#3-48 UNC Thread Dimensions and Tap Drill
#3-48 UNC is the Unified coarse thread for the number 3 size (major diameter 0.099 in, 2.51 mm): 48 threads per inch, a pitch of 0.02083 in (0.529 mm), on a 60° profile. ASME B1.1 lists it in the UNC series as a secondary size. Finer threads on the same diameter are #3-56 UNF.
- Tap drill#4776% thread, 1.99 mm
- Pitch48 TPI0.529 mm
- Major diameter0.0990 in2.515 mm
- Pitch diameter0.0855 in2.171 mm
- Minor diameter (nut)0.0764 in1.942 mm
- Tensile stress area0.00487 in²3.14 mm²
- Clearance hole#32normal fit, ASME B18.2.8
Thread profile
#3-48 UNC basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.0990 | 2.515 |
| Pitch P | 0.02083 | 0.529 |
| Pitch diameter d2 = D2 | 0.0855 | 2.171 |
| Minor diameter, internal thread D1 | 0.0764 | 1.942 |
| Minor diameter, external UNR thread d3 | 0.0742 | 1.884 |
| Height of fundamental triangle H | 0.0180 | 0.458 |
| Thread height, internal (5H/8) | 0.0113 | 0.286 |
| Thread height, external (11H/16) | 0.0124 | 0.315 |
| Crest flat of external thread (P/8) | 0.0026 | 0.066 |
| Crest flat of internal thread (P/4) | 0.0052 | 0.132 |
| Property | Value |
|---|---|
| Threads per inch | 48 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 4.44° |
| Tensile stress area As | 0.00487 in² (3.14 mm²) |
Basic (nominal) sizes per ASME B1.1, before tolerances. A UN external thread with a flat root has the same basic minor diameter as the nut (D1); d3 applies to the rounded UNR root.
#3-48 UNC limits of size (class 2A bolt, 2B nut)
Classes 2A and 2B are the general-purpose fits for commercial bolts, screws and nuts. The 2A external thread has an allowance of 0.0007 in, so its maximum size is that much below basic; the 2B nut starts at the basic size.
| External thread 2A | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Major diameter | 0.0983 | 0.0938 | 0.0045 |
| Pitch diameter | 0.0848 | 0.0825 | 0.0023 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.0845 | 0.0764 | 0.0081 |
| Pitch diameter | 0.0885 | 0.0855 | 0.0030 |
Limits of size from ASME B1.1 Table 2.
#3-48 UNC tap drill size
Tap drill for #3-48 UNC: #47 (0.0785 in, 1.99 mm)
This leaves about 76% thread in the tapped hole.
#47 is the drill listed for #3-48 UNC in standard tap drill charts. It is 0.0205 in under the 0.0990 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.076 and 0.085 in. The recommended drill falls inside that band.
Other drills that work for #3-48 UNC
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 2B limits | Thread engagement |
|---|---|---|---|---|---|
| #48 | 1.93 | 0.0760 | 85% | no | |
| 5/64" | 1.98 | 0.0781 | 77% | yes | |
| #47recommended | 1.99 | 0.0785 | 76% | yes | |
| 2 mm | 2.00 | 0.0787 | 75% | yes | |
| #46 | 2.06 | 0.0810 | 67% | yes | |
| #45 | 2.08 | 0.0820 | 63% | yes |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for #3
The clearance hole depends only on the 0.0990 in diameter, not on the thread series. The normal fit, a #32 drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | #36 | 0.1065 | 2.71 | 0.0075 |
| Normal | #32 | 0.1160 | 2.95 | 0.0170 |
| Loose | #30 | 0.1285 | 3.26 | 0.0295 |
Nominal drill sizes from ASME B18.2.8.
#3-48 UNC strength
SAE J429 starts at 1/4 in, so the SAE grades give no proof load for #3-48 UNC. For an estimate, multiply the tensile stress area of 0.00487 in² by the strength of the material: for example, 60,000 psi gives 292 lbf.
#3 pitches compared
All standard pitches on the #3 diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
| Thread | TPI | Pitch Ø (in) | Minor Ø D1 (in) | Stress area (in²) | Tap drill |
|---|---|---|---|---|---|
| #3-48 UNC | 48 | 0.0855 | 0.0764 | 0.00487 | #47 |
| #3-56 UNF | 56 | 0.0874 | 0.0797 | 0.00523 | #45 |
Closest metric thread
No metric thread is interchangeable with #3-48 UNC. The nearest is M2.5×0.45, which is 0.01 mm smaller in diameter with a pitch of 0.450 mm against 0.529 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
| Thread | Major Ø (mm) | Pitch (mm) | TPI | Ø difference (mm) |
|---|---|---|---|---|
| #3-48 UNC (this thread) | 2.515 | 0.529 | 48.00 | — |
| M2.5×0.45 | 2.500 | 0.450 | 56.44 | -0.015 |
How the #3-48 UNC dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.02083 in = 1 ÷ 48 threads per inch
- H = 0.866025 × P = 0.0180 in
- d2 = d − 0.649519 × P = 0.0990 − 0.0135 = 0.0855 in
- D1 = d − 1.082532 × P = 0.0990 − 0.0226 = 0.0764 in
- d3 = d − 1.190785 × P = 0.0990 − 0.0248 = 0.0742 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.0787² = 0.00486 in²
- Lead angle = arctan(P ÷ (π × d2)) = 4.44°
Related sizes and tools
Sources: ASME B1.1 (profile, series, limits of size and stress area), ASME B18.2.8 (clearance holes), SAE J429 (mechanical properties) and the tap drill tables of Machinery’s Handbook. How this site checks its data. Last reviewed 29 September 2026.