Thread Pitch, TPI & Fastener Identification

Plain-English Summary

Understand thread pitch, threads per inch, metric and inch fastener identification, thread gauges, diameter checks, and practical first steps before replacing or cutting threads.

Why It Matters

The wrong fastener can damage threads, loosen in service, fail under load, leak, seize, or create a false sense of security. Diameter alone does not identify a thread.

Field Rule of Thumb

If it does not start by hand, stop. A correct threaded fastener should start cleanly by hand in a clean, undamaged thread.

Core Concept

Metric pitch is the distance from one thread crest to the next in millimeters. Example: M10 × 1.5. Inch threads are commonly described by threads per inch. Example: 1/2-13 means nominal 1/2 inch with 13 threads per inch.

Worked Example

A bolt near 10 mm diameter binds after one turn with an M10 × 1.5 replacement. A pitch gauge shows the original was M10 × 1.25 fine thread. The diameter was close, but the pitch was wrong.

Common Mistakes

• Measuring diameter only.
• Forcing a close thread.
• Ignoring thread form.
• Confusing pipe thread with straight thread.
• Ignoring left-hand threads.
• Ignoring grade, coating, or material.

First Checks / Troubleshooting Flow

1. Clean the thread.
2. Inspect for damage, paint, plating, burrs, rust, or contamination.
3. Measure major diameter.
4. Use a pitch/TPI gauge.
5. Check direction and straight/tapered condition.
6. Check fastener grade, material, coating, and head style.
7. Verify against drawing, parts manual, OEM documentation, or known good sample.