Demystifying Fastener Notation: Understanding the Language of Screws, Bolts, and Nuts

Fasteners might seem simple, but their notation can be a complex language for the uninitiated. This article dives into how fasteners are described, covering drive types, head styles, diameter measurement, thread count, and thread pitch. By understanding these aspects, you’ll be able to decipher fastener codes and choose the perfect Caparo fastener for your project.

  1. Drive Type: The Grip for Your Turning Power

The drive type refers to the method used to turn the fastener. Here are some common ones:

  • Slot: A simple notch on the head, suitable for flathead screwdrivers.
  • Phillips: A star-shaped indentation requiring a Phillips screwdriver for better torque and reduced cam-out (slipping).
  • Pozidriv: Similar to Phillips but with additional features for an even stronger driver engagement.
  • Hex (Allen): A hexagonal socket requiring an Allen wrench for high torque applications.
  • Torx: A 6-point star-shaped socket offering high torque and tamper resistance.
  • Square: A square recess for a square drive wrench, commonly used in lag screws and bolts.
  1. Head Styles: Choosing the Right Fit

The head style refers to the shape of the fastener head that provides a bearing surface and allows for turning. Here are some common head styles:

  • Hex (Head): The most common, hexagonal head for use with a wrench or socket.
  • Flat Head: A countersunk head that sits flush with the material surface.
  • Button Head: A rounded head with a lower profile than a hex head.
  • Socket Head (Cap Screw): A cylindrical head with an internal socket for an Allen wrench.
  • Round Head: A dome-shaped head commonly used in machine screws and bolts.
  • Fillister Head: A flat head with a slightly rounded top, often used in decorative applications.
  1. Diameter: Sizing Up the Fastener

The diameter refers to the thickness of the fastener shaft, typically measured in millimeters (mm) or inches (“). For screws and bolts, the diameter is measured across the threads. Shank diameter can also be specified for some fasteners with a body diameter different from the threaded portion.

  1. Thread Count: How Many Threads per Inch (or Millimeter)

Thread count, also known as threads per inch (TPI) in the US or threads per millimeter (mm) for metric fasteners, indicates the number of threads per unit length on the fastener shaft. A higher thread count translates to more threads engaging with the material, offering a stronger and more secure connection.

  1. Thread Pitch: The Distance Between Threads

Thread pitch refers to the distance between corresponding points on adjacent threads. It is measured in millimeters (mm) or inches (“). A coarse thread pitch (larger distance) allows for faster assembly but may not be as strong as a fine thread pitch (smaller distance) with more threads engaging per unit length.

Putting it All Together: Deciphering Fastener Codes

Now that you understand the individual components, let’s look at how they come together in a fastener code. For example, a typical code might look like “M5 x 0.8 x 16.” Here’s how to interpret it:

  • M5: This indicates a metric bolt with a nominal diameter of 5 millimeters.
  • 0.8: This specifies the thread pitch, which in this case is 0.8 millimeters.
  • 16: This represents the length of the fastener in millimeters, measured from the underside of the head to the end of the shaft.

By understanding fastener notation, you’ll be empowered to choose the right Caparo fastener for the job. So, next time you’re at the hardware store, you can confidently navigate the fastener aisle and select the perfect screw, bolt, or nut for your project!