Fastener Glossary: Screw Thread Terminology Explained

Human beings have been using screws since 200 B.C. Considering the fact that we have built multiple civilizations off of their use, our fastener options have expanded; now, you can get your hands on custom shoulder bolts, micro fasteners, and custom screws in a variety of sizes (from miniature screws to massive fasteners). Although the concept has remained the same, the terminology has advanced throughout the years; since knowing how to read screw thread descriptions ensures that you end up with the right size for your project or product, it’s vital that you stay up-to-date on this terminology. Let’s take a look at six basic terms you may come across when researching screw threads.

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1. Major diameter: The major diameter of an external thread (like the edges of a screw) refers to the diameter of the overall shaft. This includes the height of the raised helix, and can be measured using a caliper or slot gauge; simply determine the distance between crests and you’ll have the major diameter.
2. Minor diameter: The minor diameter measures the diameter of the “root,” or innermost part of the screw. This does not include the crests of the helix. Due to accuracy issues, you’ll need to rely on specialized equipment to take this measurement.
3. Effective diameter: This term refers to the halfway point between the major and minor diameters, or halfway up the helix crest. For the same accuracy reasons, this measurement requires the use of specialized equipment.
4. Pitch: A screw thread’s pitch refers to the distance between two identical threads along the helix.
5. Flank: A screw’s helix wraps the thread around the minor diameter. The flank references the angle at which the helix is raised to form a crest on the thread.
6. Crest: The crest refers to both the height at which an external thread is raised and the depth at which an internal thread is indented (as they’re the same thing). For most common applications, screws and bolts are measured at the crests; nuts are measured at the roots.

Whether you’re looking for custom shoulder bolts or tiny screws, you’ll need to know the lingo in order to be sure you’ve purchased the right product. By understanding what the above words reference in relation to screw threads, you can rest assured you’ve made the right choice.