# Unified inch threads

Unified thread series, TPI and A/B classes, with coating, gauging and joint-performance boundaries.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, threads, unified-inch

## Identity and governing requirements

Unified inch fastening threads use a 60° form and specify pitch in threads per inch (TPI). Retain the series and class with nominal diameter. [Accu: thread-family identification](https://www.accu.co.uk/p/544-how-to-cut-tap-threads-a-complete-guide)

In **1/4-20 UNC-2A**, 1/4 is the nominal major diameter in inches, 20 is TPI, UNC is the coarse series and 2A is an external thread class. Confirm the mating internal class, handedness and delivered coating condition separately.

ASME lists **B1.1-2024**, *Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms)*. Its official preview identifies it as a revision of B1.1-2019, issued **28 June 2024**, with effectiveness one year after issuance. Record the edition required by the drawing or purchase specification; a current catalog does not silently revise an existing contract. [ASME standard record](https://www.asme.org/codes-standards/find-codes-standards/b1-1-unified-inch-screw-threads-un-unr-thread-form), [official preview](https://www.asme.org/wwwasmeorg/media/codes-standards/find-codes/c-t_b1_1_2024.pdf)

Keep the complete thread-form designation where specified. UN, UNR and UNJ should not be collapsed into an undifferentiated “inch thread” purchasing description. Obtain the actual profile and mating requirements rather than inferring them from TPI alone.

## Reading a designation

For an example external **1/4-20 UNC-2A** thread:

| Element | Meaning |
| --- | --- |
| 1/4 | Nominal major diameter, 0.250 inch |
| 20 | Twenty threads per inch |
| UNC | Unified coarse series |
| 2A | External product-thread class |

An example mating internal requirement is **1/4-20 UNC-2B**, subject to the specified fit and remaining drawing requirements. **A identifies external threads; B identifies internal threads.** Portland Bolt describes classes 1, 2 and 3 as progressively tighter tolerances; the class number is not a material-strength grade. [Portland Bolt: thread classes](https://www.portlandbolt.com/technical/faqs/thread-class-for-all-thread-rod/)

State fastener length separately. In a product description, an additional length value is not another TPI designation. Verify usable threaded length, grip length, end condition and the head-dependent length convention from the product drawing. For a tapped hole, drilling depth, full-thread depth and usable engagement are different requirements; resolve them before selecting tooling or a bolt length.

Numbered screw sizes such as #10 are also not decimal-inch diameter values. Read them through the appropriate thread table instead of interpreting “10” as 0.10 inch. Confirm handedness and any special lead requirement when those affect the assembly.

## UNC, UNF, UNEF and constant-pitch series

UNC, UNF and UNEF identify coarse, fine and extra-fine series. Accu's table gives these **1/4-inch** examples:

- **1/4-20 UNC**: 20 TPI.
- **1/4-28 UNF**: 28 TPI.
- **1/4-32 UNEF**: 32 TPI.

The nominal diameter is the same, but their mating pitches differ. “Fine” does not mean a particular class 3 fit or automatically establish manufacturing accuracy. [Accu: imperial thread chart](https://www.accu.co.uk/p/454-imperial-thread-chart-for-imperial-bolt-sizes)

Constant-pitch series must also be named correctly. Portland Bolt lists **8UN** separately from UNC and UNF and notes its use in specified construction-fastener applications. Do not assume that every large diameter uses the coarse-series TPI or that 8UN is simply another spelling of UNC. [Portland Bolt: pitch series](https://www.portlandbolt.com/technical/thread-pitch-chart/)

Choose the series against the actual joint requirements, available product standard and manufacturing capability. Avoid a blanket claim that a finer pitch prevents loosening, guarantees stronger engagement or permits shorter thread depth. Those conclusions require the material, geometry, loading and installation conditions of the actual joint.

