Reference

Thread Callout Guide

How to read and specify a thread callout, part by part, for inch (UNC/UNF) and metric threads, including series, pitch, and the fit-class letters and numbers.

A thread callout packs the whole thread definition into one string. An inch example, 1/4-20 UNC-2B, reads as nominal diameter (1/4 in) - threads per inch (20) - thread series (UNC) - class of fit (2) with A for an external thread or B for an internal thread. A metric example, M6x1-6H, reads as M (metric) - nominal diameter in mm (6) - pitch in mm (1) - tolerance class (6H). Read left to right, each piece narrows down the thread: size, how fine it is, the standard, and how tight the fit. This guide complements the tap drill chart, which tells you what to drill before you tap these threads.

Anatomy of a thread callout

Each field answers one question. Inch threads name threads per inch (TPI); metric threads name the pitch (the distance between threads) in millimeters. The class of fit comes last and controls how tight the mating threads are.

Field	Meaning	Example
Nominal diameter	The thread's nominal (major) size	1/4 in / M6 (6 mm)
Threads per inch or pitch	Inch: TPI. Metric: pitch in mm	20 TPI / 1 mm pitch
Series	The thread standard/coarseness (inch only)	UNC (coarse), UNF (fine), UNEF
Class of fit	Tolerance / tightness of the fit	2B / 6H
A or B (inch)	A = external (screw), B = internal (nut/hole)	1/4-20 UNC-2A vs 2B
Letter case (metric)	lowercase = external, UPPERCASE = internal	6g (external) vs 6H (internal)

Inch class of fit is a number (1/2/3) plus A or B. Metric tolerance class is a grade number plus a position letter; letter case, not a separate A/B, marks external vs internal.

Fit classes (tightness of the mating threads)

The class controls the clearance between the external and internal thread. Looser classes assemble easily and tolerate coating; tighter classes locate more precisely. Inch and metric use different notation for the same idea.

System	External	Internal	Fit
Inch	1A	1B	Loose - quick assembly, tolerates dirt/plating
Inch	2A	2B	General purpose - the common default
Inch	3A	3B	Tight - precision, close tolerance, no allowance
Metric	6g	6H	Medium - the common general-purpose default
Metric	4g / 4h	4H / 5H	Tighter (lower grade number = tighter)
Metric	8g	7H	Looser (higher grade number = more clearance)

Inch: A = external, B = internal; 1 loose, 2 general, 3 tight. Metric: number is the tolerance grade (6 is medium; lower is tighter), letter is the position; lowercase = external, uppercase = internal. 6g/6H is the usual metric default.

Inch thread series (UNC vs UNF vs UNEF)

For a given diameter, the inch series sets how many threads per inch you get. Coarse is the default for most work; fine and extra-fine trade some strippability for finer adjustment and better hold in thin walls.

Series	Meaning	Relative TPI	Typical use
UNC	Unified Coarse	Fewest TPI	General purpose, fast assembly, tapping in soft material
UNF	Unified Fine	More TPI	Higher strength in tension, fine adjustment, vibration
UNEF	Unified Extra Fine	Most TPI	Thin-wall tubes, fittings, limited thread depth

Same diameter, different TPI. Example: 1/4-20 UNC vs 1/4-28 UNF. Metric coarse vs fine works the same way via pitch (e.g. M6x1 coarse vs M6x0.75 fine).

Reading two full examples

Inch: 1/4-20 UNC-2B is a 1/4 in nominal diameter, 20 threads per inch, Unified Coarse series, class 2 general-purpose fit, B for an internal thread (a tapped hole). Its mating screw would be 1/4-20 UNC-2A. Metric: M6x1-6H is a metric thread, 6 mm nominal diameter, 1 mm pitch, tolerance class 6H, uppercase H marking it as an internal thread; the mating bolt would be M6x1-6g. If the pitch is omitted on a metric callout (e.g. M6), the coarse pitch is assumed.

Conventions follow the Unified inch thread standard (ASME B1.1) and ISO metric threads (ISO 965). Inch: A = external, B = internal, class 1/2/3 from loose to tight. Metric: grade number plus position letter, lowercase external / uppercase internal, 6g/6H the usual default. Confirm the class and series required for your part.

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Frequently Asked Questions

What does 1/4-20 UNC-2B mean?

It is a 1/4 in nominal diameter thread with 20 threads per inch, in the Unified Coarse (UNC) series, with a class 2 general-purpose fit. The B marks it as an internal thread (a tapped hole). The mating external thread (a screw) would be written 1/4-20 UNC-2A.

What does the A or B mean in a thread callout?

On inch (Unified) threads, A designates an external thread such as a screw or bolt, and B designates an internal thread such as a nut or tapped hole. The number before it (1, 2, or 3) is the class of fit, from loose (1) to general (2) to tight (3).

How do metric thread tolerance classes work?

A metric class like 6H or 6g has a grade number and a position letter. The number is the tolerance grade, where 6 is medium and a lower number is tighter. The letter's case tells you the thread type: lowercase is an external thread (6g) and uppercase is an internal thread (6H). 6g/6H is the common general-purpose pairing.

What is the difference between UNC and UNF threads?

UNC is Unified Coarse and UNF is Unified Fine. For the same diameter, UNF has more threads per inch, giving finer adjustment, higher tensile strength, and better hold in thin material, while UNC assembles faster and taps more easily in soft materials. Example: 1/4-20 UNC versus 1/4-28 UNF.

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