Dimensions, drill sizes and wrench sizes for 1/2-28 UNEF.
Designation
1/2-28 UNEF
Major (outside) diameter
12.7 mm · 0.5"
Pitch
0.907 mm · 28 TPI
Pitch diameter
12.111 mm · 0.4768"
Minor diameter (external)
11.587 mm · 0.4562"
Minor diameter (internal)
11.718 mm · 0.4613"
Recommended tap drill
15/32" · 11.91 mm · ≈67% thread
Clearance hole
Close 33/64" · Normal 17/32" · Loose 9/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
3/4"
Socket key (across flats)
3/8"
Close 33/64" · Normal 17/32" · Loose 9/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
1/2-28 UNEF in numbers
1/2-28 UNEF is 0.5000 in across the crests with 28 threads per inch, a 0.907 mm pitch. The basic pitch diameter is 12.111 mm and the bolt's minor diameter 11.587 mm, so the thread is 0.556 mm deep on the bolt. Its tensile stress area is 0.17 in², 20% more than the coarser 1/2-13 UNC at the same diameter. The nearest metric size, M12 × 1.75, is 0.70 mm smaller across the crests and its pitch is 0.843 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 12.7 mm and P = 0.907 mm. A smaller hole leaves more thread; the recommended 15/32" drill above gives about 67%.
Tap-drill hole diameters for 1/2-28 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
11.99
0.4722
15/32" (0.4688 in) gives 67%
65%
11.93
0.4698
15/32" (0.4688 in) gives 67%
70%
11.88
0.4675
15/32" (0.4688 in) gives 67%
75%
11.82
0.4652
15/32" (0.4688 in) gives 67%
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 1/2-28 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.17 in²
7011 lbf
58 lb·ft · 701 lb·in
SAE Grade 5
0.17 in²
10836 lbf
90 lb·ft · 1084 lb·in
SAE Grade 8
0.17 in²
15297 lbf
127 lb·ft · 1530 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. The Grade 2 row applies to bolts up to 6 in long; a longer Grade 2 bolt of the same diameter drops to 33 ksi proof.
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 1/2-28 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 1/2-28 UNEF thread, drill the hole with a 15/32" tap drill (11.91 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 12.7 mm and a pitch of 0.907 mm (28 TPI).
To pass a 1/2-28 UNEF bolt through a part rather than thread it, use a clearance hole: Close 33/64" · Normal 17/32" · Loose 9/16" - 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 1/2-28 UNEF thread?
Drill 15/32" (11.91 mm) before tapping 1/2-28 UNEF. That leaves roughly 67% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1/2-28 UNEF a coarse or fine thread?
1/2-28 UNEF is a Unified extra-fine (UNEF) thread at 28 threads per inch, used for thin-wall parts, short thread engagement and fine adjustment.
What size clearance hole does a 1/2-28 UNEF bolt need?
Close 33/64" · Normal 17/32" · Loose 9/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 1/2-28 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 1/2-28 UNEF bolt?
Estimated tightening torque for 1/2-28 UNEF runs from 58 lb·ft (SAE Grade 2) to 127 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.