32.634 mm · 1.2848"
Recommended tap drill
1-19/64" · 32.94 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.
1-3/8-12 UNF in numbers
1-3/8-12 UNF is 1.3750 in across the crests with 12 threads per inch, a 2.117 mm pitch. The basic pitch diameter is 33.55 mm and the bolt's minor diameter 32.328 mm, so the thread is 1.298 mm deep on the bolt. Its tensile stress area is 1.3147 in², 14% more than the coarser 1-3/8-6 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 1.3127 and 1.3190 in, and a class 2B nut between 1.3209 and 1.3291 in, so even the largest bolt clears the smallest nut by 0.0019 in. The nearest metric size, M36 × 4.0, is 1.08 mm larger across the crests and its pitch is 1.883 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 = 2.117 mm. A smaller hole leaves more thread; the recommended 1-19/64" drill above gives about 72%.
Tap-drill hole diameters for 1-3/8-12 UNF at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
33.27
1.3100
65%
33.14
1.3046
70%
33.00
1.2992
75%
32.86
1.2938
Class 2A and 2B limits
The commercial fit for 1-3/8-12 UNF: class 2A for the bolt, class 2B for the nut or tapped hole. Inches.
Class 2A external and class 2B internal limits of size for 1-3/8-12 UNF.
Dimension
Min
Max
2A major diameter
1.3617
1.3731
2A pitch diameter
1.3127
1.3190
2B pitch diameter
1.3209
1.3291
Matches the ASME B1.1 class 2A/2B table for this size, as printed in the data sheets of American Fastener (Cap Screws & Bolts - External Threads Class 2A) and American Fastener (Nuts - Internal Threads - Class 2B). Basic pitch diameter E = D − 0.649519 P = 1.3209; the allowance between basic and the 2A limits is 0.0019. The B1.1 formula, class 2A pitch tolerance T = 0.0015 D^(1/3) + 0.0015 √D + 0.015 P^(2/3) with the allowance 0.3 T and the 2B tolerance 1.3 T, gives the same figures for this size. Minor-diameter limits are not given here.
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 1-3/8-12 UNF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.3147 in²
32539 lbf
746 lb·ft · 8948 lb·in
SAE Grade 5
1.3147 in²
72967 lbf
1672 lb·ft · 20066 lb·in
SAE Grade 8
1.3147 in²
118324 lbf
2712 lb·ft · 32539 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 1-3/8-12 UNF 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 1-3/8-12 UNF thread, drill the hole with a 1-19/64" tap drill (32.94 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 2.117 mm (12 TPI).
To pass a 1-3/8-12 UNF 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 1-3/8-12 UNF thread?
Drill 1-19/64" (32.94 mm) before tapping 1-3/8-12 UNF. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-3/8-12 UNF a coarse or fine thread?
1-3/8-12 UNF is a Unified fine (UNF) thread at 12 threads per inch. Fine pitches have a larger stress area than coarse at the same diameter and suit thin walls, fine adjustment and vibration-sensitive joints.
What size clearance hole does a 1-3/8-12 UNF 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 1-3/8-12 UNF bolt pass through the part instead of threading into it.
How much torque should I use on a 1-3/8-12 UNF bolt?
Estimated tightening torque for 1-3/8-12 UNF runs from 746 lb·ft (SAE Grade 2) to 2712 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.