Thread Pitch – Definition & Charts

Thread pitch is the distance between adjacent threads on a fastener, measured either as threads per inch (TPI) for imperial fasteners or millimeters between threads for metric fasteners.

In this article, we’ll explore what thread pitch means, how it’s measured, and why it matters for precision machinery.

We’ll also provide detailed charts for the most common thread series so you can quickly find the specifications you need.

What is a Thread Pitch ? The Definition

In the imperial system, thread pitch is expressed as Threads Per Inch (TPI).

A bolt labeled “1/2-13” has 13 threads per inch of length.

In the metric system, pitch is the distance in millimeters between threads.

An M10 × 1.5 bolt has threads 1.5 mm apart. The smaller the pitch number, the finer the thread. The larger the number, the coarser the thread.

A coarse thread packs fewer turns into a given length, while a fine thread packs more.

That single fact decides how strongly a fastener holds, how easily it assembles, and even whether it will survive vibration.

Thread pitch affects:

• Strength: Fine threads have larger tensile stress areas, increasing strength.
• Assembly speed: Coarse threads resist cross threading, making them ideal for quick assembly.
• Vibration resistance: Fine threads hold tighter under vibration, while coarse threads tolerate dirt and damage better.
• Material thickness: Coarse threads need less engagement length; fine threads require deeper engagement to prevent stripping.

Thread Basics: Pitch, TPI, and Diameter

When you examine a threaded rod under good lighting you notice two repeating dimensions:

1. The major diameter – the outer crest-to-crest distance.
2. The distance between neighboring crests. That distance has two common names:
   • Threads Per Inch (TPI) in the imperial system
   • Thread pitch (in millimetres) in the metric system

Both numbers describe the same geometry from different angles.

Mathematically you can convert them with :

And that’s because:

A third important dimension is the minor diameter (root to root) because it limits how much metal actually carries load.

Charts often list a derived value called the tensile stress area, noted as follow :

Engineers use is with simple axial stress formulas like:

Where F is the applied tensile force.

Keep these three characters (major diameter, pitch/TPI, and tensile stress) area in mind.

Every chart you will ever read is simply a nicely formatted way to display them.

Here is a schema to picture all of it:

Exploring Thread Series: Types and Characteristics

Thread series are the categorized groups of diameter and pitch combinations, each with unique properties that make them suitable for particular tasks.

These series are measured by TPI applied to a specific diameter, and they include coarse, fine, 8-thread, and metric variations.

Understanding these helps in selecting the right fastener for the job, avoiding issues like cross-threading or stripping.

Coarse Thread Series (UNC/UNRC)

When we talk about the Unified National Coarse thread (UNC), we’re referring to the most standard thread design in North America.

It is known for being robust and easy to assemble.

Some specifications:

• Typical for general purpose bolts and nuts
• Fewer threads per inch
• Rapid assembly because it’s a more open spiral
• Less prone to cross threading

It also have a tradeoff to pay attention to: it lightly lower tensile stress area compared with a fine thread of the same diameter.

Chart :

Fine Thread Series (UNF/UNRF)

Moving into precision territory, the Unified National Fine series trades speed for strength and reliability under vibration.

These threads are often found in jobs where super precise movements are needed.

They are really strong when pulled (high tension strength) because their design gives them a bigger area to handle the stress. But, if you’re using very fine threads, you need them to grip the nut for a longer distance than coarser threads. This extra grip helps make sure they don’t strip out when under load.

Chart :

8-Thread Series – 8UN

For bolts one inch and larger, engineers often standardize on a constant 8 TPI regardless of diameter.

It is required by several important industry rules, like ASTM A193 B7, A193 B8/B8M, and A320. You’ll only find this type of thread used on diameters that are one inch or bigger.

This standard pitch simplifies design and manufacturing for larger fasteners used in high-pressure or high-temperature applications, providing a reliable and strong connection across various sizes without changing the thread count.

Metric Thread Series (ISO Coarse)

The Metric Thread Series is the international standard for screw threads, used pretty much everywhere outside of North America.

It designed to be global interchangeability, and consistent across industries and countries.

Some specifications:

• Typical for general purpose bolts, nuts, and fasteners in machinery, automotive, and construction worldwide
• Pitch measured in millimeters (larger pitch for coarse series means fewer threads per length)
• Rapid assembly thanks to the standardized design and coarser spiralLess prone to cross threading, especially in high volume manufacturing

All measurements are in metric units (mm and mm²) for accuracy, differing from the inch-based systems above.

Below is a chart that summarizes the key values:

Metric threads flip the way you approach the problem. Instead of asking, “How many turns fit into one inch?” we ask, “How far does the fastener travel in one turn?”

• A M10 × 1.5 bolt has a major diameter of 10mm and a pitch of 1.5mm.
• If you turned that bolt exactly one revolution, it would move forward 1.5mm.

Fine pitches exist in the metric world too: For exemple M10 × 1.25 or M10 × 1 are used in engines where vibration can be intense.

The lower number gives more threads over the same length, similar to the imperial UNF logic.

Metric standards such as ISO 261 list preferred pitch series the same way UNC and UNF do. The tables look different, but the engineering motivations are similar.

