Dimensions, drill sizes and wrench sizes for 1-3/8-8 UN.
Designation
1-3/8-8 UN
Major (outside) diameter
34.925 mm · 1.375"
Pitch
3.175 mm · 8 TPI
Pitch diameter
32.863 mm · 1.2938"
Minor diameter (external)
31.03 mm · 1.2216"
Minor diameter (internal)
31.488 mm · 1.2397"
Recommended tap drill
1-1/4" · 31.75 mm · ≈77% 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.
1-3/8-8 UN in numbers
1-3/8-8 UN is 1.3750 in across the crests with 8 threads per inch, a 3.175 mm pitch. The basic pitch diameter is 32.863 mm and the bolt's minor diameter 31.03 mm, so the thread is 1.947 mm deep on the bolt. Its tensile stress area is 1.2335 in², 7% more than the coarser 1-3/8-6 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 1.2844 and 1.2916 in, and a class 2B nut between 1.2938 and 1.3031 in, so even the largest bolt clears the smallest nut by 0.0022 in. The nearest metric size, M36 × 4.0, is 1.08 mm larger across the crests and its pitch is 0.825 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 = 3.175 mm. A smaller hole leaves more thread; the recommended 1-1/4" drill above gives about 77%.
Tap-drill hole diameters for 1-3/8-8 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
32.45
1.2776
65%
32.24
1.2695
70%
32.04
1.2613
75%
31.83
1.2532
Class 2A and 2B limits
The commercial fit for 1-3/8-8 UN: 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 1-3/8-8 UN.
Dimension
Min
Max
2A major diameter
1.3578
1.3728
2A pitch diameter
1.2844
1.2916
2B pitch diameter
1.2938
1.3031
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 = 1.2938; the allowance between basic and the 2A limits is 0.0022. 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 1-3/8-8 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.2335 in²
30529 lbf
700 lb·ft · 8396 lb·in
SAE Grade 5
1.2335 in²
68459 lbf
1569 lb·ft · 18826 lb·in
SAE Grade 8
1.2335 in²
111015 lbf
2544 lb·ft · 30529 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 1-3/8-8 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 1-3/8-8 UN thread, drill the hole with a 1-1/4" tap drill (31.75 mm) before tapping. That produces roughly 77% 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 3.175 mm (8 TPI).
To pass a 1-3/8-8 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 1-3/8-8 UN thread?
Drill 1-1/4" (31.75 mm) before tapping 1-3/8-8 UN. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-3/8-8 UN a coarse or fine thread?
1-3/8-8 UN is in the UN constant-pitch series: it runs 8 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 1-3/8-8 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 1-3/8-8 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 1-3/8-8 UN bolt?
Estimated tightening torque for 1-3/8-8 UN runs from 700 lb·ft (SAE Grade 2) to 2544 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.