What is the tap drill size for 1 1/4"-7 UNC?

The tap drill for 1 1/4"-7 UNC is 1 — 1.0000 in (25.400 mm). That leaves 134.7% thread engagement with a cut tap.

What is the pitch diameter of 1 1/4"-7 UNC?

The basic pitch diameter of 1 1/4"-7 UNC is 1.1572 in (29.393 mm). For a class 2B thread it runs 1.1572 in (29.393 mm) to 1.1668 in (29.637 mm).

What is the minor diameter of 1 1/4"-7 UNC?

The basic minor diameter of 1 1/4"-7 UNC is 1.0954 in (27.823 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 1 1/4"-7 UNC?

1 1/4"-7 UNC has 7 threads per inch, a pitch of 0.1429 in (3.629 mm).

What size clearance hole does a 1 1/4"-7 UNC fastener need?

close fit 1 (1.2795 in (32.499 mm)), normal fit 1 (1.3425 in (34.100 mm)), loose fit 1 (1.4370 in (36.500 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of 1 1/4"-7 UNC?

The tensile stress area of 1 1/4"-7 UNC is 0.96911 in² (625.23 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Coarse (UNC) · 1 1/4-7 UNC

1 1/4"-7 UNC Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unc/1-1-4-7 · data verified August 1, 2026

The tap drill for 1 1/4"-7 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 134.7% thread engagement with a cut tap. Basic pitch diameter is 1.1572 in (29.393 mm) and basic minor diameter is 1.0954 in (27.823 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: 1 1/4-7 UNC
TPI: 7
Standard: ASME B1.1

Tap drill · 134.7% thread · cut tap

Drill Ø: 1.0000in
Drill Ø: 25.400mm
Clearance (normal): 1

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 134.7% thread.

Basic Thread Dimensions for 1 1/4"-7 UNC

Basic thread dimensions — 1 1/4"-7 UNC
Property	Inch	Millimetre
Basic major diameter	1.2500	31.750
Threads per inch	7	—
Pitch	0.1429	3.629
Basic pitch diameter	1.1572	29.393
Basic minor diameter	1.0954	27.823
External thread root diameter	1.0799	27.430
Sharp V thread height (H)	0.1237	3.142

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 1 1/4"-7 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 — 1 1/4"-7 UNC, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	1.1572	29.393	1	1.0000	25.400	134.7
60%	1.1387	28.923	1	1.0000	25.400	134.7
65%	1.1294	28.687	1	1.0000	25.400	134.7
70%	1.1201	28.451	1	1.0000	25.400	134.7
75%default	1.1108	28.214	1	1.0000	25.400	134.7
80%	1.1015	27.978	1	1.0000	25.400	134.7
85%	1.0923	27.744	1	1.0000	25.400	134.7

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.

Clearance Hole Sizes for 1 1/4"-7 UNC

The through-hole a 1 1/4"-7 UNC fastener passes freely, not the tapped hole.

Clearance hole sizes — 1 1/4"-7 UNC
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	1	1.2795	32.499	Located joints, dowelled assemblies
Normal	1	1.3425	34.100	General assembly — the default
Loose	1	1.4370	36.500	Slotted or adjustable joints, weldments

Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch). The drill column is the nearest standard drill at or above the specified hole diameter.

Thread Class Limits for 1 1/4"-7 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 — 1 1/4"-7 UNC
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	1.2478	31.694	1.2500	31.750
Major Ø min	1.2314	31.278	1.2336	31.333
Pitch Ø max	1.1550	29.337	1.1572	29.393
Pitch Ø min	1.1476	29.149	1.1517	29.253
Pitch Ø tolerance	0.0074	0.188	0.0055	0.140
Minor Ø max	1.0622	26.980	1.0644	27.036
Minor Ø min	1.0548	26.792	1.0589	26.896
Allowance	0.0022	0.056	0.0000	0.000

Internal thread (nut / tapped hole) limits — 1 1/4"-7 UNC
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	1.2802	32.517	1.2778	32.456
Major Ø min	1.2500	31.750	1.2500	31.750
Pitch Ø max	1.1668	29.637	1.1644	29.576
Pitch Ø min	1.1572	29.393	1.1572	29.393
Pitch Ø tolerance	0.0096	0.244	0.0072	0.183
Minor Ø max	1.1229	28.522	1.1125	28.258
Minor Ø min	1.0954	27.823	1.0954	27.823
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 1 1/4"-7 UNC

The effective cross-section used to size a 1 1/4"-7 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 1 1/4"-7 UNC
Property	in²	mm²
Tensile stress area	0.96911	625.23

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

Other Thread Pitches in the Same Size as 1 1/4"-7 UNC

A 1 1/4 fastener is not always 1 1/4"-7 UNC. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 1 1/4
Thread	Series	Pitch	Tap drill
1 1/4"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1
1 1/4"-12 UNF	Unified Fine (UNF)	12 TPI	1

Sources and Method for 1 1/4"-7 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 1 1/4"-7 UNC Reference

Citation

ShopMath. "1 1/4"-7 UNC Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unc/1-1-4-7

BibTeX

@misc{shopmath-threads-unc-1-1-4-7,
  title        = {1 1/4"-7 UNC Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unc/1-1-4-7}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unc/1-1-4-7

Common Questions About 1 1/4"-7 UNC

What is the tap drill size for 1 1/4"-7 UNC?: The tap drill for 1 1/4"-7 UNC is 1 — 1.0000 in (25.400 mm). That leaves 134.7% thread engagement with a cut tap.
What is the pitch diameter of 1 1/4"-7 UNC?: The basic pitch diameter of 1 1/4"-7 UNC is 1.1572 in (29.393 mm). For a class 2B thread it runs 1.1572 in (29.393 mm) to 1.1668 in (29.637 mm).
What is the minor diameter of 1 1/4"-7 UNC?: The basic minor diameter of 1 1/4"-7 UNC is 1.0954 in (27.823 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 1 1/4"-7 UNC?: 1 1/4"-7 UNC has 7 threads per inch, a pitch of 0.1429 in (3.629 mm).
What size clearance hole does a 1 1/4"-7 UNC fastener need?: close fit 1 (1.2795 in (32.499 mm)), normal fit 1 (1.3425 in (34.100 mm)), loose fit 1 (1.4370 in (36.500 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of 1 1/4"-7 UNC?: The tensile stress area of 1 1/4"-7 UNC is 0.96911 in² (625.23 mm²). Multiply it by the fastener material's allowable stress to get a load.

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