Metric Thread Classes Explained

When you buy metric bolts, nuts and screws for a project, you will often see codes such as M12 × 1.75 – 6H or M10 × 1.5 – 6H/6g on drawings and purchase orders. Those numbers and letters after the size are the thread class. They tell you how tight or loose the mating threads should be, and how much tolerance is allowed during production according to standards like ISO 965 and AS 1275. 信息中心文件+1

For engineers, buyers and distributors in Australia, New Zealand and other regions, understanding thread class is important. It affects assembly speed on site, risk of galling, possibility to add coatings like hot-dip galvanizing, and interchangeability between suppliers. This guide explains the basics in a practical way so you can read drawings correctly and communicate clearly with your fastener supplier.

1. What does “thread class” mean?

A metric thread class defines the tolerance band for the thread diameters and the relative looseness or tightness between a bolt (external thread) and a nut or tapped hole (internal thread).

Because no manufacturing process is perfect, standards such as ISO 965-1:2013 specify how much deviation from the nominal size is allowed while still ensuring a reliable fit. 国际标准化组织+1

In simple terms:

  • A tighter thread class gives a closer fit with less play but requires more accurate machining or cold forming.
  • A looser thread class allows more clearance, which is useful for thick coatings, dirty environments or quick assembly.

When you choose a thread class, you are deciding how much clearance you want between the male and female threads.

2. How metric thread classes are written

Metric ISO screw threads are usually designated like this:

M10 × 1.5 – 6H/6g

This breaks down as:

  • M10 – metric thread with 10 mm nominal major diameter
  • 1.5 – pitch (distance between thread crests) in mm
  • 6H – internal thread tolerance class (nut or tapped hole)
  • 6g – external thread tolerance class (bolt or screw) 维基百科+1

Each thread class has two parts:

  1. Tolerance grade (number)
    • Usually from 3 (very fine tolerance) to 9 (coarse).
    • Smaller numbers = tighter tolerance band. Tameson.com+1
  2. Tolerance position (letter)
    • Upper-case letters (H, G, etc.) are used for internal threads.
    • Lower-case letters (h, g, e, f, etc.) are used for external threads. Protolabs Network+1

So in 6H:

  • 6 is the grade (medium tolerance width).
  • H is the position, usually giving zero fundamental deviation for internal threads.

In 6g:

  • 6 is again a medium grade.
  • g is a position below the basic size, giving some clearance so the bolt can enter the nut easily.

3. Common metric thread classes in fasteners

Standards for metric screw threads list many combinations, but in everyday fastener sourcing you usually see only a few:

3.1 General-purpose fit: 6H / 6g

  • 6H internal + 6g external is the most common combination for general-purpose fasteners.
  • Used for standard commercial bolts, nuts and machine screws according to ISO 965 and national standards such as AS 1275 for fasteners in Australia and New Zealand. boltbase.com+1
  • It offers a good balance between assembly ease and strength.

3.2 Close fit: 6H / 5g6g or 6H / 4g6g

  • Used when you need better alignment, for example in precision machinery or rotating equipment.
  • The external thread tolerance class 5g6g has a tighter pitch diameter tolerance than 6g, so there is less play between parts. UF MAE+1

3.3 Loose fit and coated threads: 6G / 6g or 6H / 8g

  • Standards like AS 1275-1985 include external classes 6e, 6f, 6g and 8g, and internal classes 6H and 6G for coarse pitch series fasteners from 1.6 to 64 mm diameter. 澳大利亚标准商店+1
  • Higher numbers (e.g. 8g) give more clearance, which is useful for thick coatings, slightly misaligned holes or very long engagement lengths.

3.4 Hot-dip galvanized threads

For hot-dip galvanized bolts, ISO 965-4 specifies special tolerances so that after galvanizing, the external thread can still mate with internal threads tapped to position H or G. 国际标准化组织+1

In practice, galvanized structural bolts are often supplied with a looser external tolerance and matching nuts to allow for the zinc thickness.

4. Why thread class matters for buyers and engineers

Choosing the right thread class is not just a drawing detail. It affects several practical points on your project.

4.1 Assembly speed and risk of cross-threading

  • A very tight fit may feel “nice” in theory, but on site it can slow down installers, especially when alignment is not perfect or there is some dirt on the threads.
  • A general-purpose class like 6H/6g gives enough clearance so bolts can be started by hand and run down quickly with tools.

4.2 Coatings and corrosion protection

  • If you plan to use hot-dip galvanizing or heavy mechanical zinc, the coating adds thickness to the thread flanks and crests.
  • Without extra allowance (for example, a looser external class), the nut may not run freely after coating. ISO 965-4 covers this case for hot-dip galvanized threads. 国际标准化组织+1

4.3 Strength, galling and service life

  • Too much interference or too tight a tolerance can lead to galling, especially with stainless steel fasteners. First Mold
  • A suitable thread class, combined with lubrication and correct installation torque, helps keep clamping force consistent and reduces damage during assembly.

