How to Identify a Metric Screw Thread Call Out
Whether you’re a part-time hobbyist or a seasoned mechanic, there comes a time when you have an unidentified hole, bolt, or screw. To finish your assembly, you need to know exactly what size fastener you have, or what size fastener you need. The “guess and test” method can damage the threads on the fastener and in the hole. Worse yet, you could end up with something that seems to fit, but isn’t really the right size. When that happens, mechanical failure becomes a real risk.
Fortunately, screw thread callouts are a standardized system for determining the size, thread pitch, and length of a machine screw or hole. With this information, you can quickly get all of the information you need to ensure that your fastener fits perfectly into your assembly.
Major Nominal Diameter
Simply put, this is the widest diameter of the threaded bolt, screw, nut, or hole. On metric bolts, screws, and nuts, this number is stamped on the surface of the fastener, and is preceded by the letter “M.” For example, a bolt with M3 stamped on its head will have a diameter of 3 millimeters. This measurement is taken from the high point, or crest, of one thread to the crest of the corresponding thread on the opposite side of the fastener.
Nuts and machined holes can have the same markings, though they may appear to be smaller than the stated size. Remember, the measurement is from crest to crest and, when looking into a hole or nut, you’re viewing the root, or lowest point of the threads.
This is the distance between crests along the length of the fastener. Metric fasteners are standardized at 1 millimeter of pitch. For this reason, many fasteners drop this second measurement. However, for non-standard fasteners, this number will appear after the diameter measurement. A bolt marked M3 x 1.25 is a 3 millimeter bolt with a thread pitch of 1.25 millimeters. This metric bolt will fit nuts and holes that require a fastener with a 3 millimeter diameter and a distance of 1.25 millimeters between adjacent crests.
The final measurement is the length of the metric fastener. This is the last number to appear in the callout, and generally refers to the length from the bottom of the fastener head to the end of the fastener. On flat-head or countersunk fasteners, the length is measured from the top of the head to the end of the fastener. In practice, the full callout would appear as M3 x 1.25 x 30.
For this measurement, the bolt would be 3 millimeters in diameter, have a non-standard pitch of 1.25 millimeters, and be 30 millimeters in length. A more common measurement would be M3 x 30, which would be a 3 millimeter bolt, with a standard pitch of 1 millimeter, and a length of 30 millimeters.
A Good Start
Understanding these basic callout parameters will help you find the right metric fasteners for your assembly. Of course, there are other, less frequent markings that may come up from time to time. These can affect the fastener hardness, material, coarseness of threads, and other factors. If your assembly calls for standard or non-standard metric fasteners, or if you just have a question about the markings on your fasteners or assembly, drop us a line— we’re happy to help.