Standard Thread Pitch vs. Metric Thread Pitch: The Explanation
Whenever you have fasteners that are secured by threading, you are going to have an extra measurement (the thread pitch) tacked behind the diameter measurement. Examples are ¼-20, ½-13, ½-20 for standard fasteners; and M6-1.0, M8-1.25 and M8-1.0 for metric fasteners.
Thread pitch is a measurement denoting the amount of threads per an area. For example, if you have the measurement of ½-13, this means the diameter of the nut or bolt is ½ inch and that there are 13 threads per inch. This would be a coarse thread. If you have the measurement of ½-20 it would mean the same diameter of fastener, but with 20 threads per inch. This would be a fine thread.
Metric thread pitch measurements are read differently than standard ones. While for standard thread pitch measurements the lower number is coarse thread and the higher number refers to fine thread, in metric this is opposite. M10-1.50 is a coarse thread denotation while M10-1.25 is a fine pitch. The reason is that in metric threaded fasteners the pitch is measured by the distance between the threads. So in the coarse threaded M10-1.50 fastener, there are 1.50 mm between the threads and 1.25 mm between the threads on the fine one.
I will wrap up this super basic primer on thread pitch by adding that you should never assume the vendor will know what thread pitch you want. Be sure to specify this very important dimension! One of the most common fastener ordering mishaps is ordering the wrong thread pitch.
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