When it comes to mechanical assemblies, furniture fittings, machinery, or even DIY projects, Allen bolts (also called hex socket head cap screws) are one of the most reliable fasteners. Unlike standard bolts tightened with a wrench, Allen bolts are driven with a hex key or Allen wrench, allowing for a clean, flush fit and high torque capacity.
If you’ve ever been confused by the different Allen bolt sizes, diameters, and thread types, you’re not alone. Many engineers, mechanics, and DIY enthusiasts often struggle to pick the correct size for their application. That’s where a comprehensive Allen bolt size chart comes in handy.
This guide will walk you through everything you need to know: Allen bolt dimensions, standards, applications, size conversion, and how to use an Allen bolt size chart effectively.
What Are Allen Bolts?
Allen bolts are fasteners with a cylindrical head and a hexagonal recess that requires an Allen wrench (hex key) for tightening. They are commonly used where:
Space is limited, and a wrench cannot fit.
A flush or recessed installation is needed.
Extra torque is required compared to standard screws.
A neat, professional look is important.
They are popular in:
Automotive assemblies
Furniture (like IKEA products)
Bicycles and motorcycles
Industrial machinery
Electronics
Why an Allen Bolt Size Chart Matters
Choosing the correct bolt size isn’t just about convenience — it’s about safety, durability, and performance.
Using the wrong size can strip the hex socket.
Incorrect length can either cause weak fastening or damage the material.
Mismatched threads may lead to loosening or breakage.
That’s why Allen bolt size charts are essential references. They help match the diameter, length, thread pitch, and wrench size to your specific need.
Allen Bolt Size Standards
Allen bolts are manufactured according to international standards, ensuring interchangeability across industries. The most common standards include:
ISO (International Organization for Standardization) – ISO 4762 covers hex socket head cap screws.
DIN (German Standard) – DIN 912 is a widely used metric standard.
ANSI/ASME (American Standard) – Covers inch-based Allen bolts.
This means Allen bolts can be found in both metric (mm) and imperial (inch) units.
Allen Bolt Size Chart (Metric)
Here’s a quick reference for standard metric Allen bolts (DIN 912 / ISO 4762):
| Nominal Diameter (d) | Thread Pitch (mm) | Head Diameter (mm) | Head Height (mm) | Hex Key Size (mm) |
|---|---|---|---|---|
| M3 | 0.5 | 5.5 – 6 | 3 | 2.5 |
| M4 | 0.7 | 7 – 8 | 4 | 3 |
| M5 | 0.8 | 8.5 – 9 | 5 | 4 |
| M6 | 1.0 | 10 – 11 | 6 | 5 |
| M8 | 1.25 | 13 – 14 | 8 | 6 |
| M10 | 1.5 | 16 – 17 | 10 | 8 |
| M12 | 1.75 | 18 – 20 | 12 | 10 |
| M16 | 2.0 | 24 – 26 | 16 | 14 |
| M20 | 2.5 | 30 – 32 | 20 | 17 |
💡 Example: An M8 Allen bolt means:
8 mm nominal diameter
1.25 mm thread pitch
Requires a 6 mm hex key
Allen Bolt Size Chart (Imperial / Inch)
For projects in the U.S. and some industries, inch-based Allen bolts are common.
| Nominal Size (inch) | Threads per Inch (TPI) | Head Diameter (inch) | Head Height (inch) | Hex Key Size (inch) |
|---|---|---|---|---|
| #4-40 | 40 | 0.22 | 0.09 | 1/16 |
| #6-32 | 32 | 0.28 | 0.11 | 5/64 |
| #8-32 | 32 | 0.34 | 0.13 | 9/64 |
| 1/4″-20 | 20 | 0.43 | 0.19 | 3/16 |
| 5/16″-18 | 18 | 0.52 | 0.24 | 1/4 |
| 3/8″-16 | 16 | 0.62 | 0.29 | 5/16 |
| 1/2″-13 | 13 | 0.83 | 0.41 | 3/8 |
💡 Example: A 1/4″-20 Allen bolt means:
1/4 inch diameter
20 threads per inch
Requires a 3/16 inch Allen wrench
Allen Bolt Length Selection
When selecting length:
Too short → weak grip, may fail under load.
