Metric bolts are characterised by grade, diameter, thread pitch and length in metric units, which were developed in France and formalised as an international standard as the SI, or "Systeme International." U.S. threads on bolts are specified in threads per inch with the other characteristics also in British Standard units. Other thread characteristics such as thread shape are given in units such as degrees of an angle, which are common to both systems. The SI is the only system used in Europe, while North America uses both systems.
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Check the strength of the bolt. To convert a metric bolt to one with U.S. threads the strength must be considered. A metric bolt with no markings on the head has a tensile strength of 429MPa (mega pascals) or 60,900 psi. The equivalent U.S. threaded bolt has no markings and a tensile strength of 60,000 psi.
A metric bolt marked 8.8 has a tensile strength of 830MPa or 120,350 psi. The equivalent U.S. threaded bolt has three lines radiating from the centre of the head to three of the six hexagonal corners. Its tensile strength is 120,000 psi.
A metric bolt marked 10.9 has a tensile strength of 1040MPa or 150,800 psi. The equivalent U.S. threaded bolt has six lines radiating from the centre of the head to the six hexagonal corners. Its tensile strength is 150,000 psi.
Convert the length and diameter of the bolt. The length and diameters of metric bolts are given in millimetres. The lengths and diameters of U.S. threaded bolts are given in inches.
A metric bolt 12mm in diameter and 30mm long would be 0.47244 inches in diameter and 1.18110 inches long. The conversions are given to an accuracy of five figures because a bolt that is even slightly too thick may not fit into a tightly machined hole. While five figures may be excessive, it gives a clear picture of the size relationship.
Convert the thread. The threads of metric bolts are given by the millimetres between threads. The threads of U.S. bolts are given in threads per inch. A metric bolt with a thread of 1.75 has a distance of 1.75mm between threads and therefore 25.4/1.75 = 14.51 threads per inch. Threads on U.S. bolts are integers so that a higher accuracy is not necessary.
Select the closest U.S. bolt to a metric bolt. Select the grade by checking the head for markings and select the closest U.S. equivalent. If the metric bolt has a marking of 8.8, U.S. bolts with three or six lines on the head can be substituted.
Select the diameter. Divide the millimetre diameter by 25.4 and get a five-figure number in inches. Select the closest U.S. bolt which is slightly smaller. For the 0.47244-inch bolt of the above example, select a 7/16 inch bolt, which is 0.4375 inches in diameter, or possibly a 15/32 inch bolt, which is 0.46875 inches. The length is treated the same way but the actual application will determine whether it is better to choose the next longer or next shorter corresponding U.S. bolt.
Select the thread. Divide 25.4 by the distance in millimetre between threads of the metric bolt and get the threads per inch. Select the closest U.S. bolt thread. For the metric bolt of the above example, the closest to 14.51 threads per inch would be 15.
If the factory parameters of the metric bolt are not known, the diameter and length can be measured in inches and the threads per inch can be counted. The closest U.S. bolt can then be chosen as above. If a US bolt with a diameter and threads per inch very close to the metric bolt can be found, the U.S. bolt will be an effective replacement. Usually, the U.S. bolt will not thread perfectly into a hole or nut with metric threads.
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