Dimensions, drill sizes and wrench sizes for 1-3/8-6 UNC.
Designation
1-3/8-6 UNC
Major (outside) diameter
34.925 mm · 1.375"
Pitch
4.233 mm · 6 TPI
Pitch diameter
32.175 mm · 1.2668"
Minor diameter (external)
29.731 mm · 1.1705"
Minor diameter (internal)
30.342 mm · 1.1946"
Recommended tap drill
1-7/32" · 30.96 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.
1-3/8-6 UNC in numbers
1-3/8-6 UNC is 1.3750 in across the crests with 6 threads per inch, a 4.233 mm pitch. The basic pitch diameter is 32.175 mm and the bolt's minor diameter 29.731 mm, so the thread is 2.597 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 1.2563 and 1.2643 in, and a class 2B nut between 1.2667 and 1.2771 in, so even the largest bolt clears the smallest nut by 0.0024 in. The nearest metric size, M36 × 4.0, is 1.08 mm larger across the crests and its pitch is 0.233 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 34.925 mm and P = 4.233 mm. A smaller hole leaves more thread; the recommended 1-7/32" drill above gives about 72%.
Tap-drill hole diameters for 1-3/8-6 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
31.63
1.2451
65%
31.35
1.2343
70%
31.08
1.2235
75%
30.80
1.2126
Class 2A and 2B limits
The commercial fit for 1-3/8-6 UNC: 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 1-3/8-6 UNC.
Dimension
Min
Max
2A major diameter
1.3544
1.3726
2A pitch diameter
1.2563
1.2643
2B pitch diameter
1.2667
1.2771
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 = 1.2667; the allowance between basic and the 2A limits is 0.0024. 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 1-3/8-6 UNC by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.1549 in²
28583 lbf
655 lb·ft · 7860 lb·in
SAE Grade 5
1.1549 in²
64096 lbf
1469 lb·ft · 17626 lb·in
SAE Grade 8
1.1549 in²
103939 lbf
2382 lb·ft · 28583 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 1-3/8-6 UNC 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 1-3/8-6 UNC thread, drill the hole with a 1-7/32" tap drill (30.96 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 4.233 mm (6 TPI).
To pass a 1-3/8-6 UNC 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 1-3/8-6 UNC thread?
Drill 1-7/32" (30.96 mm) before tapping 1-3/8-6 UNC. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-3/8-6 UNC a coarse or fine thread?
1-3/8-6 UNC is a Unified coarse (UNC) thread at 6 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a 1-3/8-6 UNC 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 1-3/8-6 UNC bolt pass through the part instead of threading into it.
How much torque should I use on a 1-3/8-6 UNC bolt?
Estimated tightening torque for 1-3/8-6 UNC runs from 655 lb·ft (SAE Grade 2) to 2382 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.