Unified Coarse (UNC) · 3 1/4-4 UNC
3 1/4"-4 UNC Thread — Dimensions, Tap Drill & Clearance
https://shopmath.org/threads/unc/3-1-4-4 · data verified August 1, 2026
The tap drill for 3 1/4"-4 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 692.8% thread engagement with a cut tap. Basic pitch diameter is 3.0876 in (78.425 mm) and basic minor diameter is 2.9794 in (75.677 mm). Tapped-hole figures on this page assume a class 2B thread; drill sizes for 50–85% engagement are in the tap drill table below.
- Series
- Unified Coarse (UNC)
- Designation
- 3 1/4-4 UNC
- TPI
- 4
- Standard
- ASME B1.1
Tap drill · 692.8% thread · cut tap
1
- Drill Ø
- 1.0000in
- Drill Ø
- 25.400mm
Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 692.8% thread.
Basic Thread Dimensions for 3 1/4"-4 UNC
| Property | Inch | Millimetre |
|---|---|---|
| Basic major diameter | 3.2500 | 82.550 |
| Threads per inch | 4 | — |
| Pitch | 0.2500 | 6.350 |
| Basic pitch diameter | 3.0876 | 78.425 |
| Basic minor diameter | 2.9794 | 75.677 |
| External thread root diameter | 2.9523 | 74.988 |
| Sharp V thread height (H) | 0.2165 | 5.499 |
Threads per inch has no millimetre counterpart — metric threads are specified by pitch instead. Basic dimensions are the theoretical values before any tolerance class is applied.
Tap Drill Sizes for 3 1/4"-4 UNC
Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 1 drill. Deeper engagement means more holding strength and more torque on the tap.
| Property | Target hole Ø | Nearest standard drill | Property | |||
|---|---|---|---|---|---|---|
| Thread % | in | mm | Drill | in | mm | Actual thread % |
| 50% | 3.0876 | 78.425 | 1 | 1.0000 | 25.400 | 692.8 |
| 60% | 3.0551 | 77.600 | 1 | 1.0000 | 25.400 | 692.8 |
| 65% | 3.0389 | 77.188 | 1 | 1.0000 | 25.400 | 692.8 |
| 70% | 3.0227 | 76.777 | 1 | 1.0000 | 25.400 | 692.8 |
| 75%default | 3.0064 | 76.363 | 1 | 1.0000 | 25.400 | 692.8 |
| 80% | 2.9902 | 75.951 | 1 | 1.0000 | 25.400 | 692.8 |
| 85% | 2.9740 | 75.540 | 1 | 1.0000 | 25.400 | 692.8 |
Target hole Ø is the theoretical hole for the stated engagement. The drill column is the nearest size in the standard number/letter/fractional/metric index, and actual thread % is what that real drill produces — the number that matters at the machine. Values are for cut taps. Tapped-hole figures assume a class 2B hole.
Thread Class Limits for 3 1/4"-4 UNC
Manufacturing limits for classes 2B, 3B, 2A, 3A. A gauge reading outside these bands is a reject, not a judgement call.
| Property | Class 2A | Class 3A | ||
|---|---|---|---|---|
| Limit | in | mm | in | mm |
| Major Ø max | 3.2467 | 82.466 | 3.2500 | 82.550 |
| Major Ø min | 3.2229 | 81.862 | 3.2262 | 81.945 |
| Pitch Ø max | 3.0844 | 78.344 | 3.0876 | 78.425 |
| Pitch Ø min | 3.0735 | 78.067 | 3.0795 | 78.219 |
| Pitch Ø tolerance | 0.0109 | 0.277 | 0.0082 | 0.208 |
| Minor Ø max | 2.9220 | 74.219 | 2.9252 | 74.300 |
| Minor Ø min | 2.9111 | 73.942 | 2.9171 | 74.094 |
| Allowance | 0.0033 | 0.084 | 0.0000 | 0.000 |
| Property | Class 2B | Class 3B | ||
|---|---|---|---|---|
| Limit | in | mm | in | mm |
| Major Ø max | 3.3002 | 83.825 | 3.2967 | 83.736 |
| Major Ø min | 3.2500 | 82.550 | 3.2500 | 82.550 |
| Pitch Ø max | 3.1018 | 78.786 | 3.0982 | 78.694 |
| Pitch Ø min | 3.0876 | 78.425 | 3.0876 | 78.425 |
| Pitch Ø tolerance | 0.0141 | 0.358 | 0.0106 | 0.269 |
| Minor Ø max | 3.0169 | 76.629 | 3.0094 | 76.439 |
| Minor Ø min | 2.9794 | 75.677 | 2.9794 | 75.677 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |
Tensile Stress Area of 3 1/4"-4 UNC
The effective cross-section used to size a 3 1/4"-4 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.
