How to Identify a Screw Thread

Updated March 23, 2017

Identifying screw threads is important if you need to reorder screws for a home-improvement project or if you need to repair furniture and home appliances. It is important to order screws with the correct thread as it can affect the outcome of your work. You do not have to be an expert to identify screw threads. Once you have learnt to identify screw threads, you will be able to select the correct screws for any DIY project.

Measure the thread pitch of a screw using a ruler. The thread pitch is the distance between the thread lines. Count the number of thread crests in one inch length along the screw. Start with zero for the first thread. Divide the length (one inch) with the number of threads (10) you counted. For example, if there are 10 threads in one-inch length of a screw, then the thread pitch is 0.1.

Identify the major diameter of a screw thread using a ruler. The major diameter of a screw is the diameter of the screw including the threads.

Identify the pitch diameter of a screw using an optical comparator. An optical comparator magnifies the screw onto a screen. According to Roton, the pitch diameter is "the diameter at which the thread tooth and the thread space are equal." Put the screw in the optical comparator and compare it to other screws.

Identify the minor diameter of a screw with an optical comparator. The minor diameter is the diameter of the screw without the threads.

Identify the "hand" of the thread on a screw. The hand of the screw is the way the screw turns when you screw it into something. Visit the Roton website to compare your screw threads to Figure 48 on the web page to find out whether you have a right hand or left hand screw.

Identify how many starts the screw has. A start is the distance the screw takes in one turn of a screw driver. Put a pencil tip in the thread groove of a screw and mark up the distance between the point end of the screw and where the pencil stopped after rotating the screw once. The number of threads between the beginning of the mark and the point of the screw will give you the number of starts.

Things You'll Need

  • Ruler
  • Optical comparator
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About the Author

Janos Gal has been writing since 2008. He wrote for the "Global Journalist" magazine in 2008 and for the "Estrella de Arica" daily in 2009. Gal has traveled extensively in Europe, South America and the United States. He holds a Bachelor of Arts, honors, in journalism from Edinburgh Napier University.