Metric bolts are used for various fastening purposes, such as heavy machinery assembly, automobiles and residential construction. These bolts come in two pieces, a bolt and nut, with various styled washers available to install between the bolt and material. These washers prevent stress-related damage and keep the bolt from backing out. The two most common types of bolts are imperial or metric, both of which adhere to separate sizing conventions.
Ensure the metric bolts are manufactured with a blunt end; a sharp or pointed tip indicates a screw, and should not be used in situations requiring bolt and nut fastening. Substituting a screw for a bolt may cause damage to parts the point comes in contact with, and may be unable to support the load properly.
Inspect the head of the bolt for any markings or stampings in ink or etched into the metal. Metric bolts will have the common grades under 100 millimeters in diameter stamped as a number with a single decimal point to indicate the recommended weight load, class and material. The head marking will not offer information concerning the overall length or thread count of the bolt. Any markings for common sized bolts that use radial lines to indicate the grade and material should be identified as imperial bolts, otherwise known as U.S. bolts.
Use the ruler to measure the length from the underside of the bolt's head to the end of the bolt to determine its length. Measure the diameter of the head of the bolt, taking care to lay the ruler across the center of the head to ensure an accurate reading. Determine the distances between the threads, or the raised grooves on the bolt with the ruler. Some bolts have threads spaced as low as 1 millimeter apart, so estimating the distance may be difficult. The measurements are written in millimeters following the format of diameter x thread spacing x length.