When it comes to bolts, screws, and fasteners, threads play a vital role in how parts are secured together. Choosing the correct thread size is essential for strength, safety, and compatibility. This is where a metric thread chart becomes a must-have reference.
In this guide, we’ll explore what metric threads are, why charts are important, how to read them, and provide you with a detailed breakdown of common metric thread sizes.
What is a Metric Thread?
A metric thread is a type of screw thread standardized under the ISO metric system. Unlike inch-based threads, metric threads are measured in millimeters (mm).
Metric threads are defined by two main elements:
Major Diameter (d): The outside diameter of the thread, in millimeters.
Pitch (P): The distance between adjacent thread peaks, measured in millimeters.
For example:
M10 × 1.5
M10 = 10 mm nominal diameter
1.5 = thread pitch (distance between threads)
If no pitch is specified, the standard coarse pitch is assumed.
Why Use a Metric Thread Chart?
Even experienced engineers and mechanics rely on metric thread charts for quick reference. Here’s why they matter:
Accuracy: Helps in selecting the exact thread needed.
Compatibility: Prevents mismatched nuts and bolts.
Efficiency: Saves time in industrial and DIY applications.
Standardization: Follows ISO norms, ensuring global usability.
A chart removes the guesswork and ensures fastener strength and reliability.
Thread Terminology You Should Know
Before diving into the chart, here are a few important terms:
Pitch: Distance between two thread crests.
Major Diameter: Outer diameter of the thread.
Minor Diameter: Smallest diameter of the thread (valley).
Pitch Diameter: Midway between the major and minor diameters.
Thread Angle: The angle formed between two flanks of the thread (ISO metric = 60°).
Coarse Thread: Standard pitch for general use.
Fine Thread: Smaller pitch, stronger grip, used in high-stress applications.
Metric Thread Standards
The ISO metric thread system is divided into:
Coarse Pitch Series (default): Widely used, strong, less likely to strip.
Fine Pitch Series: More threads per length, stronger hold, used in precision machinery and automotive.
Metric Thread Chart (Coarse Pitch)
Here’s a simplified version of the standard ISO metric coarse thread chart:
| Thread Size | Pitch (mm) | Major Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| M1 | 0.25 | 1.0 | 0.75 | 0.75 |
| M2 | 0.4 | 2.0 | 1.6 | 1.6 |
| M3 | 0.5 | 3.0 | 2.5 | 2.5 |
| M4 | 0.7 | 4.0 | 3.3 | 3.3 |
| M5 | 0.8 | 5.0 | 4.2 | 4.2 |
| M6 | 1.0 | 6.0 | 5.0 | 5.0 |
| M8 | 1.25 | 8.0 | 6.8 | 6.8 |
| M10 | 1.5 | 10.0 | 8.5 | 8.5 |
| M12 | 1.75 | 12.0 | 10.2 | 10.2 |
| M16 | 2.0 | 16.0 | 14.0 | 14.0 |
| M20 | 2.5 | 20.0 | 17.5 | 17.5 |
| M24 | 3.0 | 24.0 | 21.0 | 21.0 |
| M30 | 3.5 | 30.0 | 26.5 | 26.5 |
This chart is often used when selecting bolts, screws, or taps for threading holes.
Metric Fine Thread Chart
Fine threads are used when higher strength and precision are required.
| Thread Size | Fine Pitch (mm) | Major Diameter (mm) | Minor Diameter (mm) |
|---|---|---|---|
| M6 | 0.75 | 6.0 | 5.2 |
| M8 | 1.0 | 8.0 | 7.0 |
| M10 | 1.25 | 10.0 | 8.8 |
| M12 | 1.5 | 12.0 | 10.5 |
| M16 | 1.5 / 2.0 | 16.0 | 14.0 |
| M20 | 1.5 / 2.0 | 20.0 | 18.0 |
| M24 | 2.0 | 24.0 | 22.0 |
Fine pitch threads provide more resistance to loosening due to vibration, which is why they’re common in automotive and aerospace applications.
How to Read Metric Thread Sizes
Let’s break down an example:
M12 × 1.75
M12 = Major diameter (12 mm)
1.75 = Pitch (distance between threads in mm)
If it says simply M12, it means M12 × 1.75 (coarse), because coarse pitch is the default.
