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Allen bolts, also known as Socket Head Cap Screws, are among the most widely used industrial fasteners in machinery, automotive, construction, and engineering applications. Choosing the correct Allen bolt size is crucial for ensuring strength, safety, and long-term performance.
In this comprehensive guide, we provide an updated Allen Bolt Size Chart for Metric, UNC, and UNF standards, along with practical information to help engineers, procurement professionals, and manufacturers select the right bolt for their applications.
What Are Standard Allen Bolt Sizes?
Standard Allen bolts are available in:
Metric Sizes
M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30, M36
UNC Sizes
#4-40, #6-32, #8-32, #10-24, 1/4″-20, 5/16″-18, 3/8″-16, 1/2″-13
UNF Sizes
#4-48, #6-40, #8-36, #10-32, 1/4″-28, 5/16″-24, 3/8″-24, 1/2″-20
These sizes are commonly used in industrial machinery, automotive assemblies, heavy equipment, and precision engineering applications.
What is an Allen Bolt?
An Allen Bolt is a fastener with a cylindrical head and an internal hexagonal drive that is tightened using an Allen key or hex wrench.
Key Advantages
- High tightening torque
- Compact head design
- Better grip compared to conventional bolts
- Suitable for limited-space installations
- Excellent strength-to-size ratio
Because of these advantages, Allen bolts are extensively used in manufacturing industries worldwide.
Metric Allen Bolt Size Chart
| Thread Size | Thread Diameter (mm) | Standard Allen Key Size |
| M3 | 3 mm | 2.5 mm |
| M4 | 4 mm | 3 mm |
| M5 | 5 mm | 4 mm |
| M6 | 6 mm | 5 mm |
| M8 | 8 mm | 6 mm |
| M10 | 10 mm | 8 mm |
| M12 | 12 mm | 10 mm |
| M16 | 16 mm | 14 mm |
| M20 | 20 mm | 17 mm |
| M24 | 24 mm | 19 mm |
| M30 | 30 mm | 22 mm |
| M36 | 36 mm | 27 mm |
Most Popular Metric Sizes
The most frequently used Allen bolts in industrial applications are:
- M6 Allen Bolt
- M8 Allen Bolt
- M10 Allen Bolt
- M12 Allen Bolt
- M16 Allen Bolt
These sizes provide an ideal balance between strength and installation convenience
UNC Allen Bolt Size Chart
UNC stands for Unified National Coarse Thread.
These bolts are widely used in North American manufacturing industries and heavy equipment.
UNC Size | Threads Per Inch (TPI) |
#4-40 | 40 |
#6-32 | 32 |
#8-32 | 32 |
#10-24 | 24 |
1/4″-20 | 20 |
5/16″-18 | 18 |
3/8″-16 | 16 |
1/2″-13 | 13 |
UNF Allen Bolt Size Chart
UNF stands for Unified National Fine Thread.
UNF threads offer better clamping force and vibration resistance compared to coarse threads.
UNF Size | Threads Per Inch (TPI) |
#4-48 | 48 |
#6-40 | 40 |
#8-36 | 36 |
#10-32 | 32 |
1/4″-28 | 28 |
5/16″-24 | 24 |
3/8″-24 | 24 |
1/2″-20 | 20 |
Where UNC Bolts Are Used
- Agricultural machinery
- Heavy industrial equipment
- Construction machinery
- Structural assemblies
Allen Bolt Length Chart
Allen bolts are manufactured in various lengths depending on the application.
Common lengths include:
- 10 mm
- 12 mm
- 16 mm
- 20 mm
- 25 mm
- 30 mm
- 40 mm
- 50 mm
- 60 mm
- 80 mm
- 100 mm
- 150 mm
- 200 mm
Selecting the correct length ensures proper thread engagement and load distribution.
How to Measure an Allen Bolt?
To accurately identify an Allen bolt size:
Step 1
Measure the thread diameter.
Step 2
Measure the bolt length from under the head to the end.
Step 3
Identify the thread type:
- Metric
- UNC
- UNF
Step 4
Measure the hex socket size.
Following these steps helps avoid installation errors and ensures compatibility.
Allen Bolt Grades Explained
Allen bolts are available in various strength grades.
Grade | Tensile Strength |
8.8 | Medium Strength |
10.9 | High Strength |
12.9 | Extra High Strength |
Grade 8.8 Allen Bolts
Suitable for general industrial use.
Grade 10.9 Allen Bolts
Ideal for automotive and heavy engineering applications.
Grade 12.9 Allen Bolts
Used in critical high-load applications requiring maximum strength.
How to Choose the Right Allen Bolt?
When selecting an Allen bolt, consider:
Load Requirement
Higher loads require larger diameters and stronger grades.
Environmental Conditions
Use corrosion-resistant finishes for outdoor or humid environments.
Thread Standard
Choose Metric, UNC, or UNF according to project specifications.
What is the most commonly used Allen bolt size?
M6, M8, M10, and M12 are the most commonly used Allen bolt sizes in industrial applications.
What Allen key fits an M8 Allen bolt?
An M8 Allen bolt typically requires a 6 mm Allen key.
Which is stronger: Grade 8.8 or Grade 10.9 Allen bolts?
Grade 10.9 Allen bolts offer higher tensile strength and are stronger than Grade 8.8 bolts.
What is the difference between UNC and UNF Allen bolts?
UNC uses coarse threads, while UNF uses finer threads that provide better clamping force and vibration resistance.
