Metric and English screw threads are two different systems of standardisation for screws and bolts. Both systems characterise screws by two main numbers: the screw's major diameter and the frequency of the threads, or turns. The major difference, other than units, is that the metric system measures the distance or "pitch" in millimetres from thread to thread, or equivalently, how far the screw or bolt advances after one complete turn. The English system instead measures the number of threads per inch.
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Convert the diameter from millimetres to inches by dividing it by 25.4. For the example of a 2-millimeter diameter "M 2" screw, divide 2 by 25.4 to yield a 0.0787-inch diameter.
Convert pitch to threads per inch by dividing 25.4 by the pitch value. For a pitch of 0.5mm, divide 25.4 by 0.5 to yield a threads-per-inch (tpi) count of 50.8.
Recognise that metric and English threads, however close sometimes, are not perfectly interchangeable. Still, in the field of screw-on photographic filters and lenses, these are some common substitutes: M-.5 for 48 tpi, M-.75 for 32 tpi, M-.9 for 30 tpi and M-1.0 for 24 tpi.
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