This isn't a treatise on metric fasteners, but here are a couple of basics anyone working on a European car should know: Metric nuts and bolts are specified by the diameter of the shank, in millimeters, and by the thread pitch, measured as the distance between thread crests or roots(peaks or valleys), again in millimeters. (By contrast, American-type SAE thread pitches are measured as the number of threads per inch.) For bolts, you also specify the length from the underside of the head to the tip. A typical example: M8 x 1.25 x 30, which happens to be a course thread bolt 30mm long (its fine-thread equivalent would be M8 x 1.00). Note that you don’t generally specify a bolt by the head diameter. M6 fasteners usually take a 10mm wrench; M8 a 12 or 13mm wrench, and so on. One more important fact: the little number you see on the head of a bolt (usually 5.8, 8.8, 9.8 or 10.9) tells the property class (strength), and any replacement must take this into account. If you were to replace a suspension bolt marked 10.9 with a 5.8-class bolt, its lack of adequate tensile strength could result in disaster! |
