What is the tap drill size for 1 1/2"-6 UNC?

The tap drill for 1 1/2"-6 UNC is 1 — 1.0000 in (25.400 mm). That leaves 230.9% thread engagement with a cut tap.

What is the pitch diameter of 1 1/2"-6 UNC?

The basic pitch diameter of 1 1/2"-6 UNC is 1.3917 in (35.349 mm). For a class 2B thread it runs 1.3917 in (35.349 mm) to 1.4023 in (35.618 mm).

What is the minor diameter of 1 1/2"-6 UNC?

The basic minor diameter of 1 1/2"-6 UNC is 1.3196 in (33.518 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 1/2"-6 UNC?

1 1/2"-6 UNC has 6 threads per inch, a pitch of 0.1667 in (4.233 mm).

What size clearance hole does a 1 1/2"-6 UNC fastener need?

close fit 1 (1.5630 in (39.700 mm)), normal fit 1 (1.6260 in (41.300 mm)), loose fit 1 (1.7323 in (44.000 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of 1 1/2"-6 UNC?

The tensile stress area of 1 1/2"-6 UNC is 1.40525 in² (906.61 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Coarse (UNC) · 1 1/2-6 UNC

1 1/2"-6 UNC Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unc/1-1-2-6 · data verified August 1, 2026

The tap drill for 1 1/2"-6 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 230.9% thread engagement with a cut tap. Basic pitch diameter is 1.3917 in (35.349 mm) and basic minor diameter is 1.3196 in (33.518 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 1/2-6 UNC
TPI: 6
Standard: ASME B1.1

Tap drill · 230.9% 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 230.9% thread.

Basic Thread Dimensions for 1 1/2"-6 UNC

Basic thread dimensions — 1 1/2"-6 UNC
Property	Inch	Millimetre
Basic major diameter	1.5000	38.100
Threads per inch	6	—
Pitch	0.1667	4.233
Basic pitch diameter	1.3917	35.349
Basic minor diameter	1.3196	33.518
External thread root diameter	1.3015	33.058
Sharp V thread height (H)	0.1443	3.665

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 1/2"-6 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 1/2"-6 UNC, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	1.3917	35.349	1	1.0000	25.400	230.9
60%	1.3701	34.800	1	1.0000	25.400	230.9
65%	1.3593	34.526	1	1.0000	25.400	230.9
70%	1.3484	34.249	1	1.0000	25.400	230.9
75%default	1.3376	33.975	1	1.0000	25.400	230.9
80%	1.3268	33.701	1	1.0000	25.400	230.9
85%	1.3160	33.426	1	1.0000	25.400	230.9

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.

Clearance Hole Sizes for 1 1/2"-6 UNC

The through-hole a 1 1/2"-6 UNC fastener passes freely, not the tapped hole.

Clearance hole sizes — 1 1/2"-6 UNC
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	1	1.5630	39.700	Located joints, dowelled assemblies
Normal	1	1.6260	41.300	General assembly — the default
Loose	1	1.7323	44.000	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 1/2"-6 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 1/2"-6 UNC
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	1.4976	38.039	1.5000	38.100
Major Ø min	1.4794	37.577	1.4818	37.638
Pitch Ø max	1.3893	35.288	1.3917	35.349
Pitch Ø min	1.3812	35.083	1.3857	35.197
Pitch Ø tolerance	0.0081	0.206	0.0061	0.155
Minor Ø max	1.2811	32.540	1.2835	32.601
Minor Ø min	1.2730	32.334	1.2774	32.446
Allowance	0.0024	0.061	0.0000	0.000

Internal thread (nut / tapped hole) limits — 1 1/2"-6 UNC
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	1.5346	38.979	1.5320	38.913
Major Ø min	1.5000	38.100	1.5000	38.100
Pitch Ø max	1.4023	35.618	1.3996	35.550
Pitch Ø min	1.3917	35.349	1.3917	35.349
Pitch Ø tolerance	0.0105	0.267	0.0079	0.201
Minor Ø max	1.3501	34.292	1.3396	34.026
Minor Ø min	1.3196	33.518	1.3196	33.518
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 1 1/2"-6 UNC

The effective cross-section used to size a 1 1/2"-6 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 1 1/2"-6 UNC
Property	in²	mm²
Tensile stress area	1.40525	906.61

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

Other Thread Pitches in the Same Size as 1 1/2"-6 UNC

A 1 1/2 fastener is not always 1 1/2"-6 UNC. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 1 1/2
Thread	Series	Pitch	Tap drill
1 1/2"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1
1 1/2"-12 UNF	Unified Fine (UNF)	12 TPI	1

Sources and Method for 1 1/2"-6 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 1/2"-6 UNC Reference

Citation

ShopMath. "1 1/2"-6 UNC Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unc/1-1-2-6

BibTeX

@misc{shopmath-threads-unc-1-1-2-6,
  title        = {1 1/2"-6 UNC Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unc/1-1-2-6}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unc/1-1-2-6

Common Questions About 1 1/2"-6 UNC

What is the tap drill size for 1 1/2"-6 UNC?: The tap drill for 1 1/2"-6 UNC is 1 — 1.0000 in (25.400 mm). That leaves 230.9% thread engagement with a cut tap.
What is the pitch diameter of 1 1/2"-6 UNC?: The basic pitch diameter of 1 1/2"-6 UNC is 1.3917 in (35.349 mm). For a class 2B thread it runs 1.3917 in (35.349 mm) to 1.4023 in (35.618 mm).
What is the minor diameter of 1 1/2"-6 UNC?: The basic minor diameter of 1 1/2"-6 UNC is 1.3196 in (33.518 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 1/2"-6 UNC?: 1 1/2"-6 UNC has 6 threads per inch, a pitch of 0.1667 in (4.233 mm).
What size clearance hole does a 1 1/2"-6 UNC fastener need?: close fit 1 (1.5630 in (39.700 mm)), normal fit 1 (1.6260 in (41.300 mm)), loose fit 1 (1.7323 in (44.000 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of 1 1/2"-6 UNC?: The tensile stress area of 1 1/2"-6 UNC is 1.40525 in² (906.61 mm²). Multiply it by the fastener material's allowable stress to get a load.

All Unified Coarse (UNC) sizes · Every thread series