For fine threads, the pitch must always be specified:
M12 × 1.25 (fine thread, smaller spacing).
Metric vs. Imperial Threads
Many people confuse metric with imperial (inch-based) threads. Here are key differences:
Metric Threads: Measured in millimeters, pitch given in mm.
Imperial (Unified): Measured in inches, pitch given in TPI (threads per inch).
Thread Angle: Both use 60°, but sizing systems are different.
For example:
Metric: M8 × 1.25 (8 mm diameter, 1.25 mm pitch).
Imperial: 5/16″-18 UNC (5/16 inch diameter, 18 threads per inch).
Mixing the two is not recommended, as even small mismatches can damage threads.
Applications of Common Metric Threads
M1 – M3: Electronics, precision instruments.
M4 – M6: Appliances, furniture, bicycles.
M8 – M12: Automotive, construction, general machinery.
M14 – M24: Heavy-duty automotive, structural steelwork.
M30+: Industrial machinery, bridges, shipbuilding.
Metric Thread Tolerance & Fit
Threads also have tolerance classes, which define how tight or loose a screw fits into a nut.
6H/6g: Standard tolerance class (nut = 6H, bolt = 6g).
4H/4g: Tighter fit, used in precision engineering.
8H/8g: Looser fit, easier assembly for general applications.
These tolerances ensure the right balance between strength and ease of assembly.
Tips for Using Metric Thread Charts
Always check pitch: Don’t assume – M10 can be 1.5 mm (coarse) or 1.25 mm (fine).
Use proper gauges: A thread pitch gauge helps confirm sizes.
Avoid mixing systems: Stick with metric or imperial – don’t interchange.
Match nuts and bolts: Use the same thread class and pitch.
Consider strength grade: Thread size is important, but so is bolt strength (8.8, 10.9, 12.9).
Printable Metric Thread Chart
Many workshops keep a laminated metric thread chart for quick reference. This is especially useful in machine shops, automotive garages, and manufacturing facilities.
If you often work with fasteners, keeping a printable chart or digital PDF on hand saves time and prevents errors.
Practical Tips for Working with Metric Threads
Understanding metric threads is only part of the process. Applying this knowledge in real-world scenarios ensures your projects are both safe and efficient. Here are some practical tips for working with metric threads:
1. Use Thread Gauges: Thread pitch gauges are inexpensive tools that quickly verify whether a bolt or nut matches the required pitch. This prevents mistakes and ensures proper fit.
2. Keep Bolts and Nuts Organized: Many workshops organize fasteners by size, pitch, and type. Labeling storage bins with metric sizes such as M6, M8, and M10 reduces errors and saves time when selecting components.
3. Clean Threads Before Use: Dirt, rust, or debris in threads can compromise strength and make assembly difficult. Always clean threaded holes and bolts before installation.
4. Apply Lubrication When Needed: Certain applications, such as automotive or machinery, may benefit from light lubrication to reduce friction during tightening. This also prevents thread galling on stainless steel fasteners.
5. Follow Torque Specifications: Using a torque wrench ensures bolts are tightened to the manufacturer’s recommended levels. Over-tightening can strip threads or break bolts, while under-tightening can lead to loose connections.
6. Consider Thread Type for Specific Applications: Coarse threads are ideal for quick assembly and durability, whereas fine threads excel in precision applications where vibration resistance and tight tolerances are needed.
7. Know Your Materials: Thread strength depends not only on size and pitch but also on the material of the fastener. High-grade steel bolts will perform differently than aluminum or brass fasteners, so select the appropriate material for the load requirements.
By combining your understanding of metric thread charts with these practical tips, you can achieve more reliable and efficient assemblies, whether you’re working on electronics, furniture, or heavy machinery.
Final Thoughts
A metric thread chart is more than just a reference – it’s an essential tool for anyone who works with screws, bolts, nuts, and threaded holes. By understanding how to read thread sizes, pitches, and tolerances, you can avoid mismatched parts and ensure safety, durability, and performance in every project.
From small electronics to heavy machinery, metric threads are everywhere. The key is knowing how to read the chart and choose the right size.
So, the next time you’re working on a project, keep a metric thread chart close by – it will save you time, effort, and frustration.
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