Dimensions, drill sizes and wrench sizes for 7/8-20 UNEF.
Designation
7/8-20 UNEF
Major (outside) diameter
22.225 mm · 0.875"
Pitch
1.27 mm · 20 TPI
Pitch diameter
21.4 mm · 0.8425"
Minor diameter (external)
20.667 mm · 0.8137"
Minor diameter (internal)
20.85 mm · 0.8209"
Recommended tap drill
53/64" · 21.03 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).
7/8-20 UNEF in numbers
7/8-20 UNEF is 0.8750 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 21.4 mm and the bolt's minor diameter 20.667 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 0.5362 in², 16% 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 1.230 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 = 1.27 mm. A smaller hole leaves more thread; the recommended 53/64" drill above gives about 72%.
Tap-drill hole diameters for 7/8-20 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
21.24
0.8360
no standard drill within 1/64 in
65%
21.15
0.8328
53/64" (0.8281 in) gives 72%
70%
21.07
0.8295
53/64" (0.8281 in) gives 72%
75%
20.99
0.8263
53/64" (0.8281 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 7/8-20 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.5362 in²
13272 lbf
194 lb·ft · 2323 lb·in
SAE Grade 5
0.5362 in²
34185 lbf
499 lb·ft · 5982 lb·in
SAE Grade 8
0.5362 in²
48261 lbf
704 lb·ft · 8446 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 7/8-20 UNEF 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 7/8-20 UNEF thread, drill the hole with a 53/64" tap drill (21.03 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 1.27 mm (20 TPI).
To pass a 7/8-20 UNEF 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 7/8-20 UNEF thread?
Drill 53/64" (21.03 mm) before tapping 7/8-20 UNEF. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 7/8-20 UNEF a coarse or fine thread?
7/8-20 UNEF is a Unified extra-fine (UNEF) thread at 20 threads per inch, used for thin-wall parts, short thread engagement and fine adjustment.
What size clearance hole does a 7/8-20 UNEF 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 7/8-20 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 7/8-20 UNEF bolt?
Estimated tightening torque for 7/8-20 UNEF runs from 194 lb·ft (SAE Grade 2) to 704 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.