Dimensions, drill sizes and wrench sizes for #3-56 UNF.
Designation
#3-56 UNF
Major (outside) diameter
2.515 mm · 0.099"
Pitch
0.454 mm · 56 TPI
Pitch diameter
2.22 mm · 0.0874"
Minor diameter (external)
1.958 mm · 0.0771"
Minor diameter (internal)
2.024 mm · 0.0797"
Recommended tap drill
#45 · 2.08 mm · ≈73% thread
Clearance hole
Close #32 · Normal #30 · Loose #20 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Socket key (across flats)
5/64"
Close #32 · Normal #30 · Loose #20 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
#3-56 UNF in numbers
#3-56 UNF is 0.0990 in across the crests with 56 threads per inch, a 0.454 mm pitch. The basic pitch diameter is 2.22 mm and the bolt's minor diameter 1.958 mm, so the thread is 0.279 mm deep on the bolt. Its tensile stress area is 0.0052 in², 8% more than the coarser #3-48 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 0.0845 and 0.0867 in, and a class 2B nut between 0.0874 and 0.0902 in, so even the largest bolt clears the smallest nut by 0.0007 in. The nearest metric size, M2.5 × 0.45, is 0.02 mm smaller across the crests and its pitch is 0.004 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 2.515 mm and P = 0.454 mm. A smaller hole leaves more thread; the recommended #45 drill above gives about 73%.
Tap-drill hole diameters for #3-56 UNF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
2.16
0.0851
#44 (0.0860 in) gives 56%
65%
2.13
0.0839
#45 (0.0820 in) gives 73%
70%
2.10
0.0828
#45 (0.0820 in) gives 73%
75%
2.07
0.0816
#45 (0.0820 in) gives 73%
Class 2A and 2B limits
The commercial fit for #3-56 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 #3-56 UNF.
Dimension
Min
Max
2A major diameter
0.0942
0.0983
2A pitch diameter
0.0845
0.0867
2B pitch diameter
0.0874
0.0902
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 = 0.0874; the allowance between basic and the 2A limits is 0.0007. 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 #3-56 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 #3-56 UNF thread, drill the hole with a #45 tap drill (2.08 mm) before tapping. That produces roughly 73% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 2.515 mm and a pitch of 0.454 mm (56 TPI).
To pass a #3-56 UNF bolt through a part rather than thread it, use a clearance hole: Close #32 · Normal #30 · Loose #20 - 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 #3-56 UNF thread?
Drill #45 (2.08 mm) before tapping #3-56 UNF. That leaves roughly 73% thread engagement, the usual target for a strong hole that still taps cleanly.
Is #3-56 UNF a coarse or fine thread?
#3-56 UNF is a Unified fine (UNF) thread at 56 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 #3-56 UNF bolt need?
Close #32 · Normal #30 · Loose #20 - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the #3-56 UNF bolt pass through the part instead of threading into it.
What is the pitch diameter of #3-56 UNF?
The pitch diameter of #3-56 UNF is 2.22 mm (0.0874"). 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.