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How to Identify Metric Bolts

Updated May 03, 2018

Some, but not all, metric bolts are identified with an "M" at the start of the bolt sizing information, such as "M8 x 1.5 x 18" but others simply list the measurements. The measurements are expressed in millimetres. American bolts list threads per inch; metric lists the distance between threads. Metric bolts are incompatible with American bolts.

Use the first number in the measurement to identify the diameter of the bolt. For example, if the metric bolt is M13x1.7x20, it would have a diameter of 13 millimetres.

Use the second number in the measurement to identify the thread distance. For example, if the metric bolt is M13x1.7x20, it would have a thread distance of 1.7 millimetres.

Use the third number in the measurement to identify the bolt length. For example, if the metric bolt is M13x1.7x20, it would have a length of 20 millimetres.

Use the number on the head of the bolt to determine the bolt strength. Not all bolts will have a marking but the higher the number the stronger the bolt.

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About the Author

Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."