5-101.bk Page 6 Friday, March 17, 2006 11:05 AM Fastener Sizes and Types The pitch of metric fasteners is measured by determining the millimeters per thread. For example, a bolt with 0.8 pitch would have 125 complete threads in a 100-millimeter section (100 mm divided by 125 threads equals 0.8), and a bolt with 1.0 pitch would have 100 threads in a 100-millimeter section. Pitch may be measured directly, using a ruler and counting the threads. Also, thread pitch gauges are available for both U.S. and metric threads, which make it easy to check the pitch of a fastener. The first and most important fact that the mechanic must know about a fastener is whether it is a U.S.
(Inch System) or a metric thread. Next, the size of the fastener, which is usually determined by the diameter of the shank, the length of the fastener, which is usually measured from the bottom of the head to the end of the thread, and the pitch of the threads. The pitch of U.S. (Inch System) fasteners is measured by determining the number of threads per inch. The two pitches commonly used in vehicles are coarse thread, officially called Unified National Coarse (UNC), and fine thread, officially called Unified National Fine (UNF). In the U.S. system, a typical designation would be: 7/16-20 x 1.
This describes a bolt that is 7/16 inch in diameter, has 20 threads per inch, and is one inch long. The metric system is similar. A typical metric designation would be: 10 x 0.8 x 25 mm. This describes a bolt that is 10 mm in diameter, has a thread pitch of 0.8 (0.8 mm per thread), and is 25 mm long. Figure 2-3 — Flanged Capscrew Figure 2-2 — Fastener Dimensions Page 2-6 200 SPECIFICATIONS