What is the tap drill size for 1 1/2"-18 UNEF?

The tap drill for 1 1/2"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 692.8% thread engagement with a cut tap.

What is the pitch diameter of 1 1/2"-18 UNEF?

The basic pitch diameter of 1 1/2"-18 UNEF is 1.4639 in (37.183 mm). For a class 2B thread it runs 1.4639 in (37.183 mm) to 1.4704 in (37.348 mm).

What is the minor diameter of 1 1/2"-18 UNEF?

The basic minor diameter of 1 1/2"-18 UNEF is 1.4399 in (36.574 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"-18 UNEF?

1 1/2"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

What size clearance hole does a 1 1/2"-18 UNEF 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"-18 UNEF?

The tensile stress area of 1 1/2"-18 UNEF is 1.64192 in² (1059.30 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Extra Fine (UNEF) · 1 1/2-18 UNEF

1 1/2"-18 UNEF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unef/1-1-2-18 · data verified August 1, 2026

The tap drill for 1 1/2"-18 UNEF is 1 — 1.0000 in (25.400 mm) — which leaves 692.8% thread engagement with a cut tap. Basic pitch diameter is 1.4639 in (37.183 mm) and basic minor diameter is 1.4399 in (36.574 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 Extra Fine (UNEF)
Designation: 1 1/2-18 UNEF
TPI: 18
Standard: ASME B1.1

Tap drill · 692.8% 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 692.8% thread.

Basic Thread Dimensions for 1 1/2"-18 UNEF

Basic thread dimensions — 1 1/2"-18 UNEF
Property	Inch	Millimetre
Basic major diameter	1.5000	38.100
Threads per inch	18	—
Pitch	0.0556	1.411
Basic pitch diameter	1.4639	37.183
Basic minor diameter	1.4399	36.574
External thread root diameter	1.4338	36.419
Sharp V thread height (H)	0.0481	1.222

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"-18 UNEF

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"-18 UNEF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	1.4639	37.183	1	1.0000	25.400	692.8
60%	1.4567	37.000	1	1.0000	25.400	692.8
65%	1.4531	36.909	1	1.0000	25.400	692.8
70%	1.4495	36.817	1	1.0000	25.400	692.8
75%default	1.4459	36.726	1	1.0000	25.400	692.8
80%	1.4423	36.634	1	1.0000	25.400	692.8
85%	1.4387	36.543	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.

Clearance Hole Sizes for 1 1/2"-18 UNEF

The through-hole a 1 1/2"-18 UNEF fastener passes freely, not the tapped hole.

Clearance hole sizes — 1 1/2"-18 UNEF
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"-18 UNEF

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"-18 UNEF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	1.4985	38.062	1.5000	38.100
Major Ø min	1.4898	37.841	1.4913	37.879
Pitch Ø max	1.4624	37.145	1.4639	37.183
Pitch Ø min	1.4575	37.020	1.4602	37.089
Pitch Ø tolerance	0.0050	0.127	0.0037	0.094
Minor Ø max	1.4263	36.228	1.4278	36.266
Minor Ø min	1.4214	36.104	1.4241	36.172
Allowance	0.0015	0.038	0.0000	0.000

Internal thread (nut / tapped hole) limits — 1 1/2"-18 UNEF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	1.5145	38.468	1.5129	38.428
Major Ø min	1.5000	38.100	1.5000	38.100
Pitch Ø max	1.4704	37.348	1.4688	37.307
Pitch Ø min	1.4639	37.183	1.4639	37.183
Pitch Ø tolerance	0.0065	0.165	0.0048	0.122
Minor Ø max	1.4525	36.894	1.4480	36.779
Minor Ø min	1.4399	36.574	1.4399	36.574
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 1 1/2"-18 UNEF

The effective cross-section used to size a 1 1/2"-18 UNEF 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"-18 UNEF
Property	in²	mm²
Tensile stress area	1.64192	1059.30

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

Other Thread Pitches in the Same Size as 1 1/2"-18 UNEF

A 1 1/2 fastener is not always 1 1/2"-18 UNEF. 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"-12 UNF	Unified Fine (UNF)	12 TPI	1
1 1/2"-6 UNC	Unified Coarse (UNC)	6 TPI	1

Sources and Method for 1 1/2"-18 UNEF

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"-18 UNEF Reference

Citation

ShopMath. "1 1/2"-18 UNEF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unef/1-1-2-18

BibTeX

@misc{shopmath-threads-unef-1-1-2-18,
  title        = {1 1/2"-18 UNEF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unef/1-1-2-18}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unef/1-1-2-18

Common Questions About 1 1/2"-18 UNEF

What is the tap drill size for 1 1/2"-18 UNEF?: The tap drill for 1 1/2"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 692.8% thread engagement with a cut tap.
What is the pitch diameter of 1 1/2"-18 UNEF?: The basic pitch diameter of 1 1/2"-18 UNEF is 1.4639 in (37.183 mm). For a class 2B thread it runs 1.4639 in (37.183 mm) to 1.4704 in (37.348 mm).
What is the minor diameter of 1 1/2"-18 UNEF?: The basic minor diameter of 1 1/2"-18 UNEF is 1.4399 in (36.574 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"-18 UNEF?: 1 1/2"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).
What size clearance hole does a 1 1/2"-18 UNEF 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"-18 UNEF?: The tensile stress area of 1 1/2"-18 UNEF is 1.64192 in² (1059.30 mm²). Multiply it by the fastener material's allowable stress to get a load.

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