## Original pitch calculation

For an assumed **single-start** thread, axial pitch in inches is `P = 1/TPI`, and the ideal travel per revolution equals that pitch. The examples above give:

| Thread | Pitch, inch | Pitch, mm | Ideal travel in 7 revolutions |
| --- | --- | --- | --- |
| 1/4-20 | 0.050000 | 1.270000 | 0.350 inch / 8.890 mm |
| 1/4-28 | 0.035714… | 0.907143… | 0.250 inch / 6.350 mm |
| 1/4-32 | 0.031250 | 0.793750 | 0.218750 inch / 5.556250 mm |

These are original geometric conversions using 25.4 mm per inch. They assume no backlash, elastic movement or lead error. They do not specify permissible engagement, tightening turns or lead accuracy. Multi-start threads require their actual lead to be considered separately.

## Tolerance, allowance and coating state

Tolerance width and allowance are different concepts. Vermont Gage's **1 October 2019** explanation, explicitly based on **B1.1-2003**, describes allowance on class 1A/2A external threads and none on class 3A. Use that as a bounded explanation of the distinction, while obtaining current acceptance limits from the governing edition. A class designation is not a single fixed clearance in thousandths of an inch. [Vermont Gage: classes and setting plugs](https://www.vermontgage.com/support/detail/thread-classes-and-setting-plug-gages-quality-magazine)

Coating changes the dimensional stack. Establish whether the specified limits and inspection apply before or after coating, and identify any deliberate coating allowance. A plain-finish component is not automatically a substitute for a similarly labelled coated one.

As a specific manufacturing example, Portland Bolt's article, written July 3, 2007 and modified September 9, 2016, explains that its hot-dip galvanized bolts mate with nuts tapped oversize to accommodate zinc, and warns against substituting ordinary plain or zinc-plated nuts. This is coating-system guidance, not permission to enlarge every internal Unified thread or to transfer an allowance to another plating process. [Portland Bolt: galvanized nuts](https://www.portlandbolt.com/technical/faqs/galvanized-nuts-tapped-oversize/)

## Gauging and acceptance

ASME lists **B1.2-2025** for Unified inch thread gauges and gauging. Keep product-thread dimensions, gauge manufacturing requirements and the selected inspection method distinct. [ASME B1.2 record](https://www.asme.org/codes-standards/find-codes-standards/b1-2-gages-gaging-unified-inch-screw-threads)

Vermont Gage distinguishes a **2B working plug** used to inspect an internal product thread from a **2A truncated setting plug** used to set or calibrate an adjustable 2A ring gauge. The latter is not a substitute for inspecting a hole merely because it is physically a plug. Carry the full product designation through gauge selection and review calibration and wear condition. [Vermont Gage: gauge-purpose distinction](https://www.vermontgage.com/support/detail/thread-classes-and-setting-plug-gages-quality-magazine)

A caliper reading across external crests does not establish all thread characteristics. Likewise, a convenient mating screw or nut is not automatically an adequate acceptance gauge. Resolve the required functional and individual-element checks with the drawing and applicable inspection standard. Inspect burrs, damaged starts and usable extent separately when they affect assembly.

## Dimensional fit versus joint performance

A thread that assembles still needs the correct fastener material/grade, nut or tapped-material strength, engagement and installation conditions. Specify these independently of UNC/UNF/UNEF and 1/2/3 A/B classes. Bolt length and bottoming clearance must support the intended clamping arrangement rather than merely allowing initial thread engagement.

Torque requires its own basis. Bossard's guidance separates fastener type, material and head shape and states that approximate tightening tables do not replace a bolted-joint calculation. Do not derive a universal installation torque from thread size and class alone. [Bossard: preload and tightening torque](https://www.bossard.com/ch-en/knowledge-hub/resources/technical-information/preload-and-tightening-torque/)

For a useful handoff, record designation, governing editions, full-thread extent, mating component, material/grade, coating state, dimensional inspection and separate joint-performance requirements. The Vermont and Portland Bolt galvanizing articles are dated as above; the other live supplier tutorials have no verified technical revision date here. Calculations and proposed handoff fields are original guidance.