---
Understanding Bolt Head Sizes in Millimeters
Bolt heads come in various shapes and sizes, designed to accommodate different tools and applications. The size of the bolt head in mm typically refers to the diameter across flats (the distance between two parallel sides of the hexagon), the head height, and the overall dimensions. In metric standards, bolt head sizes are specified according to the bolt's diameter, thread pitch, and the head type.
Common Types of Bolt Heads
Different bolt head types serve specific purposes and are chosen based on the application. The most common bolt head styles include:
Hex Head
- The most widely used bolt head style.
- Designed with six sides for easy wrench or socket fitting.
- Suitable for general fastening applications.
Round Head
- Features a rounded top surface.
- Often used for aesthetic purposes or where a smooth appearance is desired.
Socket Head (Allen head)
- Cylindrical head with a hexagonal recess.
- Used where space is limited or for a streamlined look.
Carriage Bolt Head
- Rounded top with a square neck underneath.
- Common in wood applications.
Phillips, Pozidriv, and Other Drive Types
- Refers to the drive method rather than head shape.
- Can be combined with various head shapes.
---
Standard Bolt Head Sizes in mm
Bolt head sizes are standardized based on the bolt diameter and thread pitch. Below is a typical bolt head size chart in mm for hex head bolts, which are the most common.
Hex Bolt Head Size Chart in mm
| Bolt Diameter (d) | Across Flats (AF) | Head Height (H) | Head Width (W) | Notes |
|-------------------|-------------------|----------------|--------------|--------|
| M3 | 5.5 mm | 3.0 mm | 5.0 mm | Standard small bolt |
| M4 | 7.0 mm | 4.0 mm | 6.5 mm | Common in electronics and light assemblies |
| M5 | 8.0 mm | 5.0 mm | 8.0 mm | General purpose bolt |
| M6 | 10.0 mm | 6.0 mm | 10.0 mm | Heavy-duty applications |
| M8 | 13.0 mm | 7.0 mm | 13.0 mm | Structural and machinery use |
| M10 | 17.0 mm | 8.0 mm | 17.0 mm | High-stress applications |
| M12 | 19.0 mm | 10.0 mm | 19.0 mm | Large machinery and construction |
| M16 | 24.0 mm | 12.0 mm | 24.0 mm | Heavy equipment |
Note: The above sizes are approximate and can vary slightly depending on manufacturer specifications.
---
How to Read a Bolt Head Size Chart in mm
Understanding how to interpret the chart is crucial for selecting the correct bolt:
1. Bolt Diameter (d): The nominal diameter of the threaded part in millimeters.
2. Across Flats (AF): The distance between two parallel sides of the hexagon, indicating the size of the wrench or socket needed.
3. Head Height (H): The vertical dimension of the bolt head.
4. Head Width (W): The width across the flats, which should match the wrench or socket size.
Knowing these dimensions helps ensure compatibility with tools and components.
---
Factors Affecting Bolt Head Size Selection
Choosing the correct bolt head size involves considering several factors:
1. Application Type
- Structural vs. decorative uses.
- Heavy machinery vs. electronics.
2. Tool Compatibility
- Availability of wrenches or sockets.
- Preference for manual or power tools.
3. Material and Strength
- High-strength bolts may have specific head dimensions to accommodate torque requirements.
4. Space Constraints
- Limited access areas may require shorter head heights or socket head bolts.
5. Aesthetic Considerations
- Rounded or flush heads for visual appeal.
---
Common Bolt Head Sizes in Different Standards
Bolt sizes can vary depending on regional standards and standards organizations:
Metric (ISO)
- The most widely used standard globally.
- Sizes are specified in millimeters.
Imperial (ANSI/ASME, BS)
- Sizes are in inches but often converted to mm for compatibility.
DIN Standards
- German standards that specify precise dimensions for various bolt types.
---
Practical Tips for Using Bolt Head Size Charts
- Always measure existing bolts before replacing them.
- Use calipers for accurate measurement of bolt head dimensions.
- Check the manufacturer's specifications for specialized bolts.
- Keep a reference chart handy for quick selection.
---
Summary of Key Points
- Bolt head sizes in mm vary based on bolt diameter and head type.
- Hex head bolts are most common, with standardized across flats measurements.
- Proper selection ensures tool compatibility and secure fastening.
- Consider application, tool access, space, and aesthetics when choosing bolt head sizes.
- Always verify dimensions with precise measurements before procurement.
---
Conclusion
Having a thorough understanding of the bolt head size chart in mm is vital for any project involving fastening hardware. Whether you're working on machinery, furniture, automotive repairs, or construction, selecting the correct bolt head size ensures safety, durability, and ease of assembly. By familiarizing yourself with standard sizes, interpreting measurement charts accurately, and considering application-specific factors, you can make informed choices that enhance the quality and longevity of your work.
---
Remember: Always double-check dimensions and standards before ordering or replacing bolts to avoid mismatches and potential failure. Using the right bolt head size in mm not only improves the efficiency of your work but also guarantees the safety and reliability of your projects.
Frequently Asked Questions
What is a bolt head size chart in millimeters used for?
A bolt head size chart in millimeters is used to determine the appropriate head dimensions of various bolt sizes, ensuring compatibility with tools and fittings for assembly and maintenance.
How do I read a bolt head size chart in mm?
You read a bolt head size chart in mm by locating the bolt diameter or thread size and then referencing the corresponding head diameter, height, and shape specifications provided in the chart.
Why is it important to know the bolt head size in millimeters?
Knowing the bolt head size in millimeters is important for selecting the correct wrench or socket size, ensuring proper fit, and preventing damage during installation or removal.
Are bolt head sizes standardized in millimeters across different standards?
Yes, bolt head sizes are generally standardized according to ISO, DIN, and other international standards, making millimeter measurements consistent across manufacturers and applications.
What is the typical bolt head size for M10 and M12 bolts in mm?
For M10 bolts, the common head size is around 17 mm across flats, and for M12 bolts, it is typically about 19 mm across flats, but exact sizes can vary slightly depending on the head type.
Can a bolt head size chart help me find the right tools for installation?
Yes, a bolt head size chart provides the necessary dimensions to select the correct wrench or socket size, ensuring proper fitting and reducing the risk of stripping or damaging the bolt.
Where can I find a reliable bolt head size chart in millimeters online?
Reliable bolt head size charts in millimeters can be found on hardware supplier websites, engineering resource platforms, or standards organizations like ISO and DIN documentation.
