What is the tap drill size for 1 1/8"-18 UNEF?

The tap drill for 1 1/8"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 173.2% thread engagement with a cut tap.

What is the pitch diameter of 1 1/8"-18 UNEF?

The basic pitch diameter of 1 1/8"-18 UNEF is 1.0889 in (27.658 mm). For a class 2B thread it runs 1.0889 in (27.658 mm) to 1.0952 in (27.818 mm).

What is the minor diameter of 1 1/8"-18 UNEF?

The basic minor diameter of 1 1/8"-18 UNEF is 1.0649 in (27.049 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 1/8"-18 UNEF?

1 1/8"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

What size clearance hole does a 1 1/8"-18 UNEF fastener need?

close fit 1 (1.1575 in (29.401 mm)), normal fit 1 (1.2205 in (31.001 mm)), loose fit 1 (1.3110 in (33.299 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of 1 1/8"-18 UNEF?

The tensile stress area of 1 1/8"-18 UNEF is 0.90067 in² (581.08 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Extra Fine (UNEF) · 1 1/8-18 UNEF

1 1/8"-18 UNEF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unef/1-1-8-18 · data verified August 1, 2026

The tap drill for 1 1/8"-18 UNEF is 1 — 1.0000 in (25.400 mm) — which leaves 173.2% thread engagement with a cut tap. Basic pitch diameter is 1.0889 in (27.658 mm) and basic minor diameter is 1.0649 in (27.049 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 1/8-18 UNEF
TPI: 18
Standard: ASME B1.1

Tap drill · 173.2% 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 173.2% thread.

Basic Thread Dimensions for 1 1/8"-18 UNEF

Basic thread dimensions — 1 1/8"-18 UNEF
Property	Inch	Millimetre
Basic major diameter	1.1250	28.575
Threads per inch	18	—
Pitch	0.0556	1.411
Basic pitch diameter	1.0889	27.658
Basic minor diameter	1.0649	27.049
External thread root diameter	1.0588	26.893
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 1/8"-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 1/8"-18 UNEF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	1.0889	27.658	1	1.0000	25.400	173.2
60%	1.0817	27.475	1	1.0000	25.400	173.2
65%	1.0781	27.384	1	1.0000	25.400	173.2
70%	1.0745	27.292	1	1.0000	25.400	173.2
75%default	1.0709	27.201	1	1.0000	25.400	173.2
80%	1.0673	27.109	1	1.0000	25.400	173.2
85%	1.0637	27.018	1	1.0000	25.400	173.2

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 1/8"-18 UNEF

The through-hole a 1 1/8"-18 UNEF fastener passes freely, not the tapped hole.

Clearance hole sizes — 1 1/8"-18 UNEF
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	1	1.1575	29.401	Located joints, dowelled assemblies
Normal	1	1.2205	31.001	General assembly — the default
Loose	1	1.3110	33.299	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 1/8"-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 1/8"-18 UNEF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	1.1236	28.539	1.1250	28.575
Major Ø min	1.1148	28.316	1.1163	28.354
Pitch Ø max	1.0875	27.622	1.0889	27.658
Pitch Ø min	1.0827	27.501	1.0853	27.567
Pitch Ø tolerance	0.0048	0.122	0.0036	0.091
Minor Ø max	1.0514	26.706	1.0528	26.741
Minor Ø min	1.0466	26.584	1.0492	26.650
Allowance	0.0014	0.036	0.0000	0.000

Internal thread (nut / tapped hole) limits — 1 1/8"-18 UNEF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	1.1393	28.938	1.1377	28.898
Major Ø min	1.1250	28.575	1.1250	28.575
Pitch Ø max	1.0952	27.818	1.0936	27.777
Pitch Ø min	1.0889	27.658	1.0889	27.658
Pitch Ø tolerance	0.0062	0.158	0.0047	0.119
Minor Ø max	1.0775	27.369	1.0730	27.254
Minor Ø min	1.0649	27.049	1.0649	27.049
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 1 1/8"-18 UNEF

The effective cross-section used to size a 1 1/8"-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 1/8"-18 UNEF
Property	in²	mm²
Tensile stress area	0.90067	581.08

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

Other Thread Pitches in the Same Size as 1 1/8"-18 UNEF

A 1 1/8 fastener is not always 1 1/8"-18 UNEF. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 1 1/8
Thread	Series	Pitch	Tap drill
1 1/8"-12 UNF	Unified Fine (UNF)	12 TPI	1
1 1/8"-7 UNC	Unified Coarse (UNC)	7 TPI	63/64

Sources and Method for 1 1/8"-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 1/8"-18 UNEF Reference

Citation

ShopMath. "1 1/8"-18 UNEF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unef/1-1-8-18

BibTeX

@misc{shopmath-threads-unef-1-1-8-18,
  title        = {1 1/8"-18 UNEF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unef/1-1-8-18}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unef/1-1-8-18

Common Questions About 1 1/8"-18 UNEF

What is the tap drill size for 1 1/8"-18 UNEF?: The tap drill for 1 1/8"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 173.2% thread engagement with a cut tap.
What is the pitch diameter of 1 1/8"-18 UNEF?: The basic pitch diameter of 1 1/8"-18 UNEF is 1.0889 in (27.658 mm). For a class 2B thread it runs 1.0889 in (27.658 mm) to 1.0952 in (27.818 mm).
What is the minor diameter of 1 1/8"-18 UNEF?: The basic minor diameter of 1 1/8"-18 UNEF is 1.0649 in (27.049 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 1/8"-18 UNEF?: 1 1/8"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).
What size clearance hole does a 1 1/8"-18 UNEF fastener need?: close fit 1 (1.1575 in (29.401 mm)), normal fit 1 (1.2205 in (31.001 mm)), loose fit 1 (1.3110 in (33.299 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of 1 1/8"-18 UNEF?: The tensile stress area of 1 1/8"-18 UNEF is 0.90067 in² (581.08 mm²). Multiply it by the fastener material's allowable stress to get a load.

All Unified Extra Fine (UNEF) sizes · Every thread series