What is the tap drill size for 13/16"-20 UNEF?

The tap drill for 13/16"-20 UNEF is 49/64 — 0.7656 in (19.447 mm). That leaves 72.2% thread engagement with a cut tap.

What is the pitch diameter of 13/16"-20 UNEF?

The basic pitch diameter of 13/16"-20 UNEF is 0.7800 in (19.812 mm). For a class 2B thread it runs 0.7800 in (19.812 mm) to 0.7858 in (19.959 mm).

What is the minor diameter of 13/16"-20 UNEF?

The basic minor diameter of 13/16"-20 UNEF is 0.7584 in (19.263 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 13/16"-20 UNEF?

13/16"-20 UNEF has 20 threads per inch, a pitch of 0.0500 in (1.270 mm).

What is the tensile stress area of 13/16"-20 UNEF?

The tensile stress area of 13/16"-20 UNEF is 0.45818 in² (295.60 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Extra Fine (UNEF) · 13/16-20 UNEF

13/16"-20 UNEF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unef/13-16-20 · data verified August 1, 2026

The tap drill for 13/16"-20 UNEF is 49/64 — 0.7656 in (19.447 mm) — which leaves 72.2% thread engagement with a cut tap. Basic pitch diameter is 0.7800 in (19.812 mm) and basic minor diameter is 0.7584 in (19.263 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: 13/16-20 UNEF
TPI: 20
Standard: ASME B1.1

Tap drill · 72.2% thread · cut tap

49/64

Drill Ø: 0.7656in
Drill Ø: 19.447mm

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 72.2% thread.

Basic Thread Dimensions for 13/16"-20 UNEF

Basic thread dimensions — 13/16"-20 UNEF
Property	Inch	Millimetre
Basic major diameter	0.8125	20.637
Threads per inch	20	—
Pitch	0.0500	1.270
Basic pitch diameter	0.7800	19.812
Basic minor diameter	0.7584	19.263
External thread root diameter	0.7530	19.126
Sharp V thread height (H)	0.0433	1.100

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 13/16"-20 UNEF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 49/64 drill. Deeper engagement means more holding strength and more torque on the tap.

Tap drill sizes by thread engagement — 13/16"-20 UNEF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.7800	19.812	25/32	0.7813	19.844	48.1
60%	0.7735	19.647	25/32	0.7813	19.844	48.1
65%	0.7703	19.566	49/64	0.7656	19.447	72.2
70%	0.7670	19.482	49/64	0.7656	19.447	72.2
75%default	0.7638	19.401	49/64	0.7656	19.447	72.2
80%	0.7605	19.317	49/64	0.7656	19.447	72.2
85%	0.7573	19.235	3/4	0.7500	19.050	96.2

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.

Thread Class Limits for 13/16"-20 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 — 13/16"-20 UNEF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	0.8112	20.605	0.8125	20.637
Major Ø min	0.8030	20.396	0.8044	20.432
Pitch Ø max	0.7787	19.779	0.7800	19.812
Pitch Ø min	0.7742	19.665	0.7767	19.728
Pitch Ø tolerance	0.0044	0.112	0.0033	0.084
Minor Ø max	0.7462	18.953	0.7475	18.986
Minor Ø min	0.7418	18.842	0.7442	18.903
Allowance	0.0013	0.033	0.0000	0.000

Internal thread (nut / tapped hole) limits — 13/16"-20 UNEF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	0.8255	20.968	0.8240	20.930
Major Ø min	0.8125	20.637	0.8125	20.637
Pitch Ø max	0.7858	19.959	0.7844	19.924
Pitch Ø min	0.7800	19.812	0.7800	19.812
Pitch Ø tolerance	0.0058	0.147	0.0043	0.109
Minor Ø max	0.7699	19.555	0.7661	19.459
Minor Ø min	0.7584	19.263	0.7584	19.263
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 13/16"-20 UNEF

The effective cross-section used to size a 13/16"-20 UNEF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 13/16"-20 UNEF
Property	in²	mm²
Tensile stress area	0.45818	295.60

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

Sources and Method for 13/16"-20 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 13/16"-20 UNEF Reference

Citation

ShopMath. "13/16"-20 UNEF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unef/13-16-20

BibTeX

@misc{shopmath-threads-unef-13-16-20,
  title        = {13/16"-20 UNEF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unef/13-16-20}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unef/13-16-20

Common Questions About 13/16"-20 UNEF

What is the tap drill size for 13/16"-20 UNEF?: The tap drill for 13/16"-20 UNEF is 49/64 — 0.7656 in (19.447 mm). That leaves 72.2% thread engagement with a cut tap.
What is the pitch diameter of 13/16"-20 UNEF?: The basic pitch diameter of 13/16"-20 UNEF is 0.7800 in (19.812 mm). For a class 2B thread it runs 0.7800 in (19.812 mm) to 0.7858 in (19.959 mm).
What is the minor diameter of 13/16"-20 UNEF?: The basic minor diameter of 13/16"-20 UNEF is 0.7584 in (19.263 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 13/16"-20 UNEF?: 13/16"-20 UNEF has 20 threads per inch, a pitch of 0.0500 in (1.270 mm).
What is the tensile stress area of 13/16"-20 UNEF?: The tensile stress area of 13/16"-20 UNEF is 0.45818 in² (295.60 mm²). Multiply it by the fastener material's allowable stress to get a load.

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