Bolts or Metric Screws

A metric bolt is indicated by the letter M followed by the diameter in millimetres, for example M8, and by the thread pitch. The length is measured under the head, to the end of the screw. These data are needed to choose the nut and washer, because they must match the thread exactly.

A bolt is used together with a nut, which locks it in the through hole, while a metric screw is screwed directly into a threaded hole or into a fixed nut. The choice depends on the type of connection and on access from both sides. In metal structures the bolt with nut is the most common solution; the screw is used where one part must remain fixed.

For metal structures hex-head bolts with nut are used, often in galvanised steel, with a strength class marked on the head. The class must be chosen based on the load and the design standard, because an undersized bolt can give way. Do not replace a structural bolt with one of lower strength just because it is available.

The socket head cap screw with hex socket is used when the head must sit flush or recessed, for example in machinery and equipment. The hex-head screw is tightened with a spanner and allows a higher torque, useful in carpentry and robust connections. Choose based on access for the spanner and the appearance of the part.

The round-head bolt with square neck has a neck that enters the square hole of a plate or bar, preventing the bolt from turning while the nut is tightened. It is needed where rotation must be prevented, for example in platforms, gratings and timber or metal supports. The square hole must be the right size to avoid play.

The flanged bolt has a flange under the head that spreads the load over the surface and reduces the need for a washer. It is useful where the hole is large or the surface is not perfectly flat, such as in connections on thin sheet metal. The flange must still be checked against the material being fixed, to avoid crushing it.

For vibrations self-locking nuts with a nylon insert are used, or lock washers and toothed washers which create friction on the nut. Tightening must be to the value indicated by the manufacturer, because a loosely tightened bolt comes undone and an overtightened one can break. Check the tightening periodically on machinery subject to vibration.

The bolt must project beyond the nut by at least one complete thread, so that the nut can work correctly and carry the load. If the bolt is too short the connection will not hold; if it is too long and projects a lot, it can create a hazard and an obstruction. Choose the length based on the thickness to be passed through and the nut to be used.