Fasteners: SAE vs. METRIC, What Is Coarse and What Is Fine?
The terms “coarse” and “fine” do NOT apply to METRIC hardware. Those terms apply to SAE ("standard" or "American") hardware. For instance, a ¼” bolt comes in either ¼-20, which is “SAE standard coarse," or ¼-24, which is “SAE standard fine." The -20 and -24 represents how many THREADS PER INCH (thread pitch) are present on the fastener. The larger the number, the more threads there are per inch, thus the finer the thread. The terms fine and coarse apply because there are only two choices. These terms and quantities are arbitrary, but are indeed engineering standards established by the SAE (Society of Automotive Engineering). It is just one of many standards they have established and maintain.
When it comes to METRIC hardware, there can be up to four different thread pitches for a given size of fastener. Thread pitch in metric hardware refers to the ACTUAL DIMENSION OF EACH THREAD, and is measured in MILLIMETERS. The LARGER the number, the MORE coarse the thread pitch, OPPOSITE to the SAE system. For instance, the common 8mm bolt is an 8 x 1.25, which means each thread is 1.25mm wide. Besides the common 8 x 1.25 bolt, there also exists the finer pitch 8 x 1.0 bolt. It may be the case that an 8mm bolt only comes in those two thread pitches, thus the temptation to use the SAE terms "coarse" and "fine." However, that is not always the case with other sizes. For example, 10mm bolts are commonly available in 1.0, 1.25, and 1.5 thread pitches. Whereas almost all 10mm bolts used on German-made vehicles are of the 1.5 variety, on a Japanese-made vehicle the common pitch is 1.25. The even finer 1.0 thread is common on plumbing fittings, like the threads on oil pressure switches for example. Like just about all things in the metric system, it is actually more logical and easily applicable. In the case of thread pitch, the fact that the number given is the actual dimension of the thread itself makes it easy to determine fastener engagement, which is very useful information. One complete turn of the fastener, and it moves in or out the amount equal to its thread pitch. For example, the road wheel fasteners on an VW van (Bus, Vanagon or Eurovan, bolt, nut, or stud) measure 14 x 1.5. That means one complete turn of the fastener will make it move 1.5mm in or out. So some quick mental math and we know that ten turns equals 15mm of thread engagement, which is nice to know...
OK, so I'm a fastener nerd. What can I say...?
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