How Do You Read A Screw Thread Callout?
You have a loose screw –the threaded fastener sort–and you walk into the hardware store to obtain a replacement. There, you encounter an entire aisle of screws, nuts, washers, and other small hardware. Which one do you need? If you know a little bit about how screw sizes work, the process of finding the right part will be a lot easier. • Many different sizes. Read the numbers. They will look something like one of these: • #4-40 x .5 • 1/4-20 x 5/8 • M3-50 x 10 • The major diameter for the threaded portion of the screw. Interpret the first number. The first number gives the major, or largest, diameter. • In Unified threads (measured in inches) there are numbered diameters #0 through #10, with 0 the smallest and 10 the largest. (Diameters #12 and #14 may also be found, but are usually on older equipment and needed for repairs or restorations. #14 is close to, but not exactly the same as, 1/4-inch.) The major diameter in Unified threads = 0.060″ + 0.013″*(numbered diameter). So #2 has a
