Dimensions, drill sizes and wrench sizes for 7/8-9 UNC.
Designation
7/8-9 UNC
Major (outside) diameter
22.225 mm · 0.875"
Pitch
2.822 mm · 9 TPI
Pitch diameter
20.392 mm · 0.8028"
Minor diameter (external)
18.763 mm · 0.7387"
Minor diameter (internal)
19.17 mm · 0.7547"
Recommended tap drill
49/64" · 19.45 mm · ≈76% thread
Clearance hole
Close 57/64" · Normal 29/32" · Loose 15/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
1-5/16"
Socket key (across flats)
3/4"
Close 57/64" · Normal 29/32" · Loose 15/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
7/8-9 UNC in numbers
7/8-9 UNC is 0.8750 in across the crests with 9 threads per inch, a 2.822 mm pitch. The basic pitch diameter is 20.392 mm and the bolt's minor diameter 18.763 mm, so the thread is 1.731 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.7946 and 0.8009 in, and a class 2B nut between 0.8028 and 0.8110 in, so even the largest bolt clears the smallest nut by 0.0019 in. The nearest metric size, M22 × 2.5, is 0.23 mm smaller across the crests and its pitch is 0.322 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 22.225 mm and P = 2.822 mm. A smaller hole leaves more thread; the recommended 49/64" drill above gives about 76%.
Tap-drill hole diameters for 7/8-9 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
20.03
0.7884
25/32" (0.7813 in) gives 65%
65%
19.84
0.7812
25/32" (0.7813 in) gives 65%
70%
19.66
0.7740
25/32" (0.7813 in) gives 65%
75%
19.48
0.7668
49/64" (0.7656 in) gives 76%
Class 2A and 2B limits
The commercial fit for 7/8-9 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 7/8-9 UNC.
Dimension
Min
Max
2A major diameter
0.8592
0.8731
2A pitch diameter
0.7946
0.8009
2B pitch diameter
0.8028
0.8110
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 = 0.8028; the allowance between basic and the 2A limits is 0.0019. 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 7/8-9 UNC by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.4617 in²
11400 lbf
166 lb·ft · 1995 lb·in
SAE Grade 5
0.4617 in²
29475 lbf
430 lb·ft · 5158 lb·in
SAE Grade 8
0.4617 in²
41550 lbf
606 lb·ft · 7271 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 7/8-9 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 7/8-9 UNC thread, drill the hole with a 49/64" tap drill (19.45 mm) before tapping. That produces roughly 76% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 22.225 mm and a pitch of 2.822 mm (9 TPI).
To pass a 7/8-9 UNC bolt through a part rather than thread it, use a clearance hole: Close 57/64" · Normal 29/32" · Loose 15/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice)..
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 7/8-9 UNC thread?
Drill 49/64" (19.45 mm) before tapping 7/8-9 UNC. That leaves roughly 76% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 7/8-9 UNC a coarse or fine thread?
7/8-9 UNC is a Unified coarse (UNC) thread at 9 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a 7/8-9 UNC bolt need?
Close 57/64" · Normal 29/32" · Loose 15/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 7/8-9 UNC bolt pass through the part instead of threading into it.
How much torque should I use on a 7/8-9 UNC bolt?
Estimated tightening torque for 7/8-9 UNC runs from 166 lb·ft (SAE Grade 2) to 606 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.