![]() But if the M8 bolt had a fine pitch thread of 1mm then the diameter of the hole required for tapping would be 7mm. For example, an M8 bolt with a standard coarse thread pitch of 1.25mm would require a hole of 6.75mm diameter to be tapped. The necessary tapping hole size can be easily calculated by deducting the thread pitch from the metric bolt diameter. When it’s necessary to cut a thread then the hole size needs to be a smaller diameter to allow for the thread to be tapped. But it’s common practice to drill clearance holes that are slightly larger to allow for misalignment. So an M8 bolt would have a shaft diameter that’s slightly under 8mm which means that the bolt should fit through an 8mm hole. The defined diameter of metric bolts is actually slightly larger than the actual diameter of the bolt shaft. Machine Screw Hex Nut, Finished Hex Jam Nut Descriptions of DIN and ISO fastener standards. Here’s a list of the most common DIN and ISO numbers and what they describe. Metric bolts and fasteners are manufactured in conformance with standards set by the International Organisation for Standardisation (ISO) and the German Institute for Standardisation (DIN). ![]() For flat head bolts, the length includes the bolt head height and for dome head bolts the length is measured from the thickest, highest point on the curved head. Socket head, pan head, button head and hex head bolts are measured from the underside of the bolt head to the end of the shaft. The length of a metric bolt is measured and defined in exactly the same way as imperial, inch-based bolts and fasteners. It is worth noting that metric bolts with a coarse thread pitch have more threads per inch (they are more closely spaced) than comparable imperial bolts. Metric fasteners are available with either coarse or fine thread pitches with some metric bolt sizes offering extra fine thread pitch options. Metric Thread PitchĪs noted, the specification of metric bolts includes the definition of thread pitch, but if this is omitted then a coarse pitch is assumed. This example (M12-50) has a diameter of 12mm and a length of 50mm. Whenever the pitch dimension is omitted from a metric bolt specification then the bolt is always coarse threaded. This abbreviated format omits the pitch definition which means that the bolt has a coarse thread. And the final figure, 20, is the length in millimeters.īut it’s very common to see metric bolts specified like this: M12-50.The number 1.0 is the thread pitch of the bolt, which is the distance between threads, in millimeters.The number ‘8’ refers to the nominal diameter of the bolt shaft in millimeters. ![]() The letter ‘M’ indicates that this bolt uses a metric thread.This bolt specification breaks down like this: Here’s an example metric bolt specification: M8-1.0 x 20 For nuts the size dimensions used are simply diameter and pitch. But the size of a metric fastener is more accurately specified using diameter, pitch and length dimensions, in millimeters. Metric nuts and bolts are commonly referenced using ‘M’ sizes, for example: M3, M8, M12. Part 1 is based on ISO 965/1 and ISO 965/3 providing the principles and basic data for specifications of ISO metric screw threads.Īnd BS3643 part 2 provides tabulated diameters and tolerances for coarse and fine pitch threads, based on ISO 965/2. BS3643 Parts 1&2īS3643 defines the limits and tolerances for ISO screw thread gauges. In this brief guide we provide an introduction to standard ISO metric sizes for nuts and bolts which will hopefully help dispel any confusion. In most circumstances, it’s absolutely vital that the right size of fastening is used. Nuts and bolts are essential fastenings, critical in the safe assembly of machinery, equipment, furniture and much more.
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