Most metric bolts and screws are specified as M + nominal diameter + (pitch) + length. Once you can read the markings correctly, you’ll avoid common mistakes—especially mixing up coarse and fine threads.
This post focuses on the everyday sizes M6 / M8 / M10 and the meaning of thread pitch P.
1. What does M6 / M8 / M10 mean?
- M means a metric ISO thread.
- The number after M (for example 6, 8, 10) is the nominal major diameter of the external thread, in millimeters.
Examples:
- M6 ≈ major thread diameter ~ 6 mm
- M8 ≈ ~ 8 mm
- M10 ≈ ~ 10 mm
Note: it’s nominal—actual measured diameter can vary slightly due to tolerances.
2. What is thread pitch P?
Thread pitch (P) is the axial distance (mm) between two neighboring thread crests.
- Larger P → “coarser” thread (fewer threads per length, advances more per turn).
- Smaller P → “finer” thread (more threads per length, more precise adjustment).
Coarse vs fine threads
For common metric diameters, there is typically:
- Coarse pitch: most widely used; easy to source.
- Fine pitch: used for specific design needs (alignment, vibration behavior, limited engagement length, etc.).
Key rule of thumb:
- If pitch is not shown (e.g. M10×30), it usually implies the standard coarse pitch.
- Fine threads are typically written with pitch explicitly (e.g. M10×1.25×30).
3. How to decode a full metric fastener callout
You’ll often see one of these formats:
Format A: M10×30
- M10: 10 mm nominal diameter
- ×30: 30 mm length (measured under the head for most bolts)
- Pitch: omitted → usually coarse
Format B: M10×1.25×30
- M10: 10 mm diameter
- ×1.25: pitch P = 1.25 mm (commonly a fine pitch for M10)
- ×30: 30 mm length
Format C (extras you may encounter)
Depending on the supplier, you may also see:
- LH: left-hand thread
- Standards: DIN/ISO references (e.g. ISO 4017, DIN 933)
- Material/coating: zinc plated, stainless steel, etc.
4. Quick coarse-pitch table (common references)
Coarse pitch is the default choice in many applications. This table helps you sanity-check quickly:
| Size | Common coarse pitch P (mm) | Example notation |
|---|---|---|
| M6 | 1.0 | M6×1.0×20 (or M6×20) |
| M8 | 1.25 | M8×1.25×30 (or M8×30) |
| M10 | 1.5 | M10×1.5×40 (or M10×40) |
Quick memory aid:
- M6 → P = 1.0
- M8 → P = 1.25
- M10 → P = 1.5
5. How to measure a bolt when the label is missing
Measure nominal diameter (M)
- Use a caliper to measure the outside thread diameter.
- ~5.8–6.0 mm → likely M6; ~7.8–8.0 → M8; ~9.8–10.0 → M10.
Measure thread pitch (P)
Best method:
- Use a thread pitch gauge.
If you don’t have one:
- Count threads over a 10 mm span.
- Example: if ~8 threads fit in 10 mm, then P ≈ 10/8 = 1.25 mm.
6. Which pitch should you choose?
Practical guidance:
- Replacing hardware in common equipment: stick with coarse pitch for compatibility.
- If a nut won’t engage even though the diameter matches: verify pitch (coarse vs fine).
- For vibration: pitch is only part of the story—use correct preload and proper anti-loosening methods (lock nuts, threadlocker, spring washers, etc.).
7. Common mistakes to avoid
- M size is not length: always confirm the ×L part.
- Pitch is often omitted for coarse threads—still double-check for special assemblies.
- Don’t mix fine-pitch nuts with coarse-pitch bolts (same M, different P).
For product categories, see: Bolts.
Next: Bolt Property Classes 4.6/5.6/8.8/10.9/12.9 — Choosing by Load and Environment.
