2-1/2-4 UNC Thread Dimensions and Tap Drill
2-1/2-4 UNC is the Unified coarse thread for the 2-1/2 in (63.50 mm) diameter: 4 threads per inch, a pitch of 0.25000 in (6.350 mm), on a 60° profile. ASME B1.1 lists it in the UNC series.
- Tap drill2-1/4"77% thread, 57.15 mm
- Pitch4 TPI6.350 mm
- Major diameter2.5000 in63.500 mm
- Pitch diameter2.3376 in59.376 mm
- Minor diameter (nut)2.2294 in56.626 mm
- Tensile stress area4 in²2580 mm²
Thread profile
2-1/2-4 UNC basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 2.5000 | 63.500 |
| Pitch P | 0.25000 | 6.350 |
| Pitch diameter d2 = D2 | 2.3376 | 59.376 |
| Minor diameter, internal thread D1 | 2.2294 | 56.626 |
| Minor diameter, external UNR thread d3 | 2.2023 | 55.939 |
| Height of fundamental triangle H | 0.2165 | 5.499 |
| Thread height, internal (5H/8) | 0.1353 | 3.437 |
| Thread height, external (11H/16) | 0.1488 | 3.781 |
| Crest flat of external thread (P/8) | 0.0312 | 0.794 |
| Crest flat of internal thread (P/4) | 0.0625 | 1.587 |
| Property | Value |
|---|---|
| Threads per inch | 4 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 1.95° |
| Tensile stress area As | 4 in² (2580 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.
2-1/2-4 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 0.0031 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 | 2.4969 | 2.4731 | 0.0238 |
| Pitch diameter | 2.3345 | 2.3241 | 0.0104 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 2.267 | 2.229 | 0.0380 |
| Pitch diameter | 2.3511 | 2.3376 | 0.0135 |
Limits of size from ASME B1.1 Table 2.
2-1/2-4 UNC tap drill size
Tap drill for 2-1/2-4 UNC: 2-1/4" (2.2500 in, 57.15 mm)
This leaves about 77% thread in the tapped hole.
2-1/4" is the drill listed for 2-1/2-4 UNC in standard tap drill charts. It is 0.2500 in under the 2.5000 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 2.229 and 2.267 in. The recommended drill falls inside that band.
Other drills that work for 2-1/2-4 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 |
|---|---|---|---|---|---|
| 57 mm | 57.00 | 2.2441 | 79% | yes | |
| 2-1/4"recommended | 57.15 | 2.2500 | 77% | yes | |
| 2-5/16" | 58.74 | 2.3125 | 58% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for 2-1/2 in
ASME B18.2.8 covers sizes up to 1-1/2 in, so there is no standard clearance hole for the 2-1/2 in size. Size the hole from the fit the joint needs.
2-1/2-4 UNC strength
SAE J429 stops at 1-1/2 in, so the SAE grades give no proof load for 2-1/2-4 UNC. Bolts this large are made to specifications such as ASTM A307, A449 or A354, whose strengths depend on diameter. For an estimate, multiply the tensile stress area of 4 in² by the strength of the material: for example, 60,000 psi gives 240,000 lbf.
Closest metric thread
No metric thread is interchangeable with 2-1/2-4 UNC. The nearest is M64×6, which is 0.50 mm larger in diameter with a pitch of 6.000 mm against 6.350 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
How the 2-1/2-4 UNC dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.25000 in = 1 ÷ 4 threads per inch
- H = 0.866025 × P = 0.2165 in
- d2 = d − 0.649519 × P = 2.5000 − 0.1624 = 2.3376 in
- D1 = d − 1.082532 × P = 2.5000 − 0.2706 = 2.2294 in
- d3 = d − 1.190785 × P = 2.5000 − 0.2977 = 2.2023 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 2.2564² = 3.99883 in²
- Lead angle = arctan(P ÷ (π × d2)) = 1.95°
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.