Although they are found everywhere and rarely given much consideration, threaded objects are one of the most basic elements of modern construction and engineering. Threading makes screws usable, as simple machines and threads are the "glue" that holds together much of the equipment that people use on a daily basis.
Threads come in many shapes and sizes, and when working with fasteners or machinery, it is important to understand the characteristics of the threaded object that you are dealing with. This is where thread gages come in, but before using them it is important to understand how those basic characteristics are measured.
When dealing with a threaded fasteners, there are two important terms to keep in mind: thread length and thread pitch. Additionally, it's key to remember that the threads themselves are the raised parts of the helical portion of the fastener. The top of each thread is generally known as its crest.
The characteristics of most fasteners are carefully chosen by engineers to suit the particular application they are being used for. Of these, the two most important characteristics are the thread length and thread pitch. Thread length is a fairly simple concept; fasteners are either full-length or partial-length. Full-length fasteners have a threaded portion which extends all the way to the head, while partial-length fasteners have a "flat" section of shank between the head and the thread.
Thread pitch or thread count are two different terms that describe essentially the same thing: the density of threads on a fastener. Thread count, given in threads per inch, is typically used in the US, and it measures the total number of thread crests in one inch of the fastener. Thread pitch, used mostly in metric countries, measures the distance in millimeters from one thread crest to the next.
The Role of Thread Gages
Thread gages are used to check the thread pitch of a threaded object, either for inspection or to determine the pitch of an unknown fastener. Thread gages come in a variety of styles and types, depending on their intended use. For manufacturing purposes, the most common types of gages are simply designed to determine if a particular part meets whatever specifications are required. In this case, the gage is not used to measure the pitch so much as to confirm that it is correct.
For use in the field (such as in automotive shops) it is more common to use multi-piece gage sets that can check both internal and external threads. For tools such as these, fasteners can be checked using the external thread side of the tool, while the internal thread side can be used for threaded holes. This makes it relatively simple to confirm the particular characteristics of a fastener or hole in order to source proper replacement parts.