What is the tap drill size for 7/8"-14 UNF?

The tap drill for 7/8"-14 UNF is 13/16 — 0.8125 in (20.637 mm). That leaves 67.4% thread engagement with a cut tap.

What is the pitch diameter of 7/8"-14 UNF?

The basic pitch diameter of 7/8"-14 UNF is 0.8286 in (21.046 mm). For a class 2B thread it runs 0.8286 in (21.046 mm) to 0.8357 in (21.227 mm).

What is the minor diameter of 7/8"-14 UNF?

The basic minor diameter of 7/8"-14 UNF is 0.7977 in (20.262 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 7/8"-14 UNF?

7/8"-14 UNF has 14 threads per inch, a pitch of 0.0714 in (1.814 mm).

What size clearance hole does a 7/8"-14 UNF fastener need?

close fit 57/64 (0.9055 in (23.000 mm)), normal fit 59/64 (0.9370 in (23.800 mm)), loose fit 1 (1.0315 in (26.200 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of 7/8"-14 UNF?

The tensile stress area of 7/8"-14 UNF is 0.50947 in² (328.69 mm²). Multiply it by the fastener material's allowable stress to get a load.

Why does the 7/8"-14 UNF tap drill here differ from the chart on the wall?

Computed 75% target 0.8054 in. Nearest standard drill is 13/16 (0.8125 in, off by 0.0071 in); the traditional published chart lists 51/64 (0.797 in, off by 0.0084 in, 84.1% thread). Both are shown; neither overrides the other.

Unified Fine (UNF) · 7/8-14 UNF

7/8"-14 UNF Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unf/7-8-14 · data verified August 1, 2026

The tap drill for 7/8"-14 UNF is 13/16 — 0.8125 in (20.637 mm) — which leaves 67.4% thread engagement with a cut tap. Basic pitch diameter is 0.8286 in (21.046 mm) and basic minor diameter is 0.7977 in (20.262 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 Fine (UNF)
Designation: 7/8-14 UNF
TPI: 14
Standard: ASME B1.1

Tap drill · 67.4% thread · cut tap

13/16

Drill Ø: 0.8125in
Drill Ø: 20.637mm
Clearance (normal): 59/64

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 67.4% thread.

Basic Thread Dimensions for 7/8"-14 UNF

Basic thread dimensions — 7/8"-14 UNF
Property	Inch	Millimetre
Basic major diameter	0.8750	22.225
Threads per inch	14	—
Pitch	0.0714	1.814
Basic pitch diameter	0.8286	21.046
Basic minor diameter	0.7977	20.262
External thread root diameter	0.7899	20.064
Sharp V thread height (H)	0.0619	1.572

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 7/8"-14 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 13/16 drill. Deeper engagement means more holding strength and more torque on the tap.

Tap drill sizes by thread engagement — 7/8"-14 UNF, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.8286	21.046	53/64	0.8281	21.034	50.5
60%	0.8193	20.810	13/16	0.8125	20.637	67.4
65%	0.8147	20.693	13/16	0.8125	20.637	67.4
70%	0.8100	20.574	13/16	0.8125	20.637	67.4
75%default	0.8054	20.457	13/16	0.8125	20.637	67.4
80%	0.8008	20.340	51/64	0.7969	20.241	84.2
85%	0.7961	20.221	51/64	0.7969	20.241	84.2

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 7/8"-14 UNF

The through-hole a 7/8"-14 UNF fastener passes freely, not the tapped hole.

Clearance hole sizes — 7/8"-14 UNF
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	57/64	0.9055	23.000	Located joints, dowelled assemblies
Normal	59/64	0.9370	23.800	General assembly — the default
Loose	1	1.0315	26.200	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 7/8"-14 UNF

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 — 7/8"-14 UNF
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	0.8734	22.184	0.8750	22.225
Major Ø min	0.8630	21.920	0.8647	21.963
Pitch Ø max	0.8270	21.006	0.8286	21.046
Pitch Ø min	0.8216	20.869	0.8245	20.942
Pitch Ø tolerance	0.0054	0.137	0.0041	0.104
Minor Ø max	0.7806	19.827	0.7822	19.868
Minor Ø min	0.7752	19.690	0.7781	19.764
Allowance	0.0016	0.041	0.0000	0.000

Internal thread (nut / tapped hole) limits — 7/8"-14 UNF
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	0.8924	22.667	0.8906	22.621
Major Ø min	0.8750	22.225	0.8750	22.225
Pitch Ø max	0.8357	21.227	0.8339	21.181
Pitch Ø min	0.8286	21.046	0.8286	21.046
Pitch Ø tolerance	0.0070	0.178	0.0053	0.135
Minor Ø max	0.8135	20.663	0.8067	20.490
Minor Ø min	0.7977	20.262	0.7977	20.262
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 7/8"-14 UNF

The effective cross-section used to size a 7/8"-14 UNF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 7/8"-14 UNF
Property	in²	mm²
Tensile stress area	0.50947	328.69

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

Other Thread Pitches in the Same Size as 7/8"-14 UNF

A 7/8 fastener is not always 7/8"-14 UNF. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size 7/8
Thread	Series	Pitch	Tap drill
7/8"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	53/64
7/8"-9 UNC	Unified Coarse (UNC)	9 TPI	49/64

Sources and Method for 7/8"-14 UNF

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 7/8"-14 UNF Reference

Citation

ShopMath. "7/8"-14 UNF Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unf/7-8-14

BibTeX

@misc{shopmath-threads-unf-7-8-14,
  title        = {7/8"-14 UNF Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unf/7-8-14}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unf/7-8-14

Common Questions About 7/8"-14 UNF

What is the tap drill size for 7/8"-14 UNF?: The tap drill for 7/8"-14 UNF is 13/16 — 0.8125 in (20.637 mm). That leaves 67.4% thread engagement with a cut tap.
What is the pitch diameter of 7/8"-14 UNF?: The basic pitch diameter of 7/8"-14 UNF is 0.8286 in (21.046 mm). For a class 2B thread it runs 0.8286 in (21.046 mm) to 0.8357 in (21.227 mm).
What is the minor diameter of 7/8"-14 UNF?: The basic minor diameter of 7/8"-14 UNF is 0.7977 in (20.262 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 7/8"-14 UNF?: 7/8"-14 UNF has 14 threads per inch, a pitch of 0.0714 in (1.814 mm).
What size clearance hole does a 7/8"-14 UNF fastener need?: close fit 57/64 (0.9055 in (23.000 mm)), normal fit 59/64 (0.9370 in (23.800 mm)), loose fit 1 (1.0315 in (26.200 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of 7/8"-14 UNF?: The tensile stress area of 7/8"-14 UNF is 0.50947 in² (328.69 mm²). Multiply it by the fastener material's allowable stress to get a load.
Why does the 7/8"-14 UNF tap drill here differ from the chart on the wall?: Computed 75% target 0.8054 in. Nearest standard drill is 13/16 (0.8125 in, off by 0.0071 in); the traditional published chart lists 51/64 (0.797 in, off by 0.0084 in, 84.1% thread). Both are shown; neither overrides the other.

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