Dimensions, drill sizes and wrench sizes for 1-8 UNC.
Designation
1-8 UNC
Major (outside) diameter
25.4 mm · 1"
Pitch
3.175 mm · 8 TPI
Pitch diameter
23.338 mm · 0.9188"
Minor diameter (external)
21.505 mm · 0.8466"
Minor diameter (internal)
21.963 mm · 0.8647"
Recommended tap drill
7/8" · 22.23 mm · ≈77% thread
Clearance hole
Close 1" · Normal 1-1/32" · Loose 1-1/16" - 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)
1-1/2"
Socket key (across flats)
3/4"
Close 1" · Normal 1-1/32" · Loose 1-1/16" - 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.
1-8 UNC in numbers
1-8 UNC is 1.0000 in across the crests with 8 threads per inch, a 3.175 mm pitch. The basic pitch diameter is 23.338 mm and the bolt's minor diameter 21.505 mm, so the thread is 1.947 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.9100 and 0.9168 in, and a class 2B nut between 0.9188 and 0.9276 in, so even the largest bolt clears the smallest nut by 0.0020 in. The nearest metric size, M24 × 3.0, is 1.40 mm smaller across the crests and its pitch is 0.175 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 25.4 mm and P = 3.175 mm. A smaller hole leaves more thread; the recommended 7/8" drill above gives about 77%.
Tap-drill hole diameters for 1-8 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
22.93
0.9026
29/32" (0.9063 in) gives 58%
65%
22.72
0.8945
57/64" (0.8906 in) gives 67%
70%
22.51
0.8863
57/64" (0.8906 in) gives 67%
75%
22.31
0.8782
7/8" (0.8750 in) gives 77%
Class 2A and 2B limits
The commercial fit for 1-8 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 1-8 UNC.
Dimension
Min
Max
2A major diameter
0.9830
0.9980
2A pitch diameter
0.9100
0.9168
2B pitch diameter
0.9188
0.9276
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 = 0.9188; the allowance between basic and the 2A limits is 0.0020. 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 1-8 UNC by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.6057 in²
15000 lbf
250 lb·ft · 3000 lb·in
SAE Grade 5
0.6057 in²
38625 lbf
644 lb·ft · 7725 lb·in
SAE Grade 8
0.6057 in²
54525 lbf
909 lb·ft · 10905 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 1-8 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 1-8 UNC thread, drill the hole with a 7/8" tap drill (22.23 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 25.4 mm and a pitch of 3.175 mm (8 TPI).
To pass a 1-8 UNC bolt through a part rather than thread it, use a clearance hole: Close 1" · Normal 1-1/32" · Loose 1-1/16" - 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 1-8 UNC thread?
Drill 7/8" (22.23 mm) before tapping 1-8 UNC. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1-8 UNC a coarse or fine thread?
1-8 UNC is a Unified coarse (UNC) thread at 8 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a 1-8 UNC bolt need?
Close 1" · Normal 1-1/32" · Loose 1-1/16" - 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 1-8 UNC bolt pass through the part instead of threading into it.
How much torque should I use on a 1-8 UNC bolt?
Estimated tightening torque for 1-8 UNC runs from 250 lb·ft (SAE Grade 2) to 909 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.
Each grade is a row on the torque chart, worked out at four nut factors from the diameter and clamp load, with the figures the published charts print beside it.