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).
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 are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables - not hand-entered. 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.