How to Identify Thread Types on Screws and Bolts

🔩 What Are the Most Common Thread Standards?

Before identifying a thread, it helps to know what types are most likely:

• Metric Threads (ISO): Measured in millimeters (e.g. M8×1.25), widely used in Europe and Asia.

• Unified Threads (UNC/UNF): Inch-based U.S. standard. UNC = coarse, UNF = fine.

• British Standard (BSW/BSF): Older imperial threads, common in older machinery.

• NPT (Tapered Pipe Threads): Often used for fluid or gas connections.

🧠 Tip: Most fasteners fall into metric or unified systems. If you're working on international machinery, metric is more likely.

📏 How Can You Measure Thread Pitch Accurately?

Thread pitch is the distance between threads—key to identifying the type.

• Metric Threads: Pitch is measured in millimeters (e.g. 1.0 mm = distance between crests).

• Imperial Threads: Measured in TPI (Threads Per Inch)—count the threads in one inch.

Common Tools:

• Thread Pitch Gauge: The most reliable tool for identifying pitch.

• Caliper: For measuring outer diameter (OD).

• Steel Ruler: Can help in a pinch, but less precise.

⚠️ Mixing metric and imperial threads may feel like a “near fit”—but it’s a trap. They won’t seat properly and may damage each other.

❓ What’s the Difference Between Metric and Imperial Threads?

Some clues to distinguish them quickly:

🔎 How to Distinguish Threads Without Measuring Tools?

If you're in the field without gauges or calipers, try these:

1. Check the Head Markings
   Metric bolts often include an “M” (e.g. M8). Imperial bolts use fractional numbers.

2. Look at the Thread Profile
   Metric threads are usually more “flat” on top, while imperial may appear more rounded.

3. Trial Fit with Known Nuts
   Try fitting the bolt into a known nut of either system—if it binds early, it’s probably the wrong type.

4. Use Reference Parts
   Keep labeled samples (e.g. M6 vs 1/4-20) as a field guide.

🧰 What Tools Help You Identify Threads Correctly?

If you regularly work with mixed standards, these tools are invaluable:

• 🧵 Thread Pitch Gauges – Metric and imperial sets are available.

• 🔧 Nut & Bolt Checkers – Plates with multiple tapped holes and studs.

• 📐 Digital Calipers – For measuring outer diameters precisely.

• 📄 Thread Standard Charts – Laminated quick guides are useful for on-site reference.

🛠️ Still Unsure? Fasto Can Assist with Thread Identification

Identifying threads can be surprisingly tricky—especially when dealing with mixed systems, non-standard fasteners, or legacy equipment.

At Fasto, we don’t just sell fasteners—we help customers match, measure, and customize. If you're unsure whether your bolt is UNC or M8, or need compatible mating parts across standards, our team can help.

We even offer thread matching consultation and can manufacture custom fasteners based on samples or specs.

Let’s eliminate the guesswork—get in touch and make thread identification one less thing to worry about.

📧 info@fasto.cn
🌐 Explore our catalog or request samples today!