Dimensions, drill sizes and wrench sizes for #3-48 UNC.
Designation
#3-48 UNC
Major (outside) diameter
2.515 mm · 0.099"
Pitch
0.529 mm · 48 TPI
Pitch diameter
2.171 mm · 0.0855"
Minor diameter (external)
1.865 mm · 0.0734"
Minor diameter (internal)
1.942 mm · 0.0764"
Recommended tap drill
#47 · 1.99 mm · ≈76% 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).
#3-48 UNC in numbers
#3-48 UNC is 0.0990 in across the crests with 48 threads per inch, a 0.529 mm pitch. The basic pitch diameter is 2.171 mm and the bolt's minor diameter 1.865 mm, so the thread is 0.325 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.0825 and 0.0848 in, and a class 2B nut between 0.0855 and 0.0885 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 0.079 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 = 0.529 mm. A smaller hole leaves more thread; the recommended #47 drill above gives about 76%.
Tap-drill hole diameters for #3-48 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
2.10
0.0828
#45 (0.0820 in) gives 63%
65%
2.07
0.0814
#46 (0.0810 in) gives 67%
70%
2.03
0.0801
#46 (0.0810 in) gives 67%
75%
2.00
0.0787
#47 (0.0785 in) gives 76%
Class 2A and 2B limits
The commercial fit for #3-48 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 #3-48 UNC.
SAE J429 tabulates grades from 1/4 in upwards, so no proof load or tightening torque is published for numbered sizes.
Put #3-48 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 #3-48 UNC thread, drill the hole with a #47 tap drill (1.99 mm) before tapping. That produces roughly 76% 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 0.529 mm (48 TPI).
To pass a #3-48 UNC 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 #3-48 UNC thread?
Drill #47 (1.99 mm) before tapping #3-48 UNC. That leaves roughly 76% thread engagement, the usual target for a strong hole that still taps cleanly.
Is #3-48 UNC a coarse or fine thread?
#3-48 UNC is a Unified coarse (UNC) thread at 48 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a #3-48 UNC 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 #3-48 UNC bolt pass through the part instead of threading into it.
What is the pitch diameter of #3-48 UNC?
The pitch diameter of #3-48 UNC is 2.171 mm (0.0855"). 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.