What is the tap drill size for 7/8"-9 UNC?

The tap drill for 7/8"-9 UNC is 49/64 — 0.7656 in (19.447 mm). That leaves 75.8% thread engagement with a cut tap.

What is the pitch diameter of 7/8"-9 UNC?

The basic pitch diameter of 7/8"-9 UNC is 0.8028 in (20.391 mm). For a class 2B thread it runs 0.8028 in (20.391 mm) to 0.8110 in (20.599 mm).

What is the minor diameter of 7/8"-9 UNC?

The basic minor diameter of 7/8"-9 UNC is 0.7547 in (19.169 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"-9 UNC?

7/8"-9 UNC has 9 threads per inch, a pitch of 0.1111 in (2.822 mm).

What size clearance hole does a 7/8"-9 UNC 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"-9 UNC?

The tensile stress area of 7/8"-9 UNC is 0.46173 in² (297.89 mm²). Multiply it by the fastener material's allowable stress to get a load.

Unified Coarse (UNC) · 7/8-9 UNC

7/8"-9 UNC Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/unc/7-8-9 · data verified August 1, 2026

The tap drill for 7/8"-9 UNC is 49/64 — 0.7656 in (19.447 mm) — which leaves 75.8% thread engagement with a cut tap. Basic pitch diameter is 0.8028 in (20.391 mm) and basic minor diameter is 0.7547 in (19.169 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 Coarse (UNC)
Designation: 7/8-9 UNC
TPI: 9
Standard: ASME B1.1

Tap drill · 75.8% thread · cut tap

49/64

Drill Ø: 0.7656in
Drill Ø: 19.447mm
Clearance (normal): 59/64

Assumes 75% thread engagement with a cut tap and a class 2B hole. That drill actually leaves 75.8% thread.

Basic Thread Dimensions for 7/8"-9 UNC

Basic thread dimensions — 7/8"-9 UNC
Property	Inch	Millimetre
Basic major diameter	0.8750	22.225
Threads per inch	9	—
Pitch	0.1111	2.822
Basic pitch diameter	0.8028	20.391
Basic minor diameter	0.7547	19.169
External thread root diameter	0.7427	18.865
Sharp V thread height (H)	0.0962	2.443

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"-9 UNC

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 — 7/8"-9 UNC, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.8028	20.391	51/64	0.7969	20.241	54.1
60%	0.7884	20.025	25/32	0.7813	19.844	65.0
65%	0.7812	19.843	25/32	0.7813	19.844	65.0
70%	0.7740	19.660	25/32	0.7813	19.844	65.0
75%default	0.7667	19.474	49/64	0.7656	19.447	75.8
80%	0.7595	19.291	49/64	0.7656	19.447	75.8
85%	0.7523	19.108	3/4	0.7500	19.050	86.6

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"-9 UNC

The through-hole a 7/8"-9 UNC fastener passes freely, not the tapped hole.

Clearance hole sizes — 7/8"-9 UNC
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"-9 UNC

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"-9 UNC
Property	Class 2A		Class 3A
Limit	in	mm	in	mm
Major Ø max	0.8731	22.177	0.8750	22.225
Major Ø min	0.8592	21.824	0.8611	21.872
Pitch Ø max	0.8009	20.343	0.8028	20.391
Pitch Ø min	0.7946	20.183	0.7981	20.272
Pitch Ø tolerance	0.0063	0.160	0.0047	0.119
Minor Ø max	0.7288	18.512	0.7307	18.560
Minor Ø min	0.7225	18.352	0.7259	18.438
Allowance	0.0019	0.048	0.0000	0.000

Internal thread (nut / tapped hole) limits — 7/8"-9 UNC
Property	Class 2B		Class 3B
Limit	in	mm	in	mm
Major Ø max	0.8992	22.840	0.8972	22.789
Major Ø min	0.8750	22.225	0.8750	22.225
Pitch Ø max	0.8110	20.599	0.8090	20.549
Pitch Ø min	0.8028	20.391	0.8028	20.391
Pitch Ø tolerance	0.0082	0.208	0.0061	0.155
Minor Ø max	0.7776	19.751	0.7681	19.510
Minor Ø min	0.7547	19.169	0.7547	19.169
Allowance	0.0000	0.000	0.0000	0.000

Tensile Stress Area of 7/8"-9 UNC

The effective cross-section used to size a 7/8"-9 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — 7/8"-9 UNC
Property	in²	mm²
Tensile stress area	0.46173	297.89

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

Other Thread Pitches in the Same Size as 7/8"-9 UNC

A 7/8 fastener is not always 7/8"-9 UNC. 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"-14 UNF	Unified Fine (UNF)	14 TPI	13/16

Sources and Method for 7/8"-9 UNC

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"-9 UNC Reference

Citation

ShopMath. "7/8"-9 UNC Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/unc/7-8-9

BibTeX

@misc{shopmath-threads-unc-7-8-9,
  title        = {7/8"-9 UNC Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/unc/7-8-9}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/unc/7-8-9

Common Questions About 7/8"-9 UNC

What is the tap drill size for 7/8"-9 UNC?: The tap drill for 7/8"-9 UNC is 49/64 — 0.7656 in (19.447 mm). That leaves 75.8% thread engagement with a cut tap.
What is the pitch diameter of 7/8"-9 UNC?: The basic pitch diameter of 7/8"-9 UNC is 0.8028 in (20.391 mm). For a class 2B thread it runs 0.8028 in (20.391 mm) to 0.8110 in (20.599 mm).
What is the minor diameter of 7/8"-9 UNC?: The basic minor diameter of 7/8"-9 UNC is 0.7547 in (19.169 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"-9 UNC?: 7/8"-9 UNC has 9 threads per inch, a pitch of 0.1111 in (2.822 mm).
What size clearance hole does a 7/8"-9 UNC 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"-9 UNC?: The tensile stress area of 7/8"-9 UNC is 0.46173 in² (297.89 mm²). Multiply it by the fastener material's allowable stress to get a load.

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