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Difference between Imperial and metric bolts

    Metric bolts and Imperial bolts differ in several aspects, including their units of measurement, sizing standards,and applications.Here’s a detailed breakdown of the key differences between metric and imperial bolts.

    Imperial Bolt
    Image B

    Image D – 1/2 inch bolt

    Imperial Bolts (Image B)

    Imperial bolts, in the context of fasteners and hardware, refer to bolts that are sized and measured using the Imperial system of units. The Imperial system, also known as the British Imperial System or the U.S. customary system, employs inches and fractions of an inch for measurements.

    Key characteristics of Imperial bolts include:

    • Diameter: The diameter of an Imperial bolt is measured in inches. Some of the common diameters include 1/4”, 3/8“,1/2 ”, and so on. (“ represents inches)
    • Threads per Inch (TPI): Instead of pitch, Imperial bolts are described by the number of threads per inch. This indicates how many threads are there in a one-inch length along the bolt. For example, a bolt with 13 TPI means there are 13 threads in one inch. (Image D)
    • Length: The length of an Imperial bolt is measured from the bottom of the head to the end of the bolt and is typically expressed in inches.

    Imperial bolts may also have different grades, i.e. Grade 2, Grade 5, or Grade 8, indicating different levels of strength. The grading system is often marked on the head of the bolt.

    1. Measured in inches for diameter and length. The number of threads in one inch is expressed in terms of threads per inch, or TIP.
    2. Numbers are used such as Grade 2, Grade 5, or Grade 8, indicating different levels of tensile strength of the bolt.
    3. Conforms to various standards depending on the region, such as ANSI (American National Standards Institute) or SAE (Society of Automotive Engineers) standards.
    4. It is commonly used in the United States and a few other countries following the imperial system. Found in many industries, including manufacturing, automobile, and construction.
    5. Similarly, converting from imperial to metric bolts requires attention to the different units and grading systems.
    6. Requires imperial tools for installation and removal.
    7. May vary regionally, leading to differences in specifications.

    Metric Bolts (Image A)

    Metric bolts are fasteners that adhere to the metric system of measurement, which is based on the International System of Units (SI). These bolts are commonly used in many countries worldwide, especially in Europe and Asia, where the metric system is the standard for measurements. Metric bolts are characterized by their dimensions, which are specified in millimetres. The three dimensions typically specified for metric bolts are diameter, pitch, and length.

    Metric Bolt
    Image A

    Image C – M12 Bolt

    • Diameter: The diameter of a metric bolt is measured in millimetres and refers to the width of the bolt shaft, excluding the threads. Common metric bolt diameters include 6mm, 8mm, 10mm, and so on. The diameter is a crucial parameter as it determines the size of the hole the bolt will fit into.

    • Pitch: Instead of threads per inch (TPI) as in the imperial system, metric bolts use pitch to describe the distance between adjacent threads. The pitch is measured in millimetres, and it represents the axial distance the bolt advances with one complete turn. For instance, a bolt having a 1.0 mm pitch would advance 1 mm axially for each full turn.

    • Length: The length of the bolt is measured from the bottom of the head to the end of the bolt and is also expressed in millimetres. The length is an essential consideration to ensure that the bolt is of adequate size for the specific application. Apart from these dimensions, metric bolts may also be classified by their grade of strength. The strength/ grade is typically indicated by a numerical marking on the bolt head, such as 8.8, 10.9, or 12.9. These numbers represent the tensile strength of the bolt in megapascals (MPa). Higher the number the stronger the bolt.

    Here’s a brief overview of some common metric bolt grades:

    • Grade 4.6: Low to medium carbon steel; suitable for general applications.
    • Grade 8.8: Medium carbon steel; provides good strength and is commonly used in automotive applications.
    • Grade 10.9: High-strength steel; used in applications requiring higher tensile strength.
    • Grade 12.9: Alloy steel; offers the highest tensile strength among metric bolt grades
    1. Measured in millimetres (mm) for diameter, pitch, and length.
    2. Thread pitch is used to specify the distance between threads in millimetres.
    3. Graded using a numerical system (e.g., 8.8, 10.9, 12.9) indicating the bolt’s tensile strength in megapascals (MPa).
    4. Conform to metric standards like ISO (International Organization for Standardization) standards.
    5. Widely used in countries where the metric system is standard, such as Europe and Asia. Widespread in mechanical, construction, electrical and automotive applications.     
    6. More widely used globally, especially in regions that follow the metric system.
    7. Require metric tools for installation and removal.
    8. Converting between metric and imperial bolts may require careful consideration due to the differences in measurement units and grading systems.
    9. It’s part of a globally standardised system (ISO standards) that facilitates consistency in specifications.

    Metric bolts find application in various industries, including automotive, construction, machinery, and electronics. They are used to join components together securely and are a fundamental element in the assembly and construction of structures, machines, and equipment. 

    It’s important to understand these differences for selecting the right bolts for a given application and to avoid compatibility issues. Also, it’s crucial to use the appropriate tools and follow the relevant standards when working with metric or imperial bolts to ensure proper fit, functionality, and safety.

    It’s important to keep in mind that different parts of the world use different measurement systems – some use the imperial system while others use the metric system. Before deciding which type of bolts to use, it’s important to know who you’re doing business with and what their preferred system is. It’s also important to consider future maintenance needs, not just the present.

    This blog post should have clarified some of the finer differences between a ‘Metric bolt’ and an ‘Imperial bolt’. If you are looking to start a career in electrical, we at AUSINET – a Nationally Recognised Training Organization offer specialised electrical training for beginners with courses such as Certificate 2 and pathways to become an electrician through the Certificate 3 in Electrotechnology Electrician . Also read our blog on ‘How to find an electrical apprenticeship’ for further information.