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Metric vs. Imperial Hex Keys: Which to Choose? by Hi-Spec

Metric vs. Imperial Hex Keys: Which to Choose?

When it comes to DIY projects, having the right tools is essential. One such tool that every handy person should have in their toolbox is a set of hex keys. However, with two different measurement systems, metric and imperial, it can be challenging to decide which one to choose. 

This article aims to provide an in-depth comparison between metric and imperial hex keys and help you make the right decision for your needs.

Table of Contents

  1. Hex Key Basics
    1.1. What are Hex Keys?
    1.2. Common Uses of Hex Keys 
  2. Metric and Imperial Measurement Systems
    2.1. History and Origin
    2.2. SAE and Metric Units in DIY 
  3. Metric Hex Keys
    3.1. Overview and Sizes
    3.2. Advantages
    3.3. Disadvantages 
  4. Imperial Hex Keys
    4.1. Overview and Sizes
    4.2. Advantages
    4.3. Disadvantages 
  5. How to Choose Between Metric and Imperial Hex Keys
    5.1. Consider Your Project Needs
    5.2. Evaluate Your Tool Collection
    5.3. Assess Compatibility with Existing Equipment 
  6. Conclusion 

Hex Key Basics

What are Hex Keys?

Hex keys, also known as Allen keys or Allen wrenches, are small L-shaped or T-shaped hand tools used to drive bolts and screws with hexagonal sockets. They come in various sizes, typically measured in inches (imperial) or millimeters (metric). Made of steel, these tools are known for their durability and versatility.

Common Uses of Hex Keys

Hex keys are widely used across various industries and DIY projects. Some common uses include:

  • Assembling furniture: Flat-pack furniture often comes with hex bolts and screws that require hex keys for assembly. 
  • Repairing bicycles: Hex keys are used to adjust and tighten various parts of a bicycle, such as the seat, handlebars, and brakes. 
  • Automotive maintenance: Many car parts, such as engine components and suspension systems, use hex bolts and screws that require hex keys for installation and removal. 

Metric and Imperial Measurement Systems

History and Origin

The imperial system, also known as the British Imperial System or Standard American Engineering (SAE), originated in the United Kingdom and was widely adopted in the United States. It uses units such as inches, feet, and pounds for measurements.

On the other hand, the metric system is an international decimalized system of measurement that uses units like meters, grams, and liters. It was first introduced in France in the late 18th century and has since been adopted by most countries worldwide.

SAE and Metric Units in DIY

In DIY projects, you may encounter both SAE and metric units depending on the equipment and materials you use. For example, some power tools and fasteners use imperial measurements, while others use metric. This can make it necessary to have both metric and imperial hex keys in your toolbox to avoid compatibility issues.

Related Article: Best DIY Hex Wrench Set

Metric Hex Keys

Overview and Sizes

Metric hex keys use the metric system for sizing and are measured in millimeters. They come in various sizes, typically ranging from 1.5 mm to 22 mm. Some common metric hex key sizes include 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm.

Advantages

  • Universality: The metric system is the global standard for measurements, making metric hex keys more widely used and recognized. 
  • Easier size identification: With a decimal-based system, it's easier to identify the correct size of a metric hex key compared to an imperial one. 
  • Greater precision: Metric hex keys offer more precise sizing due to smaller increments between sizes. 

Disadvantages

  • Limited compatibility with SAE equipment: Using metric hex keys on SAE fasteners can cause damage and vice versa. 
  • Less availability in certain regions: In countries like the United States, where the imperial system is prevalent, it may be challenging to find metric hex keys or replacement fasteners. 

Imperial Hex Keys

Overview and Sizes

Imperial hex keys use the imperial system for sizing and are measured in inches. They come in various sizes, typically ranging from 0.028 inches to 3/8 inch. Some common imperial hex key sizes include 1/16 inch, 1/8 inch, 3/16 inch, and 1/4 inch.

Advantages

  • Compatibility with SAE equipment: If you frequently work with SAE fasteners and tools, having imperial hex keys is essential for proper fit and function. 
  • Availability in certain regions: In countries that predominantly use the imperial system, such as the United States, imperial hex keys are more readily available. 

Disadvantages

  • Limited global recognition: The imperial system is not as widely recognized as the metric system, which can make it challenging to find compatible tools and fasteners when traveling or working with international equipment. 
  • Less precision: Due to larger increments between sizes, imperial hex keys may not offer the same level of precision as their metric counterparts. 

How to Choose Between Metric and Imperial Hex Keys

Consider Your Project Needs

Evaluate the types of projects you typically work on and the fasteners they require. If you mostly work with metric fasteners, a metric hex key set would be more suitable. Conversely, if you mainly work with SAE fasteners, an imperial hex key set would be more appropriate.


Evaluate Your Tool Collection

Assess your existing tool collection to determine which measurement system is more prevalent. If most of your tools and equipment use the imperial system, it may be more practical to stick with imperial hex keys. Similarly, if your tools are predominantly metric, opting for metric hex keys would be more beneficial.


Assess Compatibility with Existing Equipment

If you have a mix of metric and imperial tools and equipment, consider investing in both metric and imperial hex key sets to ensure compatibility and avoid damage caused by using the wrong size.


Avoiding Damage from Using Wrongly Sized Hex Keys

Using a hex key that is not the correct size for the fastener can cause damage to both the tool and the fastener. To prevent this, always ensure that you are using the right-sized hex key for the job. Here are some tips to help you avoid using the wrong size:

  • Check the fastener's markings or documentation: Many fasteners have markings that indicate their size and measurement system. Consult any accompanying documentation or look for markings on the fastener itself to determine the correct size.
  • Test the fit before applying force: Before tightening or loosening a fastener, gently insert the hex key into the socket to test the fit. If it feels too loose or too tight, try a different size. The correct-sized hex key should fit snugly without excessive play or resistance.
  • Keep both metric and imperial hex keys on hand: As mentioned earlier, having both metric and imperial hex key sets in your toolbox can help ensure that you always have the correct size for any given fastener.
  • Organize your hex keys: Keep your hex keys organized and clearly labeled to make it easy to find the right size when needed. This can help prevent accidentally grabbing the wrong size and causing damage.

Conclusion

Choosing between metric and imperial hex keys ultimately depends on your specific needs, project requirements, and existing tools.
By carefully considering the advantages and disadvantages of each system and evaluating your toolbox, you can make an informed decision that will serve you well in your DIY endeavors. 

No matter which measurement system you choose, investing in high-quality hex keys will ensure durability and reliability for years to come.

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