Metric Thread Tolerances - Your Complete Guide (2024)

A Guide to ISO 965-1 and the 6H/6g Tolerances

When working with fasteners like Nuts and Bolts, precision is crucial. One of the key aspects of fastener design and selection is understanding metric thread tolerances.

In this post, we’ll explore the ISO 965-1 specification and the significance of the 6H and 6g tolerances. My goal is to provide a clear, understandable and comprehensive guide to help you understand this technical topic.

What are Metric Thread Tolerances?

In simple terms, metric thread tolerances define the acceptable limits of variation in metric thread dimensions.

These tolerances ensure the threaded parts fit together correctly and function as intended. Without proper tolerances, threads might be too tight, causing difficulty in assembly, or too loose, resulting in unreliable connections.

ISO 965-1 Specification

This now brings me to the ISO 965-1 standard.

ISO 965-1 is an international standard that was created to outline the general rules for the production and acceptance of metric screw threads. This standard ensures uniformity and compatibility across different applications and industries. ISO 965-1 specifically addresses:

• Basic Profile: The basic geometry of the thread, including the crest, root, and flanks, in accordance with ISO 68-1.
• Tolerance Classes: These specify the permissible deviations from the basic profile.
• Major Diameter: This is the largest diameter of a thread. It is the outside diameter for external threads (like those on a hex bolt or a machine screw). On the other hand for internal threads (like those in a nut), it is the diameter of the hole at the largest points of the thread.
• Minor Diameter: This is the smallest diameter of a thread. For external threads like a bolt, it is the diameter of the root on the thread (the bottom of the thread groove). Whereas for internal threads, it is the diameter of the crest of the thread (the top of the thread).
• Pitch Diameter: This is the diameter of an imaginary cylinder that passes through the thread profile so that the widths of the thread and the space between threads are equal. It is crucial to ensure that mating threads fit together properly.

Understanding 6H and 6g in Metric Thread Tolerances

Now I will move on to the 6H and 6g tolerance classes. Within the ISO 965-1 standard, the tolerance classes 6H and 6g are commonly used. These classes define the limits for internal and external threads.

• 6H Tolerance (Internal Threads): The 6H tolerance is used for nuts or the internal threads of a bolt and nut assembly. It ensures that the internal threads fit precisely with the external threads. The ‘H’ designation indicates that the tolerance is on the higher side of the nominal diameter, ensuring a tighter fit.
• 6g Tolerance (External Threads): This class is used for bolts or the external threads of a bolt and nut assembly. It provides a slightly looser fit to accommodate variations in manufacturing. The ‘g’ designation indicates that the tolerance is on the lower side of the nominal diameter, allowing for a bit more play.

However these 2 tolerance classes are part of a broader system. Other classes include 4H, 5H, 7H for internal threads and 4g, 5g, 7g for external threads. The lower numbers mean tighter tolerances, while higher numbers indicate looser tolerances.

Coarse Pitch Tolerances vs Fine Pitch Tolerances

When you are dealing with metric threads, you may notice they come in 2 types. You will often find bolts with a coarse pitch thread or bolts with a fine pitch thread. The most common standard is the coarse pitch.

Each type has its specific tolerances. Coarse pitch threads have a larger pitch (the distance between threads), while fine pitch threads have a smaller pitch. The finer the pitch, the closer together the threads are, which affects the tolerance ranges.

External Thread Tolerances

We have put together a chart for you to reference the metric thread tolerances of both fine pitch and coarse pitch metric bolts and screws. This will show you the minimum and maximum diameters of the major diameter, pitch diameter and minor diameter.

For a reminder of what these 3 are, have a look above under the ISO 965-1 Specification header

Continued

The Importance of Metric Thread Tolerances in Manufacturing

Below I will explain a few situations in different industries where the importance of strict metric thread tolerances are needed. From manufacturing to assembly, here are a few reasons why they are so important.

1. Interchangeability: Proper tolerances ensure that parts manufactured in different locations and companies fit together seamlessly. This is vital in global supply chains.
2. Functionality: Threads that are too tight or loose can compromise the functionality of the assembly. Tight threads can cause excessive wear and difficulty in assembly. On the other hand, loose threads can lead to instability and failure of the assembly.
3. Quality Control: Tolerances provide a standard for quality control, ensuring that all parts meet a minimum standard of precision and accuracy. This reduces the risk of defects and increases the reliability of the final product.
4. Cost Efficiency: Adhering to specified tolerances helps reduce manufacturing costs by minimising waste and rework. This also ensures consistent production of parts, reducing variability and improving efficiency

The Need For Metric Thread Tolerances In Different Industries

The 6H and 6g metric thread tolerances are widely used by manufacturers in the production of metric fasteners,. These are especially important in various industries for different reasons. Here are some examples:

• The Manufacturing Of Machines: Machine Screws used in the assembly of machines must be made to strict thread tolerances. As a result, issues are avoided when it comes to maintenance or replacement of parts.
• Automotive Industry: In the automotive industry, precise thread tolerances are essential for the assembly of engines, transmissions, and other critical components. The 6H tolerance ensures that internal threads in engine blocks or transmission housings fit perfectly with the external threads of Hexagon bolts or Flanged Hex bolts, providing a secure and reliable connection.
• Heavy Machinery: In heavy machinery, such as construction equipment and industrial machines. Robust and precise thread tolerances are crucial on bolts. They maintain the integrity of the machinery under heavy loads and continuous use.
• Engineering: The engineering industry relies on precision-made fasteners. From Socket Screws to nuts and bolts. Engineers depend on the accuracy of tolerances to ensure a proper design and build is achieved.

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