Dimensions, drill sizes and wrench sizes for #2-56 UNC.
Designation
#2-56 UNC
Major (outside) diameter
2.184 mm · 0.086"
Pitch
0.454 mm · 56 TPI
Pitch diameter
1.89 mm · 0.0744"
Minor diameter (external)
1.628 mm · 0.0641"
Minor diameter (internal)
1.693 mm · 0.0667"
Recommended tap drill
#50 · 1.78 mm · ≈69% thread
Clearance hole
Close #38 · Normal #32 · Loose #25 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)
5/64"
Close #38 · Normal #32 · Loose #25 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
#2-56 UNC in numbers
#2-56 UNC is 0.0860 in across the crests with 56 threads per inch, a 0.454 mm pitch. The basic pitch diameter is 1.89 mm and the bolt's minor diameter 1.628 mm, so the thread is 0.278 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.0717 and 0.0738 in, and a class 2B nut between 0.0744 and 0.0772 in, so even the largest bolt clears the smallest nut by 0.0006 in. The nearest metric size, M2 × 0.4, is 0.18 mm smaller across the crests and its pitch is 0.054 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 2.184 mm and P = 0.454 mm. A smaller hole leaves more thread; the recommended #50 drill above gives about 69%.
Tap-drill hole diameters for #2-56 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
1.83
0.0721
#49 (0.0730 in) gives 56%
65%
1.80
0.0709
#50 (0.0700 in) gives 69%
70%
1.77
0.0697
#50 (0.0700 in) gives 69%
75%
1.74
0.0686
#50 (0.0700 in) gives 69%
Class 2A and 2B limits
The commercial fit for #2-56 UNC: 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 #2-56 UNC.
Dimension
Min
Max
2A major diameter
0.0813
0.0854
2A pitch diameter
0.0717
0.0738
2B pitch diameter
0.0744
0.0772
Matches the ASME B1.1 class 2A/2B table for this size, as printed in the data sheets of American Fastener (Cap Screws & Bolts - External Threads Class 2A) and American Fastener (Nuts - Internal Threads - Class 2B). Basic pitch diameter E = D − 0.649519 P = 0.0744; the allowance between basic and the 2A limits is 0.0006. The B1.1 formula, class 2A pitch tolerance T = 0.0015 D^(1/3) + 0.0015 √D + 0.015 P^(2/3) with the allowance 0.3 T and the 2B tolerance 1.3 T, gives the same figures for this size. Minor-diameter limits are not given here.
Tightening torque estimate
SAE J429 tabulates grades from 1/4 in upwards, so no proof load or tightening torque is published for numbered sizes.
Put #2-56 UNC 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 #2-56 UNC thread, drill the hole with a #50 tap drill (1.78 mm) before tapping. That produces roughly 69% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 2.184 mm and a pitch of 0.454 mm (56 TPI).
To pass a #2-56 UNC bolt through a part rather than thread it, use a clearance hole: Close #38 · Normal #32 · Loose #25 - 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 #2-56 UNC thread?
Drill #50 (1.78 mm) before tapping #2-56 UNC. That leaves roughly 69% thread engagement, the usual target for a strong hole that still taps cleanly.
Is #2-56 UNC a coarse or fine thread?
#2-56 UNC is a Unified coarse (UNC) thread at 56 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a #2-56 UNC bolt need?
Close #38 · Normal #32 · Loose #25 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the #2-56 UNC bolt pass through the part instead of threading into it.
What is the pitch diameter of #2-56 UNC?
The pitch diameter of #2-56 UNC is 1.89 mm (0.0744"). It is the diameter where the thread ridge and groove are equally wide, and the reference dimension for checking thread fit.
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.