4.4 Interchangeability and multiple suppliers

  • Using internationally recognised thread classes defined in ISO 965 and AS 1275 makes it easier to source equivalent parts from different suppliers or regions. 国际标准化组织+1
  • When your drawings clearly specify M16 × 2 – 6H/6g, a fastener manufacturer can match that requirement even if they have never worked on your project before.

5. Typical thread classes for common applications

5.1 Structural steel and construction

For structural bolting in buildings, bridges and infrastructure, designers often use:

  • Coarse pitch ISO metric threads, e.g. M16, M20, M24.
  • General-purpose thread class 6H/6g, aligned with ISO 965 and local standards such as AS 1275 and Australian structural fastener specifications. codehub.building.govt.nz+1

This combination offers robust performance, good assembly speed and compatibility with many commercial bolt and nut products.

5.2 Machinery, equipment and precision assemblies

In machinery with rotating shafts, bearing housings or precision frames, a closer fit may be needed to control run-out or vibration.

  • Designers sometimes specify 5g6g or another tighter class for external threads, paired with 6H for internal threads. www-eng.lbl.gov+1
  • This is common for high-precision studs, adjustment screws or parts that are repeatedly assembled and disassembled.

5.3 Outdoor, marine and infrastructure with coatings

For coastal and marine environments in Australia, New Zealand and the Pacific region, corrosion protection is critical. Common choices include:

  • Hot-dip galvanized structural bolts with a suitable allowance according to ISO 965-4. 国际标准化组织+1
  • Stainless steel fasteners with standard 6H/6g or slightly looser external threads to reduce the risk of galling.

When you send a fastener enquiry, it is helpful to state not only the material (e.g. carbon steel, stainless A2/A4), but also the expected coating and the required thread class.

6. How thread classes appear on drawings and orders

To avoid misunderstandings between design, purchasing and manufacturing, thread information should appear clearly on technical drawings. Common examples:

  • M10 × 1.5 – 6H – internal thread in a tapped hole or nut.
  • M12 – 6g – external thread such as a bolt or stud.
  • M20 × 2.5 – 6H/6g – complete fit between nut (6H) and bolt (6g). 瑞巴建设+1

For buyers, key points to include on your RFQs and purchase orders are:

  • Thread designation (e.g. M16, M20 × 2.5, fine pitch if needed).
  • Thread class (e.g. 6H, 6g, 6H/6g).
  • Material and strength class (e.g. carbon steel 8.8, 10.9, stainless steel).
  • Surface treatment (zinc plated, mechanical zinc, hot-dip galvanized, etc.).
  • Reference standards (ISO 965, AS 1275, and any project-specific standards).
  • Required quantities, packing type and any small-pack branding requirements.

If you are planning ongoing projects or mixed fastener lists, you are welcome to send your drawings and BOM so a fastener manufacturer can help you check that the thread classes are consistent across all items.

7. Quality control and inspection of thread classes

Correct thread class must be verified during production. In a typical QC process:

  • Go / no-go gauges (plug gauges for nuts and threaded holes, ring gauges for bolts and screws) are used to confirm pitch diameter and functional fit according to the specified thread class. McGill University+1
  • Thread micrometers and optical or vision systems may be used for more detailed measurement of major, minor and pitch diameters.
  • Sampling plans are set based on batch size, application requirements and customer agreements.

When you discuss quality requirements with your fastener supplier, you can ask for:

  • Confirmation of which gauges and reference standards they use for the specified thread classes.
  • Dimensional inspection reports for critical items or first-off samples.
  • Clear marking of boxes and labels with thread size, pitch, class, material and coating, to reduce mix-up risk in your warehouse.

8. Practical tips for buyers and engineers

To make thread classes work for you in real projects, here are some practical suggestions:

  • Always include the thread class on drawings and RFQs. Do not rely on “standard commercial” unless you are sure everyone understands that you mean 6H/6g.
  • Match thread class to coating. If you need heavy coatings, talk to your supplier about suitable external classes or special galvanized threads.
  • Standardise across your project. Where possible, keep one or two thread classes for most items. This simplifies stock management and reduces the chance of mismatched parts.
  • Work with suppliers who understand both standards and applications. They can help you balance cost, ease of production and assembly performance.

If you need metric bolts, nuts, washers or screws with different thread classes in one shipment, a flexible manufacturer can integrate cold forming parts, machined parts and stamped components in one package to support your project list and small-pack requirements.

You can explore typical fastener categories at:

9. Conclusion: turn thread classes into a sourcing advantage

Metric thread classes such as 6H, 6g, 5g6g and 8g are not just abstract codes in ISO 965 and AS 1275. They directly affect how easily your fasteners assemble, how they behave after coating, and how interchangeable they are between different suppliers. By understanding the basics of thread class and specifying them clearly, you reduce risk and make procurement more predictable.

If you are preparing a new project, a stock program or a customised fastener list, you are welcome to share your drawings and requirements for discussion. A fastener manufacturer can recommend suitable thread classes, materials and coatings based on the working conditions of your application.

For more information on metric threads and tolerance systems, you can also refer to:

If you want to discuss a specific fastener, project or drawing, you can contact the team via:

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