Thread Measuring Tools
Screws, nuts, pipes, plumbing fittings and myriad other things have threads. You will find objects with male thread and with female thread. Some fittings have what is called common thread, while others have fine thread. The pitch and the direction of the thread can also vary. Thread-measuring tools will give you the specifics of each of these attributes. You can use these tools so that you don't have to try a lot of bolts to fit that unknown nut or to frustratingly find the right size nut only to discover that it only will make two turns because the pitch is wrong.
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Caliper
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A caliper is a device to measure either the outside or inside dimensions of an object. Over the years, calipers have evolved from simple screw-down tools to today's extremely accurate digital devices. In 2011, you can use calipers to measure to the nearest 1/100,000 inch. Calipers help you determine the outside diameter of a male thread or the inside diameter of a female thread. Six-inch digital calipers begin in the $10 to $15 price range in 2011 dollars. A fractional eight-inch digital caliper can cost you as much as $50.
Pitch Gauge
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A thread pitch gauge is a simple and inexpensive hand-held tool. You use it to measure the number of threads per inch (English system) or threads per centimeter (metric system). You can also find the pitch of a thread with a ruler by carefully counting the threads in the space of an inch or a centimeter. You will find pitch gauges for under $10 at most hardware stores. Using a pitch gauge eliminates the frustration of starting a bolt and having it jam after only two turns.
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Thread Identification Charts
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Once you know the diameter and the pitch of your thread, go to a thread identification chart (see Resources). With this type of chart, you can find the angle between each of the threads of your fitting. If you need to convert U.S. thread standards to British standards or the metric system, these charts are also very helpful. The charts allow you to go to your hardware store and cross-reference nuts bolts and other fittings to get the exact fit that you need.
Micrometer
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You can use a micrometer to measure the inside or outside diameter of your fitting if you don't have a caliper. Micrometers are somewhat more accurate than the caliper, but are commensurately more expensive. A laboratory grade micrometer will cost you almost $100. There also are gauges made that will measure both thread diameter as well as the pitch. These tools, while being convenient, can also run in the $100 price range.
Care
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Whatever tool you use, your measurement will only be as accurate as the care you put into it. All of these tools are precision instruments and must be used correctly to get an accurate measurement.
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References
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