What is the tap drill size for 2 1/2"-4 UNC?

The tap drill for 2 1/2"-4 UNC is 1 — 1.0000 in (25.400 mm). That leaves 461.9% thread engagement with a cut tap.

What is the pitch diameter of 2 1/2"-4 UNC?

The basic pitch diameter of 2 1/2"-4 UNC is 2.3376 in (59.375 mm). For a class 2B thread it runs 2.3376 in (59.375 mm) to 2.3511 in (59.718 mm).

What is the minor diameter of 2 1/2"-4 UNC?

The basic minor diameter of 2 1/2"-4 UNC is 2.2294 in (56.627 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 2 1/2"-4 UNC?

2 1/2"-4 UNC has 4 threads per inch, a pitch of 0.2500 in (6.350 mm).

What is the tensile stress area of 2 1/2"-4 UNC?

The tensile stress area of 2 1/2"-4 UNC is 3.99883 in² (2579.88 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Coarse (UNC) · 2 1/2-4 UNC

2 1/2"-4 UNC Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unc/2-1-2-4 · data verified August 1, 2026

The tap drill for 2 1/2"-4 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 461.9% thread engagement with a cut tap. Basic pitch diameter is 2.3376 in (59.375 mm) and basic minor diameter is 2.2294 in (56.627 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: 2 1/2-4 UNC
TPI: 4
Standard: ASME B1.1

Tap drill · 461.9% thread · cut tap

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 461.9% thread.

Basic Thread Dimensions for 2 1/2"-4 UNC

Basic thread dimensions — 2 1/2"-4 UNC
Property	Inch	Millimetre
Basic major diameter	2.5000	63.500
Threads per inch	4	—
Pitch	0.2500	6.350
Basic pitch diameter	2.3376	59.375
Basic minor diameter	2.2294	56.627
External thread root diameter	2.2023	55.938
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 2 1/2"-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.

Tap drill sizes by thread engagement — 2 1/2"-4 UNC, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	2.3376	59.375	1	1.0000	25.400	461.9
60%	2.3051	58.550	1	1.0000	25.400	461.9
65%	2.2889	58.138	1	1.0000	25.400	461.9
70%	2.2727	57.727	1	1.0000	25.400	461.9
75%default	2.2564	57.313	1	1.0000	25.400	461.9
80%	2.2402	56.901	1	1.0000	25.400	461.9
85%	2.2240	56.490	1	1.0000	25.400	461.9

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 2 1/2"-4 UNC

Manufacturing limits for classes 2B, 3B, 2A, 3A. A gauge reading outside these bands is a reject, not a judgement call.

External thread (bolt / screw) limits — 2 1/2"-4 UNC
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	2.4969	63.421	2.5000	63.500
Major Ø min	2.4731	62.817	2.4762	62.895
Pitch Ø max	2.3345	59.296	2.3376	59.375
Pitch Ø min	2.3242	59.035	2.3299	59.179
Pitch Ø tolerance	0.0104	0.264	0.0078	0.198
Minor Ø max	2.1721	55.171	2.1752	55.250
Minor Ø min	2.1618	54.910	2.1675	55.054
Allowance	0.0031	0.079	0.0000	0.000

Internal thread (nut / tapped hole) limits — 2 1/2"-4 UNC
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	2.5496	64.760	2.5462	64.674
Major Ø min	2.5000	63.500	2.5000	63.500
Pitch Ø max	2.3511	59.718	2.3477	59.632
Pitch Ø min	2.3376	59.375	2.3376	59.375
Pitch Ø tolerance	0.0135	0.343	0.0101	0.257
Minor Ø max	2.2669	57.579	2.2594	57.389
Minor Ø min	2.2294	56.627	2.2294	56.627
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 2 1/2"-4 UNC

The effective cross-section used to size a 2 1/2"-4 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 2 1/2"-4 UNC
Property	in²	mm²
Tensile stress area	3.99883	2579.88

The formula this value comes from is in the sources section below.

Sources and Method for 2 1/2"-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 this data is built and checked →

How to Cite This 2 1/2"-4 UNC Reference

Citation

ShopMath. "2 1/2"-4 UNC Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unc/2-1-2-4

BibTeX

@misc{shopmath-threads-unc-2-1-2-4,
  title        = {2 1/2"-4 UNC Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unc/2-1-2-4}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unc/2-1-2-4

Common Questions About 2 1/2"-4 UNC

What is the tap drill size for 2 1/2"-4 UNC?: The tap drill for 2 1/2"-4 UNC is 1 — 1.0000 in (25.400 mm). That leaves 461.9% thread engagement with a cut tap.
What is the pitch diameter of 2 1/2"-4 UNC?: The basic pitch diameter of 2 1/2"-4 UNC is 2.3376 in (59.375 mm). For a class 2B thread it runs 2.3376 in (59.375 mm) to 2.3511 in (59.718 mm).
What is the minor diameter of 2 1/2"-4 UNC?: The basic minor diameter of 2 1/2"-4 UNC is 2.2294 in (56.627 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 2 1/2"-4 UNC?: 2 1/2"-4 UNC has 4 threads per inch, a pitch of 0.2500 in (6.350 mm).
What is the tensile stress area of 2 1/2"-4 UNC?: The tensile stress area of 2 1/2"-4 UNC is 3.99883 in² (2579.88 mm²). Multiply it by the fastener material's allowable stress to get a load.

All Unified Coarse (UNC) sizes · Every thread series