Why Pitch Choices Affect Strength and Assembly

There is multiple points to be aware of when it come to make a pitch choice.

1. How Strong is the Thread?

Imagine you’re trying to pull a nail out of wood. The more wood there is around the nail, the harder it is to pull out, right? It’s the same with threads.

Tensile Strength and Stress Area just means how much metal is left at the bottom of the thread grooves. The more metal there is, the stronger the thread is against being pulled apart.

Fine threads win here: Threads with smaller, more numerous grooves (fine threads) have more metal around their core than threads with bigger, deeper grooves (coarse threads), even if they’re the same overall size.

Here is some math :

This means fine threads are better at resisting a pulling force.

2. How Many Twists to Prevent Stripping?

That refers to how many times you need to turn a screw before it really holds tight.

Here is what to consider :

Coarse threads have deeper roots, they need fewer engaged turns. Because coarse threads have deeper grooves, each turn gets a stronger “bite.” So, you don’t need to turn a coarse thread as many times to get a good, strong hold compared to a fine thread.

Shear Stripping and Engagement Length: This is about how much the thread needs to be “gripping” inside another part to prevent it from stripping or shearing off.

3. How Easy Is It to Put Together?

Coarse threads slip together quickly and forgive a bit of grit, paint, or minor mis-alignment.

Fine threads demand cleaner starts.

When a race team changes tires in seconds, they use a very coarse wheel stud, when a machinist sets gib screws in a lathe, fine UNF or metric fine is a good bet for that.

4. Vibration Resistance

It is how well a thread stays put when there’s a lot of shaking or vibration.

The sides of the grooves on fine threads are less steep than on coarse threads. This means it’s harder for vibrations to push the thread loose.

If you add a special nut that resists turning (a locknut), fine threads are incredibly good at staying tight, even with lots of vibration.

How to Measure Threads?

You don’t need to be an expert with a lot of expensive equipment to figure out what kind of thread you’re looking at. Even a beginner can get the right measurements with some simple and affordable tools.

One of the easiest methods is to use a thread gauge.

This is a small tool that looks like a pocket knife, but it’s filled with little metal leaves that have teeth on them. You just try each leaf against the threads on your screw or bolt.

When you find the one that fits perfectly into the grooves with no gaps, you’ve found your match. The leaf will have the measurement stamped right on it.

On more premium threads, such as API Threads or ACME Threads, you might need to have exact dimensions, especially to check wear or in a quality control process.

This level of precision often requires advanced tools like optical comparators or dedicated thread micrometers.

For internal or difficult to access threads, the use of impression taking technology like Plastiform allows you to easily measure the thread profile externally.

Conclusion

In this article we covered what Thread Pitch is and some important charts to identify those.

The Thread pitch is the distance between threads, expressed differently in regions around the world as Threads Per Inch (TPI) for imperial standards and in millimeters for the metric system.

This single measurement dictates a fastener’s core characteristics.

The choice between coarse threads (like UNC) and fine threads (like UNF) is very important and should be considered based on your project needs.

• Coarse threads offer unmatched speed and ease of assembly, forgiving minor imperfections and requiring fewer turns to secure.
• Fine threads provide superior tensile strength and excellent resistance to vibration, making them essential for precision and applications exposed to intense stresses.
• Specialized series like 8UN and ISO Metric system provide standardized solutions for specific industrial needs.

During production, or even during the threads life, you will use different ways of measure your threads, including the pitch. It can be with a simple gauge, high-tech measuring machines or even making a precision replica with an impression compound

Selecting the right thread pitch is a critical engineering decision for your systems and its maintenance is as important for the whole assembly to keep going as expected.

Frequently Asked Questions

1. Why would I choose coarse threads (UNC) if fine threads (UNF) are technically stronger?

You would choose coarse threads for their practical advantages. They assemble much faster, are less likely to get cross-threaded, can tolerate dirt or minor damage, and don’t need to be screwed in as far to get a secure grip (less engagement length). They are ideal for general construction and applications where speed is more important than maximum tensile strength.

2. For imperial threads, does a higher TPI number mean the thread is finer or coarser?

A higher TPI number means the thread is finer. For example, a 1/2-20 bolt (20 TPI) has more threads packed into one inch than a 1/2-13 bolt (13 TPI), making the 1/2-20 the finer thread.

3. What is the main difference in how metric and imperial pitch are described?

The main difference is the frame of reference. The imperial system counts how many threads fit into a fixed distance (Threads Per Inch). The metric system measures the distance from one thread crest to the next in millimeters.

4. When is the 8-Thread (8UN) series typically used?

The 8UN series, with its constant 8 threads per inch, is used specifically for larger fasteners with diameters of 1 inch and greater. It’s a standard often required in high-pressure and high-temperature industrial applications, like those governed by ASTM standards.

5. What should I do if I need to measure internal threads that I can’t see or reach with a gauge?

For hard-to-reach internal threads, the best solution is to use an impression-making compound like Plastiform. This putty-like substance creates a perfect, solid replica of the internal threads, which you can then easily remove and measure externally with calipers or a thread gauge.