Dimensions, drill sizes and wrench sizes for 1-1/4-8 UN.
Designation
1-1/4-8 UN
Major (outside) diameter
31.75 mm · 1.25"
Pitch
3.175 mm · 8 TPI
Pitch diameter
29.688 mm · 1.1688"
Minor diameter (external)
27.855 mm · 1.0966"
Minor diameter (internal)
28.313 mm · 1.1147"
Recommended tap drill
1-1/8" · 28.58 mm · ≈77% thread
Clearance hole
Close 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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)
1-7/8"
Close 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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.
1-1/4-8 UN in numbers
1-1/4-8 UN is 1.2500 in across the crests with 8 threads per inch, a 3.175 mm pitch. The basic pitch diameter is 29.688 mm and the bolt's minor diameter 27.855 mm, so the thread is 1.947 mm deep on the bolt. Its tensile stress area is 0.9997 in², 3% more than the coarser 1-1/4-7 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 1.1597 and 1.1667 in, and a class 2B nut between 1.1688 and 1.1780 in, so even the largest bolt clears the smallest nut by 0.0021 in. The nearest metric size, M33 × 3.5, is 1.25 mm larger across the crests and its pitch is 0.325 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 31.75 mm and P = 3.175 mm. A smaller hole leaves more thread; the recommended 1-1/8" drill above gives about 77%.
Tap-drill hole diameters for 1-1/4-8 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
29.28
1.1526
65%
29.07
1.1445
70%
28.86
1.1363
75%
28.66
1.1282
Class 2A and 2B limits
The commercial fit for 1-1/4-8 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 1-1/4-8 UN.
Dimension
Min
Max
2A major diameter
1.2329
1.2479
2A pitch diameter
1.1597
1.1667
2B pitch diameter
1.1688
1.1780
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 = 1.1688; the allowance between basic and the 2A limits is 0.0021. The B1.1 formula lands 0.0001 in away from the table on one or more of these cells for this size, so the table's figures are printed. 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 1-1/4-8 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.9997 in²
24743 lbf
515 lb·ft · 6186 lb·in
SAE Grade 5
0.9997 in²
55484 lbf
1156 lb·ft · 13871 lb·in
SAE Grade 8
0.9997 in²
89974 lbf
1874 lb·ft · 22493 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 1-1/4-8 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 1-1/4-8 UN thread, drill the hole with a 1-1/8" tap drill (28.58 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 31.75 mm and a pitch of 3.175 mm (8 TPI).
To pass a 1-1/4-8 UN bolt through a part rather than thread it, use a clearance hole: Close 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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 1-1/4-8 UN thread?
Drill 1-1/8" (28.58 mm) before tapping 1-1/4-8 UN. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-1/4-8 UN a coarse or fine thread?
1-1/4-8 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 1-1/4-8 UN bolt need?
Close 1-17/64" · Normal 1-9/32" · Loose 1-5/16" - 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 1-1/4-8 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 1-1/4-8 UN bolt?
Estimated tightening torque for 1-1/4-8 UN runs from 515 lb·ft (SAE Grade 2) to 1874 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.