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UN-8 Thread Pitch Chart
This UN-8 Thread Chart provides a quick reference for Unified National 8 Thread Series thread sizes and is designed for engineers, machinists, manufacturers, maintenance technicians and workshop professionals working with larger diameter imperial threads. Whether you're selecting the right tap, die or drill bit, this chart helps you find the specifications you need quickly and accurately.
| Diameter | TPI | Drill Size (mm) |
|---|---|---|
| 1" | 14 | 23.5 mm |
| 1-1/8" | 8 | 25.3 mm |
| 1-1/4" | 8 | 28.5 mm |
| 1-3/8" | 8 | 31.6 mm |
| 1-1/2" | 8 | 34.8 mm |
| 1-5/8" | 8 | 38.0 mm |
| 1-3/4" | 8 | 41.2 mm |
| 1-7/8" | 8 | 44.3 mm |
| 2" | 8 | 47.5 mm |
| 2-1/4" | 8 | 55.0 mm |
| 2-1/2" | 8 | 61.3 mm |
What is a UN-8 Thread?
UN-8 stands for Unified National 8 Thread Series. It is part of the Unified Thread Standard (UTS) system used for inch-based fasteners and tooling applications. UN-8 threads have a constant 8 threads per inch (TPI) regardless of the thread diameter, making them ideal for larger diameter applications where consistent thread pitch is essential.
UN-8 threads use a 60° thread profile, the standard geometry for all Unified National threads. The UN-8 series is commonly used for larger diameter fasteners where a coarse, strong thread is required. These threads provide excellent strength, durability and resistance to damage in heavy-duty applications, making them a trusted choice across industrial, engineering and manufacturing sectors.
Understanding the Unified Thread Series
The Unified Thread Standard includes several thread series, each designed for specific applications and requirements:
- UNC (Unified National Coarse) – General-purpose coarse threads used in most fastening applications.
- UNF (Unified National Fine) – Finer threads for improved clamping force and precision in tighter tolerances.
- UNEF (Unified National Extra Fine) – Extra fine threads for limited space and high-precision applications.
- UN-8 Thread Series – Constant 8 TPI threads specifically engineered for larger diameter applications and heavy-load fastening.
Selecting the correct thread series is important for achieving proper fit, strength and performance in your application. UN-8 threads excel where larger diameters and consistent pitch are required, offering reliability in demanding industrial environments.
How to Read the UN-8 Thread Chart
The UN-8 Thread Chart above displays the key specifications you need for thread selection and tooling:
- Diameter – The nominal thread size in imperial inches (for example, 1", 1-1/4", 1-1/2").
- TPI (Threads Per Inch) – The number of threads per inch; UN-8 threads maintain 8 TPI across all sizes in this series.
- Drill Size (mm) – The recommended tap drill size in millimetres, essential for creating the correct pilot hole before tapping.
Use this reference to select the correct UN-8 tap drill size for your application, ensuring proper thread formation and strength in your finished holes.
Common Applications
UN-8 threads are found in a wide range of industrial and engineering applications where larger diameter fasteners and consistent thread pitch are required:
- Heavy machinery and industrial equipment
- Large mechanical assemblies and structural connections
- Engineering and manufacturing applications
- Construction equipment and site machinery
- Agricultural machinery and farm equipment
- Maintenance and repair work in industrial settings
- High-load fastening applications requiring strength and durability
The UN-8 Thread Series is trusted across these sectors for its reliability, strength and consistent performance in demanding applications.
Frequently Asked Questions
What does UN-8 mean in thread specifications?
UN-8 refers to the Unified National 8 Thread Series, where "8" indicates 8 threads per inch. This constant pitch is maintained across all diameter sizes in the UN-8 series.
Is UN-8 the same as UNC?
No. UNC (Unified National Coarse) and UN-8 are different thread series. While both are part of the Unified Thread Standard, UN-8 is specifically designed for larger diameter applications with a constant 8 TPI pitch, whereas UNC varies in pitch depending on diameter.
What is the difference between UN-8 and UNF threads?
UN-8 threads have 8 threads per inch and are designed for larger diameter fasteners in heavy-duty applications. UNF (Unified National Fine) threads have a finer pitch and are used where improved clamping force and precision are required in smaller diameter applications.
Are UN-8 threads interchangeable with metric threads?
No. UN-8 threads are imperial (inch-based) and are not directly interchangeable with metric threads. Always verify thread specifications before selecting fasteners or tooling.
Where are UN-8 threads commonly used?
UN-8 threads are commonly found in heavy machinery, industrial equipment, large mechanical assemblies, construction equipment, agricultural machinery and high-load fastening applications where larger diameter imperial threads are required.