Introduction to Standardized Fastener Thread Types

Huyett Marketing Department
05/16/2024

Introduction to Fastener Threads Blog Hero Image

Bolts, nuts, and screws are among the most common threaded fasteners in use. Each category includes a wide variety of subtypes; while these subtypes perform roughly the same tasks, there are key differences in their designs that make their form, fit, and function unique. One of these essential characteristics for screws, nuts and bolts is their thread type.

There are differences between these threads and threads for pipes and other fasteners (lifting and rigging hardware, threaded rods and studs, etc.). If you want to learn more about pipe threads for pipe plugs and grease fittings, check out our Grease Fitting Threads Guide.

Importance of Understanding Thread Types

Thread type refers to the specific way that threads are designed to incorporate unique dimensions, tolerances, shapes, and patterns. Many different types of threads exist, with the most common being governed by standards like the UTS or ISO/DIN. These standards communicate expected dimensions and design elements to which the threads must conform. However, there is also a wide variety of fastener thread types that are crafted outside of standards and are meant for specific applications. It is important to know what kind of threads apply to your fasteners for a few reasons:

Some applications require regulated and standardized fasteners. This is common in automotive, machinery, and construction work, where the application’s construction and maintenance procedures must be predictable for safety, clarity, and conformity.
Mating fasteners need to have the same thread types. If incompatible threads are forced together, it will cause damage and the connection will not hold as expected.
Different thread types offer their own benefits. Thread types vary because they must support a wide range of application needs. They are designed to support specific materials, facilitate unique material displacement, provide varying levels of holding power, accommodate different torque requirements, etc. You need to know what type of thread will best support your application and which types may not.

Before delving into different screw, nut, and bolt thread types and what they mean, we’ll take a look at the basic fastener thread anatomy to help contextualize the types later on.

Fastener Thread Anatomy

All fastener threads will either be male (external) or female (internal).

Male Threads: Those that exist on an external shank (like a bolt or screw)
Female Threads: Those that exist inside a bore or hole (like a nut or tapped hole)

Both thread designations will have the same characteristics and measurements, except the inside diameter (ID) and the outside diameter (OD) will be reversed (more on this below).

At its base, the anatomy of any thread is the same. You will have the following characteristics:

Root: The inner base of the thread
Crest: The outermost point of the thread
Depth: The distance between the crest and the root
Flank: The face of the angle
Angle: The angle measured from the root outwards along opposing flanks

Fastener Thread Measurements

You will also use similar thread measurements, although these are not usually called out in uncommon thread types. Additionally, metric fasteners technically use the pitch measurement, while inch fasteners use the TPI .

Pitch: The distance between thread crests (measured in mm)
Threads per Inch (TPI): The number of threads within one inch of the shank
- While TPI and pitch have these technical definitions, the terms are often used interchangeably in casual settings. Make sure you know which technical measurement is being referenced.
Inside Diameter (ID)
- Male Threads: The diameter from opposing root to root
- Female Threads: The diameter from opposing crest to crest
Outside Diameter (OD)
- External Threads: The diameter from the opposing crest to crest
- Internal Threads: The diameter from opposing root to root

Most standardized screws, nuts, and bolts will call out sizes by the outside diameter and the pitch/TPI. While the depth and angle are measurable dimensions, they don’t change a fastener’s size – their dimensions only influence standardization.

To learn more about how to read inch vs. metric diameter and thread sizes in a part description, read our Introduction to Nuts and Bolts Sizes guide.

How Do Thread Types Differ?

Thread styles vary from each other in five main ways:

Angle: Specific thread types will usually feature a consistent angle, but the angle can change between types.
Depth: While a normal thread depth is the pitch multiplied by 0.5 (0.5xpitch), this dimension can change and is sometimes the one difference between standards.
Shape: Most standardized threads are fundamentally based on a trapezoid (with varying degrees of sharpness). However, some threads are engineered to be square, round, saw-toothed, buttressed, etc.
Pattern: The most common thread pattern is a single spiral turning clockwise down a fastener shank or bore, but some threads are parallel, others turn counterclockwise, some feature hi-lo threads, etc. At times, fasteners will have multiple patterns on one component. Fasteners with abnormal patterns are typically unstandardized.
Standards: Thread standards designate specific combinations of angles, depths, shapes, patterns, dimensions, etc.; some threads are not standardized by an organization, even though they are consistent as a type.

Thread Types by Standards

The most common types of threads are the ones that are designated by standards and specifications, which is what this guide covers. As mentioned above, there are plenty of unstandardized options. While these options are high-quality threads that serve important purposes, it is a complicated category that requires its own deep-dive. This guide will establish the baseline of standardized threads.

The most common standards that govern bolt, nut, and screw threads are:

Unified Thread Standards (UTS)
ISO/DIN
British Standard Whitworth (BSW)
ACME

Coarse and Fine Threads

UTS, ISO/DIN, and BSW use a spectrum of coarse and fine thread specifications as categories to help designate sizing.

Coarse Threads: Lower TPI/greater pitch
Fine Threads: Higher TPI/smaller pitch
Extra Fine Threads: Even higher TPI/smaller pitches than fine threads

Each standard defines their coarse/fine thread designation differently, but each list is independently consistent. For example, using the UTS standard, a fastener with a 3/8” OD will always have a coarse, fine, and extra fine TPI of 16, 24, or 32 (respectively).

Coarse Fine and Extra Fine Threads Blog Image

Unified Thread Standard (UTS)

Often referred to as “standard,” the Unified Thread Standard (UTS) is regulated by the American Society of Mechanical Engineers (ASME). You may also see the UTS noted as an “ASME/ANSI” standard (the American National Standards Institute (ANSI) is responsible for validating standards of different organizations). For more information on how fastener standards work, check out our Guide to Standards in the Fastener Industry.

