More about Metric Bolts

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Metric bolts are threaded fasteners whose length, thread pitch, thread length and thread length are offered in metric measurement, such as one millimeter (a possible thread pitch measurement). Additionally, thread sizes are denoted in various “M” (or MJ) numbers, which help manufacturers and suppliers specify sizes. These sizes will vary based on the type of fastener being used; a screw with an M4 thread size, for example, has a 2.8 millimeter head height in hexagon head cap screw, while a socket head cap screw with an M4 thread size has a 4 millimeter head height. A typical delineation for a metric bolt will be as follows: M6 x 1.5, which indicates the thread profile, fastener nominal size and thread pitch. Other numbers, such as external thread tolerance class and engagement length group, may also be supplied.

Metric bolts generally have right handed threads, but can be ordered with left handed threads. They can be fully or partially threaded; a partially threaded bolt has an area beneath the head that is bare from threads. This enables the bolt to be locked in place, but offer less friction within the work piece. Bolt varieties are either hex, heavy hex, bent, heavy hex structural, hex transmission tower or round head square neck. They differ from screws in that bolts can accept a non-tapered nut and are manufactured to highly precise specifications. Metric bolt specification standards are set and determined by the ISO (International Organization for Standardization), which is a world-wide organization dedicated to creating industrial and commercial standards. They are produced in various classes, or grades, such as 4.6, 8.8 and 10.9, which indicate the maximum torque that the bolt can withstand (bolt length also factors in to maximum torque). These grades are marked on the bolt head, and often on the accompanying nuts.