| Property | in² | mm² |
|---|---|---|
| Tensile stress area | 7.09891 | 4579.93 |
The formula this value comes from is in the sources section below.
Sources and Method for 3 1/4"-4 UNC
- Basic dimensionscomputed
NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1; series membership: Series membership: threadlib UIS_thread.csv (BSD-3-Clause); cross-checked against NBS H28 (1969) Part I Table 2.21; clearance holes: ISO 273 (metric) and ASME B18.2.8 (inch) via FreeCAD FeatureHole.cpp (LGPL-2.1+), cross-checked against bd_warehouse clearance_hole_sizes.csv (Apache-2.0)
Verified against NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in
Computed from the standard's formulas, then checked against published tables.
- Tap drill sizescomputed
Machinery's Handbook %-thread formula (geometry-derived); nearest-standard-drill-to-75% rule verified at 36/39 published chart rows in docs/research/data-sourcing.md §3.4
Verified against Wikipedia Unified Thread Standard, preferred cutting tap drill size columns (CC BY-SA 4.0) — 36/39 published UNC+UNF rows reproduced
Computed from the standard's formulas, then checked against published tables.
- Class limitscomputed
NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1
Verified against NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in
Computed from the standard's formulas, then checked against published tables.
- Formula
Tap drill hole diameter, inch threads
hole Ø = D − (%thread ÷ 100) × (1.29904 ÷ n) D = basic major diameter (in) n = threads per inch
1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° Unified profile.
Actual thread engagement of the chosen drill
%thread = (D − drill Ø) × n ÷ 1.29904 × 100
The drill column rounds to a real drill; this is the engagement that drill actually leaves.
Tensile stress area, Unified inch threads
At = 0.7854 × (D − 0.9743 ÷ n)²
ASME B1.1 / NBS H28 form. Reproduces the stored value exactly for this thread.
- Assumptions
- The headline tap drill assumes 75% thread engagement, the general shop default. Every other engagement from 50% to 85% is in the tap drill table — change the assumption and the number moves.
- Tap drill figures are for cut taps. Drill diameters are nominal; a worn or run-out drill cuts oversize and reduces engagement further.
- Tapped-hole figures assume a class 2B internal thread, the general-purpose fit.
- Dataset scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.
- Data last verified
- August 1, 2026
How to Cite This 3 1/4"-4 UNC Reference
Citation
ShopMath. "3 1/4"-4 UNC Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unc/3-1-4-4
BibTeX
@misc{shopmath-threads-unc-3-1-4-4,
title = {3 1/4"-4 UNC Thread — Dimensions, Tap Drill & Clearance},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/threads/unc/3-1-4-4}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/threads/unc/3-1-4-4
Common Questions About 3 1/4"-4 UNC
What is the tap drill size for 3 1/4"-4 UNC?
- The tap drill for 3 1/4"-4 UNC is 1 — 1.0000 in (25.400 mm). That leaves 692.8% thread engagement with a cut tap.
What is the pitch diameter of 3 1/4"-4 UNC?
- The basic pitch diameter of 3 1/4"-4 UNC is 3.0876 in (78.425 mm). For a class 2B thread it runs 3.0876 in (78.425 mm) to 3.1018 in (78.786 mm).
What is the minor diameter of 3 1/4"-4 UNC?
- The basic minor diameter of 3 1/4"-4 UNC is 2.9794 in (75.677 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.
What is the thread pitch of 3 1/4"-4 UNC?
- 3 1/4"-4 UNC has 4 threads per inch, a pitch of 0.2500 in (6.350 mm).
What is the tensile stress area of 3 1/4"-4 UNC?
- The tensile stress area of 3 1/4"-4 UNC is 7.09891 in² (4579.93 mm²). Multiply it by the fastener material's allowable stress to get a load.