The UTS includes specific dimensions, regulations for how to notate things, and advanced formulas. At its most basic core, it outlines a thread “template” that consists of a sharp trapezoidal thread shape and a 60° angle, with all measurements given in imperial units (including the TPI, which is sometimes called the pitch in everyday use but is actually given as threads/in .).

UTS thread sizes will appear as either a number between 0-12 or an inch increment, followed by a dash and then the TPI. The first number refers to the OD, which will appear as 0-12 until it reaches 1/4in (where inch increments take over). The TPI will appear as a number. For example:

#10 – 32: 0.19in. OD – 32 TPI
5/16” - 18: 5/16in. OD – 18 TPI

In the UTS, you will see coarse, fine, extra fine, and other UTS-specific descriptors written as either the full word/phrase or in acronyms:

UNC: Unified National Coarse
UNF: Unified National Fine
UNEF: Unified National Extra Fine
UNS: Unified National Special
- These are defined between the manufacturer and the supplier
8-UN: 8 Thread Series
- Less common category of a constant TPI of 8 for any OD over 1in.
12-UN: 12 Thread Series
- Like 8UN, but less common and requires an OD over 1 ½in.

You may not always see UNC, UNF, UNEF, etc . written in a thread description, or you may only see those acronyms. It is important to be familiar with the OD/TPI relationships so you can fill in the information you’ll need; for example, if the OD is 7/16in. and the designation is UNC, the TPI is 14. If it is UNF, the TPI will be 20.

UTS threads are also divided into 5 classes, which are distinguished by tighter tolerances and allowances as the number increases:

Classes 1-3 are most common, with 3 featuring the tighter tolerances
- A: External male threads
- B: Internal female threads
- Example: When discussing thread fit, a typical bolt is 2A (class 2 external) and a nut is 2B (class 2 internal)
4 is theoretical and obsolete
5 is a wrench fit

ISO/DIN Thread Standards

The International Organization for Standardization (ISO) owns almost 25,000 global standards in almost every industry in existence. However, when it comes to fasteners, many of the standards started out in the German national standard system (DIN). Over time, ISO took ownership of national standards to represent them on a unified, global scale. Hence, many DIN fastener standards are seen (or known) as ISO/DIN or just ISO , although they reference the same specification.

The ISO/DIN thread standards 261 and 262 govern general-purpose metric threads. There are other ISO/DIN standards that supplement this governance with additional regulations (for example, ISO 965-1 can apply specific tolerances to general-purpose threads and the design is defined in ISO 68-1), but 261 and 262 are the base specifications.

ISO/DIN threads are very similar to UTS threads, but they are measured in metric units. The shape and angle mirror the UTS standards and they feature a “normal” thread depth (approx. 0.5xpitch). There are several complex formulas that govern major and minor diameters, the cut-off point of the trapezoid tip, and other dimensions, but the main difference between ISO and UTS threads is the metric/imperial measurements.

These thread sizes will be called out with the OD first, followed by the thread pitch (distance between crests in millimeters), rather than the TPI. You may or may not see Coarse or Fine in the description (metric threads won’t use an acronym, just the word/phrase). For example:

M1.6 x 0.2 Fine: M1.6 = 1.6mm OD; 0.2 = pitch in mm; Fine = fine threads

In rare instances, you might see a metric Extra Fine or Extra Extra Fine thread , but these are very uncommon. In fact, most metric threads will be coarse; if there is no specific designation, coarse is assumed.

British Standard Whitworth Threads

The British Standard Whitworth thread standard (BSW) was the first ever national thread standard established in 1841, following a series of company-specific industry standards that prompted the need for a consolidated approach. As the oldest national thread standard, it served as the model for other standards like the UTS, but it does have a few key differences.

British Whitworth Thread Diagram Blog Image

Like the other standards, BSW is based on a series of complex formulas. But the basic template features rounded crests and roots, a 55° angle, and a thread depth of (approx. 0.64xpitch), which is different than normal depth.

It is important to note the following about the BSW standard:

BSW and UNC thread sizes are very similar, but different enough to cause connection issues if used together – ensure you only connecting fasteners of the same standard.
While still recognized and used in the industry, the BSW standard is slowly phasing out and being superseded by ISO.
BSW threads are coarse by default; fine threads (BSF) are not nearly as common.
These threads are measured with imperial units and should call out a TPI, but you will sometimes see a pitch given in both inch and metric units.

ACME Threads

The ACME thread design was developed in the 1890s as an improvement to the square thread. With a trapezoidal shape, ACME threads are stronger, more resistant to wear and tear, and easier to manufacture.

ACME is not a standard itself – it is the shortened name of the original manufacturer, ACME Screw Machine Company. It is, however, governed by ASME/ANSI standards and has a general template and complex formulas like the other thread standards. These threads feature a 29° angle, a normal thread depth (approx. 0.5xpitch), and a wide-base trapezoidal shape.

There are a few important points to know about ACME threads:

They are divided into 3 classes:
- General Purpose
- Centralizing (tighter tolerances than general purpose)
- Stub (same as centralizing but with a shorter depth)
They use imperial unit measurements
Not all trapezoidal threads are ACME – for example, metric trapezoidal threads appear very similar, but are governed by different standards and measurements

Uncommon Thread Styles

Beyond standardized threads, there are a wide range of styles designed to accommodate specific application needs (wood and deck installation, metal connections, applications that need to control material displacement, etc.). These styles combine thread directions, shapes, distances, depths, and tips to form unique designs, including but not limited to:

Reverse Threads
Double Reverse Threads
Flat Threads
Dual Threads
Hi-Lo Threads

While these styles are high-quality and consistent, they are not usually covered by standards and introduce significant complexity that is beyond the scope of this guide.