Too long → may damage internal parts or protrude unnecessarily.
Rule of Thumb:
For steel parts, thread engagement should be at least 1 x bolt diameter.
For aluminum or softer metals, aim for 1.5 x diameter.
How to Read an Allen Bolt Size
An Allen bolt size usually looks like this:
Metric: M8 × 30 – 8.8
M8 = 8 mm diameter
30 = 30 mm length
8.8 = strength grade
Imperial: 1/4″-20 × 1.5″
1/4″ = diameter
20 = threads per inch
1.5″ = length
Common Applications of Allen Bolts
Furniture: Flat-pack furniture often uses Allen bolts for easy assembly.
Bicycles & Motorcycles: Handlebar clamps, brake rotors, and engine covers.
Automotive: Engine assemblies, chassis parts, and aftermarket accessories.
Industrial Machines: Securing precision components.
Electronics: Securing PC cases, mounts, and brackets.
Tips for Using Allen Bolts
Always use the correct hex key size (too small = rounding, too big = won’t fit).
Apply anti-seize or thread locker depending on application.
Use high-strength bolts (grade 12.9) for heavy loads.
Replace damaged bolts immediately — stripped sockets are unsafe.
Allen Bolt Size Conversion (Quick Reference)
Sometimes you need to convert metric ↔ imperial:
| Metric Bolt | Closest Imperial Equivalent |
|---|---|
| M3 | #4 (0.112″) |
| M4 | #8 (0.164″) |
| M5 | 3/16″ (0.190″) |
| M6 | 1/4″ (0.250″) |
| M8 | 5/16″ (0.312″) |
| M10 | 3/8″ (0.375″) |
Additional Tips and Considerations for Allen Bolts
1. Material Matters
Allen bolts come in different materials such as stainless steel, carbon steel, and alloy steel. Stainless steel bolts resist corrosion, making them ideal for outdoor or humid environments. Carbon steel provides higher strength but may require coatings to prevent rust. Choosing the right material ensures durability for your application.
2. Coatings and Surface Treatments
Many Allen bolts are coated with zinc, black oxide, or other finishes to improve corrosion resistance. Black oxide bolts offer mild protection and a sleek appearance, while zinc plating provides better protection in harsh environments. Consider the working environment before selecting a coating.
3. Torque Considerations
Allen bolts rely on proper torque for optimal performance. Over-tightening can strip the hex socket, while under-tightening can lead to loosening. Always follow manufacturer torque specifications, especially in automotive or machinery assemblies.
4. Head Type Selection
Though most Allen bolts have a standard hex socket head, other types exist, like flat countersunk heads. Countersunk heads allow a flush finish on the surface, which is ideal for furniture or precision machinery. Matching the head type to your application improves aesthetics and load distribution.
5. Thread Engagement
Proper thread engagement is critical. For steel parts, aim for at least 1× the bolt diameter; for softer metals like aluminum, 1.5× the diameter is recommended. Correct engagement ensures maximum strength and prevents stripping or loosening.
6. Compatibility with Nuts and Washers
Even with the correct bolt size, pairing it with a matching nut and washer ensures secure fastening. Washers help distribute load and prevent surface damage. Always consult the size chart to match components accurately.
7. Environment and Load Considerations
Take into account exposure to vibration, high temperatures, or chemicals. Using bolts suited for the specific conditions enhances performance and longevity, reducing maintenance needs.
Final Thoughts
Allen bolts are small but mighty components that play a huge role in engineering, furniture, automotive, and countless other industries. Having a clear Allen bolt size chart makes it much easier to choose the right fastener for the job, preventing costly mistakes and ensuring strong, durable assemblies.
Whether you’re working with metric M-series bolts or imperial inch-based bolts, always double-check the diameter, length, and hex key size before installation. A properly selected Allen bolt means safety, reliability, and a professional finish.
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