How to Determine Thread Length on Bolts

Updated July 19, 2017

Hex-head cap screws and bolts come in all sizes. Measuring the length of the thread is important in order to provide the proper retention to the attached connection. While bolt thread, length and diameters come in a variety of sizes, between standard and metric measurements, installing the incorrect bolt can lead to problems. Because of the wide diversity in bolt sizes, there are tools in the field to help the do-it-yourself enthusiast or the seasoned repair person.

Insert the bolt into the bolt diameter gauge to determine its thickness. Go between a metric and standard gauge to properly identify the diameter. Some bolts may fit into both gauges, but one of them will allow a wiggling motion of the bolt. The one the bolt fits in more snugly identifies the bolt as metric or standard diameter. Other bolt diameters are easier to identify because they will only fit into one or the other.

Measure the length of the thread on a tap bolt that employs threads along the entire shaft of the bolt. Use the metric gauge if the diameter is metric and the standard if the diameter was in standard measurement. If the bolt diameter gauge is not equipped with a metric or standard ruler, use a measuring tape. Measure from the bottom of the bolt head to the end of the bolt shaft.

Calculate shoulder bolt thread length by applying this formula: A shoulder bolt uses a non-threaded section known as a shoulder just beneath the cap or hex head. If the bolt diameter is 1/2 inch, multiply the diameter by 2 and then add 1/2 inch. The example of a 1/2-inch diameter shoulder bolt will have a 1 1/2-inch thread length no matter how long the bolt is.

Use the pitch gauge to determine the pitch accuracy of the bolt. This is to determine if the right bolt is going to be threaded into the connection correctly. Since metric and standard are somewhat similar, it's easy to install the wrong bolt and cross-thread the bolt, damaging either the bolt, the mating threads of the connection or both. Again, go between the metric and standard thread pitch gauges, but it's safe to assume that a metric diameter bolt will employ a metric thread pitch and a standard diameter bolt will employ a standard thread pitch.

Use the measuring tape to count how many threads per inch are present along the threaded shaft of the bolt, if you don't have a thread pitch gauge. The amount of threads per inch will determine the bolt thread pitch number applied to the bolt for standard-sized bolts. For metric, the distance in millimetres between the top crest of one thread to the top crest of the adjoining thread will determine whether the metric thread is coarse or fine.

Things You'll Need

  • Bolt diameter gauge, metric and standard
  • Measuring tape (optional)
  • Thread pitch gauge; metric and standard
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About the Author

Jody L. Campbell spent over 15 years as both a manager and an under-car specialist in the automotive repair industry. Prior to that, he managed two different restaurants for over 15 years. Campbell began his professional writing career in 2004 with the publication of his first book.