Dimensions, drill sizes and wrench sizes for 7/8-14 UNF.
Designation
7/8-14 UNF
Major (outside) diameter
22.225 mm · 0.875"
Pitch
1.814 mm · 14 TPI
Pitch diameter
21.047 mm · 0.8286"
Minor diameter (external)
19.999 mm · 0.7874"
Minor diameter (internal)
20.261 mm · 0.7977"
Recommended tap drill
13/16" · 20.64 mm · ≈67% 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).
7/8-14 UNF in numbers
7/8-14 UNF is 0.8750 in across the crests with 14 threads per inch, a 1.814 mm pitch. The basic pitch diameter is 21.047 mm and the bolt's minor diameter 19.999 mm, so the thread is 1.113 mm deep on the bolt. Its tensile stress area is 0.5095 in², 10% more than the coarser 7/8-9 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 0.8216 and 0.8270 in, and a class 2B nut between 0.8286 and 0.8356 in, so even the largest bolt clears the smallest nut by 0.0016 in. The nearest metric size, M22 × 2.5, is 0.23 mm smaller across the crests and its pitch is 0.686 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 = 1.814 mm. A smaller hole leaves more thread; the recommended 13/16" drill above gives about 67%.
Tap-drill hole diameters for 7/8-14 UNF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
20.81
0.8193
13/16" (0.8125 in) gives 67%
65%
20.69
0.8147
13/16" (0.8125 in) gives 67%
70%
20.58
0.8101
13/16" (0.8125 in) gives 67%
75%
20.46
0.8054
13/16" (0.8125 in) gives 67%
Class 2A and 2B limits
The commercial fit for 7/8-14 UNF: 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 7/8-14 UNF.
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Proof-load preload and estimated tightening torque for 7/8-14 UNF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.5095 in²
12600 lbf
184 lb·ft · 2205 lb·in
SAE Grade 5
0.5095 in²
32475 lbf
474 lb·ft · 5683 lb·in
SAE Grade 8
0.5095 in²
45825 lbf
668 lb·ft · 8019 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 7/8-14 UNF 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 7/8-14 UNF thread, drill the hole with a 13/16" tap drill (20.64 mm) before tapping. That produces roughly 67% 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 1.814 mm (14 TPI).
To pass a 7/8-14 UNF 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 7/8-14 UNF thread?
Drill 13/16" (20.64 mm) before tapping 7/8-14 UNF. That leaves roughly 67% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 7/8-14 UNF a coarse or fine thread?
7/8-14 UNF is a Unified fine (UNF) thread at 14 threads per inch. Fine pitches have a larger stress area than coarse at the same diameter and suit thin walls, fine adjustment and vibration-sensitive joints.
What size clearance hole does a 7/8-14 UNF 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 7/8-14 UNF bolt pass through the part instead of threading into it.
How much torque should I use on a 7/8-14 UNF bolt?
Estimated tightening torque for 7/8-14 UNF runs from 184 lb·ft (SAE Grade 2) to 668 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.