Dimensions, drill sizes and wrench sizes for 1-1/4-18 UNEF.
Designation
1-1/4-18 UNEF
Major (outside) diameter
31.75 mm · 1.25"
Pitch
1.411 mm · 18 TPI
Pitch diameter
30.833 mm · 1.2139"
Minor diameter (external)
30.019 mm · 1.1818"
Minor diameter (internal)
30.222 mm · 1.1899"
Recommended tap drill
1-13/64" · 30.56 mm · ≈65% 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.
1-1/4-18 UNEF in numbers
1-1/4-18 UNEF is 1.2500 in across the crests with 18 threads per inch, a 1.411 mm pitch. The basic pitch diameter is 30.833 mm and the bolt's minor diameter 30.019 mm, so the thread is 0.866 mm deep on the bolt. Its tensile stress area is 1.1232 in², 16% more than the coarser 1-1/4-7 UNC at the same diameter. The nearest metric size, M33 × 3.5, is 1.25 mm larger across the crests and its pitch is 2.089 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 = 1.411 mm. A smaller hole leaves more thread; the recommended 1-13/64" drill above gives about 65%.
Tap-drill hole diameters for 1-1/4-18 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
60%
30.65
1.2067
65%
30.56
1.2031
70%
30.47
1.1995
75%
30.38
1.1959
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 1-1/4-18 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
1.1232 in²
27799 lbf
579 lb·ft · 6950 lb·in
SAE Grade 5
1.1232 in²
62338 lbf
1299 lb·ft · 15585 lb·in
SAE Grade 8
1.1232 in²
101089 lbf
2106 lb·ft · 25272 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 1-1/4-18 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 1-1/4-18 UNEF thread, drill the hole with a 1-13/64" tap drill (30.56 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 31.75 mm and a pitch of 1.411 mm (18 TPI).
To pass a 1-1/4-18 UNEF 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 1-1/4-18 UNEF thread?
Drill 1-13/64" (30.56 mm) before tapping 1-1/4-18 UNEF. That leaves roughly 65% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-1/4-18 UNEF a coarse or fine thread?
1-1/4-18 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 1-1/4-18 UNEF 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 1-1/4-18 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 1-1/4-18 UNEF bolt?
Estimated tightening torque for 1-1/4-18 UNEF runs from 579 lb·ft (SAE Grade 2) to 2106 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.