Dimensions, drill sizes and wrench sizes for 7/8-12 UN.
Designation
7/8-12 UN
Major (outside) diameter
22.225 mm · 0.875"
Pitch
2.117 mm · 12 TPI
Pitch diameter
20.85 mm · 0.8209"
Minor diameter (external)
19.628 mm · 0.7728"
Minor diameter (internal)
19.934 mm · 0.7848"
Recommended tap drill
51/64" · 20.24 mm · ≈72% 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).
7/8-12 UN in numbers
7/8-12 UN is 0.8750 in across the crests with 12 threads per inch, a 2.117 mm pitch. The basic pitch diameter is 20.85 mm and the bolt's minor diameter 19.628 mm, so the thread is 1.299 mm deep on the bolt. Its tensile stress area is 0.4949 in², 7% more than the coarser 7/8-9 UNC at the same diameter. The nearest metric size, M22 × 2.5, is 0.23 mm smaller across the crests and its pitch is 0.383 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 22.225 mm and P = 2.117 mm. A smaller hole leaves more thread; the recommended 51/64" drill above gives about 72%.
Tap-drill hole diameters for 7/8-12 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
20.57
0.8100
13/16" (0.8125 in) gives 58%
65%
20.44
0.8046
51/64" (0.7969 in) gives 72%
70%
20.30
0.7992
51/64" (0.7969 in) gives 72%
75%
20.16
0.7938
51/64" (0.7969 in) gives 72%
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 7/8-12 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.4949 in²
12249 lbf
179 lb·ft · 2144 lb·in
SAE Grade 5
0.4949 in²
31550 lbf
460 lb·ft · 5521 lb·in
SAE Grade 8
0.4949 in²
44541 lbf
650 lb·ft · 7795 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 7/8-12 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 7/8-12 UN thread, drill the hole with a 51/64" tap drill (20.24 mm) before tapping. That produces roughly 72% 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 2.117 mm (12 TPI).
To pass a 7/8-12 UN 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 7/8-12 UN thread?
Drill 51/64" (20.24 mm) before tapping 7/8-12 UN. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 7/8-12 UN a coarse or fine thread?
7/8-12 UN is in the UN constant-pitch series: it runs 12 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 7/8-12 UN 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 7/8-12 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 7/8-12 UN bolt?
Estimated tightening torque for 7/8-12 UN runs from 179 lb·ft (SAE Grade 2) to 650 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.