ISO 261 is an international standard that defines the nominal diameters and pitch sizes of metric screw threads. It specifies coarse and fine pitch series for different thread sizes, ensuring compatibility across industries.
This standard is essential for fastener selection, mechanical design, and manufacturing, as it ensures global standardization of metric screw threads.
ISO 261 works alongside:
- ISO 68-1 (Thread Profile Definition)
- ISO 724 (Basic Thread Dimensions)
- ISO 965 (Thread Tolerances)
Technical Details
ISO 261 classifies metric thread sizes into:
- Coarse Threads (widely used for general applications and fast assembly)
- Fine Threads (higher precision, better resistance to loosening, used in automotive and aerospace applications)
Each thread size is defined by:
- Nominal Diameter (M)
- Pitch (P)
- Major Diameter (D / d)
- Minor Diameter (D₁ / d₁)
- Thread Depth
ISO 261 Thread Size Tables
Standard Metric Thread Sizes (Coarse Pitch)
| Nominal Diameter (M) | Pitch (P) (mm) | Major Diameter (D/d) (mm) | Minor Diameter (D₁/d₁) (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| M1 | 0.25 | 1.00 | 0.75 | 0.8 |
| M1.6 | 0.35 | 1.60 | 1.25 | 1.3 |
| M2 | 0.40 | 2.00 | 1.60 | 1.7 |
| M2.5 | 0.45 | 2.50 | 2.05 | 2.1 |
| M3 | 0.50 | 3.00 | 2.40 | 2.5 |
| M4 | 0.70 | 4.00 | 3.30 | 3.4 |
| M5 | 0.80 | 5.00 | 4.20 | 4.3 |
| M6 | 1.00 | 6.00 | 5.00 | 5.2 |
| M8 | 1.25 | 8.00 | 6.75 | 6.8 |
| M10 | 1.50 | 10.00 | 8.50 | 8.8 |
| M12 | 1.75 | 12.00 | 10.20 | 10.5 |
| M16 | 2.00 | 16.00 | 13.90 | 14.0 |
| M20 | 2.50 | 20.00 | 17.30 | 17.5 |
| M24 | 3.00 | 24.00 | 20.90 | 21.0 |
🔹 Note: Coarse threads are commonly used in general-purpose fasteners due to their durability and easy assembly.
Fine Pitch Metric Threads (ISO 261)
| Nominal Diameter (M) | Fine Pitch (mm) | Major Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| M8 × 1 | 1.00 | 8.00 | 7.00 | 7.1 |
| M10 × 1.25 | 1.25 | 10.00 | 8.75 | 8.8 |
| M12 × 1.5 | 1.50 | 12.00 | 10.50 | 10.8 |
| M16 × 1.5 | 1.50 | 16.00 | 14.50 | 14.8 |
| M20 × 1.5 | 1.50 | 20.00 | 18.50 | 18.8 |
🔹 Note: Fine-pitch threads have greater resistance to loosening and are used in precision applications, including automotive, aerospace, and hydraulic systems.
Applications of ISO 261 Metric Threads
ISO 261 standard metric threads are used in various industries, including:
- Automotive (engine bolts, wheel lugs)
- Aerospace (fasteners for aircraft components)
- Industrial machinery (threaded rods, mechanical connections)
- Construction (structural bolts, anchor fasteners)
The standardization ensures interchangeability and consistency in manufacturing.
Related Metric Thread Standards
After reviewing ISO 261, you may also find the following metric thread standards useful:
- ISO 68-1 – Metric Thread Profile (Defines the fundamental shape of metric threads.)
- ISO 262 – Preferred Metric Thread Sizes (Specifies commonly used metric thread sizes.)
- ISO 724 – Basic Dimensions of Metric Threads (Defines the fundamental dimensions for metric screw threads.)
- ISO 965 – Metric Thread Tolerances (Specifies tolerances for external and internal metric threads.)
- ISO 1502 – Thread Gauges for Metric Threads (Provides details on thread inspection using gauges.)
These links allow users to navigate easily between related standards.
Important Notice on Standards Validity
This information is for reference purposes only. Users should consult official sources and updated standards to ensure compliance with current regulations.
The validity of standards may change over time, so it is recommended to verify them through recognized engineering and standardization organizations (ISO, DIN, ANSI, ASME, etc.).
Check with relevant standardization bodies in your region to confirm whether a specific standard is still valid and applicable. Regulations and industry requirements may vary based on region and application.