Dimensions, drill sizes and wrench sizes for 1-1/8-8 UN.
Designation
1-1/8-8 UN
Major (outside) diameter
28.575 mm · 1.125"
Pitch
3.175 mm · 8 TPI
Pitch diameter
26.513 mm · 1.0438"
Minor diameter (external)
24.68 mm · 0.9716"
Minor diameter (internal)
25.138 mm · 0.9897"
Recommended tap drill
1" · 25.4 mm · ≈77% thread
Clearance hole
Close 1-9/64" · Normal 1-5/32" · Loose 1-3/16" - 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)
1-11/16"
Close 1-9/64" · Normal 1-5/32" · Loose 1-3/16" - 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.
1-1/8-8 UN in numbers
1-1/8-8 UN is 1.1250 in across the crests with 8 threads per inch, a 3.175 mm pitch. The basic pitch diameter is 26.513 mm and the bolt's minor diameter 24.68 mm, so the thread is 1.947 mm deep on the bolt. Its tensile stress area is 0.7905 in², 4% more than the coarser 1-1/8-7 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 1.0348 and 1.0417 in. The nearest metric size, M30 × 3.5, is 1.43 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 = 28.575 mm and P = 3.175 mm. A smaller hole leaves more thread; the recommended 1" drill above gives about 77%.
Tap-drill hole diameters for 1-1/8-8 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
26.10
1.0276
no standard drill within 1/64 in
65%
25.89
1.0195
no standard drill within 1/64 in
70%
25.69
1.0113
1" (1.0000 in) gives 77%
75%
25.48
1.0032
1" (1.0000 in) gives 77%
Class 2A limits
The commercial fit for 1-1/8-8 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 1-1/8-8 UN.
Dimension
Min
Max
2A major diameter
1.1079
1.1229
2A pitch diameter
1.0348
1.0417
Matches the ASME B1.1 class 2A table for this size, as printed in the data sheets of American Fastener (Cap Screws & Bolts - External Threads Class 2A). The class 2B row is left out because the source sheet misprints it. Basic pitch diameter E = D − 0.649519 P = 1.0438; the allowance between basic and the 2A limits is 0.0021. 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 1-1/8-8 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.7905 in²
19564 lbf
367 lb·ft · 4402 lb·in
SAE Grade 5
0.7905 in²
43870 lbf
823 lb·ft · 9871 lb·in
SAE Grade 8
0.7905 in²
71141 lbf
1334 lb·ft · 16007 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 1-1/8-8 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 1-1/8-8 UN thread, drill the hole with a 1" tap drill (25.4 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 28.575 mm and a pitch of 3.175 mm (8 TPI).
To pass a 1-1/8-8 UN bolt through a part rather than thread it, use a clearance hole: Close 1-9/64" · Normal 1-5/32" · Loose 1-3/16" - 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 1-1/8-8 UN thread?
Drill 1" (25.4 mm) before tapping 1-1/8-8 UN. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-1/8-8 UN a coarse or fine thread?
1-1/8-8 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 1-1/8-8 UN bolt need?
Close 1-9/64" · Normal 1-5/32" · Loose 1-3/16" - 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 1-1/8-8 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 1-1/8-8 UN bolt?
Estimated tightening torque for 1-1/8-8 UN runs from 367 lb·ft (SAE Grade 2) to 1334 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.