Vernier caliper A Vernier caliper (Figure 2) is the most helpful tool for measuring the major diameter of a threaded fastener, whether the threads are internal or external. The upper jaws on top of the caliper’s head (Figure 2 labeled A) can measure internal thread diameters, and the lower jaws (Figure 2 labeled B) can measure external thread diameters. The main scale (Figure 2 labeled C) shows the integer value of the measurement. This scale can be in centimeters or inches. The Vernier scale shows the decimal value of the measurement. On a metric scale, the Vernier scale represents 1 millimeter. The Vernier scale has 25 increments of 0.025 inches on an imperial scale.
When measuring the major diameter of a threaded fastener, it's essential to first know if the thread is tapered. If a visual inspection cannot determine this, use the caliper to measure the fastener's first, fourth, and last threads. If the diameter changes across the fastener, the thread is tapered. If the diameter remains constant, the thread is straight or parallel. If the thread is tapered, measure the major diameter at the 4th or 5th thread to get the thread’s true major diameter. If the thread is straight, measure any thread to find the major diameter. If measuring the major diameter of an external thread, place the caliper's jaws on the thread's crest. If measuring the major diameter of an internal thread, place the jaws on the thread's groove. To measure the length of a bolt, measure the head's bottom to the threading's end. The following instructions describe using a Vernier caliper to measure a threaded fastener.
Figure 3 shows a pitch gauge measuring a thread. Pitch gauges can be metric or imperial. A pitch gauge has several leaves with a number stamped on it. The number indicates the pitch. When identifying an unknown thread, it's important to have an imperial and metric gauge. There are similarities between metric and imperial threads that may lead to a false positive. For example, a metric pitch gauge may appear to match some imperial threads. An imperial gauge will have a closer match and provide the correct pitch.
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