How to Identify an Unknown Thread

Verified by Rachel Mayfield, Supply Chain Analyst - April 2026

How to Identify an Unknown Thread

Identifying unknown threads in a workshop can save you time and money by preventing costly ordering mistakes. Here's how to do it step-by-step for the six main thread systems used in UK workshops: Metric Coarse, Metric Fine, UNC (US Coarse), UNF (US Fine), BSP Parallel, and BSP Taper.

Step 1: Measure Major Diameter with Calipers

Start by measuring the major diameter of the unknown thread using calipers. This is the outermost surface of the threads. For example, an M10x1.5 metric coarse thread has a major diameter of exactly 10mm.

Step 2: Measure Pitch with Thread Pitch Gauge or Count Threads Per Inch

Next, measure the pitch of the thread. You can do this with a thread pitch gauge or by counting the number of threads per inch (TPI) if you're dealing with UNC or UNF threads. For instance, an M10x1.5 metric coarse thread has a 1.5mm pitch.

Step 3: Check Profile

The profile angle is important for identifying the correct thread type. Metric and unified threads have a 60-degree profile, while BSP threads typically have a 55-degree profile.

Step 4: Cross-Reference Diameter + Pitch Against Standard Tables

Once you've got your measurements, cross-reference them against standard tables to identify the thread system. For example, an M8x1.25 is metric coarse, whereas a 3/8-16 UNC has a diameter of 9.525mm and 16 TPI.

Quick Reference Guide

Here's a quick reference guide for common threads:

• M10x1.5: Major diameter = 10mm; pitch = 1.5mm
• 3/8-16 UNC: Major diameter = 9.525mm (approx.); TPI = 16

Common Confusion Points

#### M10 vs. 3/8 Inch

M10 and 3/8-inch threads are often mistaken for each other due to their similar sizes. However, an M10 thread has a major diameter of exactly 10mm (0.394 inches), whereas a 3/8-inch UNC thread measures about 9.525mm in major diameter. They won't interchangeably fit.

#### BSP vs. NPT

Both BSP and NPT are pipe threads, but they have different profile angles: BSP has a 55-degree angle, while NPT is 60 degrees. This means that although they look similar, fitting them incorrectly can lead to leaks or other issues.

Additional Tips for Identifying Threads

• Thread Depth: Deeper threads indicate metric coarse or fine threads. At How to Identify an Unknown Thread, - Surface Finish: Rougher surfaces often suggest older manufacturing standards like BSP threads.

Using Thread Standards in the UK and International Markets

When dealing with international projects, remember that thread standards can vary. For instance, ISO 228-1 defines metric threads, while ASME B1.1 covers UNC and UNF threads. Understanding these differences helps avoid ordering errors and ensures parts fit correctly.

Trade Bodies for Thread Standards

Several trade bodies provide valuable resources on thread standards:

• British Industrial Fasteners Association (BIFA) offers detailed information on fastener standards.
• The British Pump Manufacturers' Association (BPMA) provides guidance on pipe threads like BSP and NPT.
• British Threading Manufacturers' Association (BTMA) focuses on threading equipment.

Conclusion

Identifying unknown threads accurately can save your workshop significant time and money. By following the steps above, you'll be able to quickly determine thread types and ensure compatibility across different systems. Keep these key points in mind: measure major diameter first, then check pitch and profile angle, and finally cross-reference against standard tables.

By mastering this skill, you'll reduce downtime caused by incorrect parts and maintain efficient operations in your workshop or manufacturing facility.