What is the tap drill size for 1 5/8"-18 UNEF?

The tap drill for 1 5/8"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 866.0% thread engagement with a cut tap.

What is the pitch diameter of 1 5/8"-18 UNEF?

The basic pitch diameter of 1 5/8"-18 UNEF is 1.5889 in (40.358 mm). For a class 2B thread it runs 1.5889 in (40.358 mm) to 1.5954 in (40.523 mm).

What is the minor diameter of 1 5/8"-18 UNEF?

The basic minor diameter of 1 5/8"-18 UNEF is 1.5649 in (39.748 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 5/8"-18 UNEF?

1 5/8"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

What is the tensile stress area of 1 5/8"-18 UNEF?

The tensile stress area of 1 5/8"-18 UNEF is 1.93808 in² (1250.37 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Extra Fine (UNEF) · 1 5/8-18 UNEF

1 5/8"-18 UNEF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unef/1-5-8-18 · data verified August 1, 2026

The tap drill for 1 5/8"-18 UNEF is 1 — 1.0000 in (25.400 mm) — which leaves 866.0% thread engagement with a cut tap. Basic pitch diameter is 1.5889 in (40.358 mm) and basic minor diameter is 1.5649 in (39.748 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 5/8-18 UNEF
TPI: 18
Standard: ASME B1.1

Tap drill · 866.0% 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 866.0% thread.

Basic Thread Dimensions for 1 5/8"-18 UNEF

Basic thread dimensions — 1 5/8"-18 UNEF
Property	Inch	Millimetre
Basic major diameter	1.6250	41.275
Threads per inch	18	—
Pitch	0.0556	1.411
Basic pitch diameter	1.5889	40.358
Basic minor diameter	1.5649	39.748
External thread root diameter	1.5588	39.593
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 5/8"-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 5/8"-18 UNEF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	1.5889	40.358	1	1.0000	25.400	866.0
60%	1.5817	40.175	1	1.0000	25.400	866.0
65%	1.5781	40.084	1	1.0000	25.400	866.0
70%	1.5745	39.992	1	1.0000	25.400	866.0
75%default	1.5709	39.901	1	1.0000	25.400	866.0
80%	1.5673	39.809	1	1.0000	25.400	866.0
85%	1.5637	39.718	1	1.0000	25.400	866.0

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 1 5/8"-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 5/8"-18 UNEF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	1.6235	41.237	1.6250	41.275
Major Ø min	1.6148	41.016	1.6163	41.054
Pitch Ø max	1.5874	40.320	1.5889	40.358
Pitch Ø min	1.5824	40.193	1.5852	40.264
Pitch Ø tolerance	0.0050	0.127	0.0038	0.097
Minor Ø max	1.5513	39.403	1.5528	39.441
Minor Ø min	1.5463	39.276	1.5491	39.347
Allowance	0.0015	0.038	0.0000	0.000

Internal thread (nut / tapped hole) limits — 1 5/8"-18 UNEF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	1.6395	41.643	1.6379	41.603
Major Ø min	1.6250	41.275	1.6250	41.275
Pitch Ø max	1.5954	40.523	1.5938	40.483
Pitch Ø min	1.5889	40.358	1.5889	40.358
Pitch Ø tolerance	0.0065	0.165	0.0049	0.124
Minor Ø max	1.5775	40.069	1.5730	39.954
Minor Ø min	1.5649	39.748	1.5649	39.748
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 1 5/8"-18 UNEF

The effective cross-section used to size a 1 5/8"-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 5/8"-18 UNEF
Property	in²	mm²
Tensile stress area	1.93808	1250.37

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

Sources and Method for 1 5/8"-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 5/8"-18 UNEF Reference

Citation

ShopMath. "1 5/8"-18 UNEF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unef/1-5-8-18

BibTeX

@misc{shopmath-threads-unef-1-5-8-18,
  title        = {1 5/8"-18 UNEF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unef/1-5-8-18}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unef/1-5-8-18

Common Questions About 1 5/8"-18 UNEF

What is the tap drill size for 1 5/8"-18 UNEF?: The tap drill for 1 5/8"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 866.0% thread engagement with a cut tap.
What is the pitch diameter of 1 5/8"-18 UNEF?: The basic pitch diameter of 1 5/8"-18 UNEF is 1.5889 in (40.358 mm). For a class 2B thread it runs 1.5889 in (40.358 mm) to 1.5954 in (40.523 mm).
What is the minor diameter of 1 5/8"-18 UNEF?: The basic minor diameter of 1 5/8"-18 UNEF is 1.5649 in (39.748 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 5/8"-18 UNEF?: 1 5/8"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).
What is the tensile stress area of 1 5/8"-18 UNEF?: The tensile stress area of 1 5/8"-18 UNEF is 1.93808 in² (1250.37 mm²). Multiply it by the fastener material's allowable stress to get a load.

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