ISO 965 is the international standard that defines tolerances and deviations for metric screw threads. It ensures that internal (nut) and external (bolt) threads fit together correctly within specified limits.
This standard is essential in engineering, manufacturing, and quality control, as it ensures threaded components function properly without excessive looseness or tightness.
ISO 965 is closely related to:
- ISO 68-1 (Metric Thread Profile)
- ISO 261 (Standard Metric Thread Sizes)
- ISO 724 (Basic Dimensions of Metric Threads)
Technical Details
ISO 965 defines tolerance grades for metric threads and ensures compatibility between mating parts.
Main Tolerance Classes for Metric Threads:
- External Threads (Bolts & Screws): 4g, 6g, 6e, 8g (Common: 6g – general use)
- Internal Threads (Nuts & Tapped Holes): 4H, 5H, 6H, 7H (Common: 6H – standard fit)
Key Concepts in Thread Tolerancing:
✅ Tolerance Class (6H, 6g, etc.) – Defines the acceptable limits for thread dimensions
✅ Deviation (es, EI, etc.) – Difference between the actual and nominal size
✅ Allowance (Fundamental Deviation) – The intentional difference to ensure proper assembly
ISO 965 Tolerance Classes and Their Fit
| Tolerance Class | Application | Fit Type |
|---|---|---|
| 4H / 4g | Precision instruments | Very tight fit |
| 5H / 5g | High-precision mechanical parts | Close fit |
| 6H / 6g | Standard fasteners, general engineering | Normal fit |
| 7H / 7g | Coarse threads, heavy-duty applications | Loose fit |
| 8H / 8g | Structural connections, rough machining | Extra loose fit |
🔹 Note: The most commonly used fit is 6H (nut) and 6g (bolt), which ensures a balance between strength and manufacturability.
Recommended Tolerances for Standard Metric Threads (ISO 965)
| Nominal Diameter (M) | Tolerance Class (External – 6g) | Tolerance Class (Internal – 6H) |
|---|---|---|
| M3 | ±0.030 mm | ±0.015 mm |
| M4 | ±0.040 mm | ±0.020 mm |
| M5 | ±0.050 mm | ±0.025 mm |
| M6 | ±0.060 mm | ±0.030 mm |
| M8 | ±0.070 mm | ±0.035 mm |
| M10 | ±0.080 mm | ±0.040 mm |
| M12 | ±0.100 mm | ±0.050 mm |
| M16 | ±0.120 mm | ±0.060 mm |
| M20 | ±0.140 mm | ±0.070 mm |
🔹 Note: Tighter tolerances improve precision but may increase manufacturing complexity and cost.
Applications of ISO 965 Metric Thread Tolerances
ISO 965 tolerances are widely used in:
- Machinery & Engineering (ensuring precise assembly in mechanical components)
- Aerospace & Automotive (critical fasteners where precision is required)
- Structural & Heavy Machinery (loose fits for easy assembly under load)
Correct tolerances prevent thread binding, improve wear resistance, and ensure interchangeability.
Related Metric Thread Standards
After reviewing ISO 965, you may also find the following standards useful:
- ISO 68-1 – Metric Thread Profile (Defines the fundamental shape of metric threads.)
- ISO 261 – Standard Metric Thread Sizes (Covers the full range of standard metric thread sizes.)
- ISO 262 – Preferred Metric Thread Sizes (Specifies the most commonly used metric thread sizes.)
- ISO 724 – Basic Dimensions of Metric Threads (Defines the fundamental dimensions for metric screw threads.)
- ISO 1502 – Thread Gauges for Metric Threads (Provides details on thread inspection using gauges.)
These links allow users to easily navigate 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.