How to Read a Screw Thread Callout

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Unlike self-tapping screws, machine screws need be carefully matched in both length and thread pitch to their intended holes or nuts. A screw thread callout is a system for describing the characteristics of a machine screw. The ability to read screw thread callouts will allow you to quickly find the screw sizes you're looking for.

  • Figure out which measurement system you're using. The two most common systems are the Unified Thread Standard, used in North America, and the ISO Metric Screw Thread, used in Europe and most places in the world. Metric numbers are preceded by the letter "M."

  • Read the first number. The first number indicates the outside diameter of the screw. For Unified threads, diameters are numbered "#1" through "#10" (screws 1/4 inch and larger are given in fractional inches). In the case of Metric threads, the diameter is equivalent to the first number in millimeters. (So a screw numbered "M4" would have an outside diameter of 4 millimeters.)

  • Read the second number. The second number indicates the pitch of the screw. In the case of Unified threads, there is a fine thread and a coarse thread for each diameter size of screw. In the case of Metric threads, a pitch number may not be given. This means that the screw uses the standard coarse pitch of 1 millimeter per 360 degrees of rotation.

  • Check to see if there is a third number after an "X." If there is, this number specifies the length of the screw, given, depending on the system, in either inches or millimeters.

Tips & Warnings

  • A quick way to check if 2 screws have matching pitches is to lay them against one another, with their heads facing in opposite directions. If their threads interlock with each other, then the 2 screws have the same pitch.
  • A screw pitch gauge is a handy tool used for measuring screw pitches. It contains folding leaves that have teeth on their size corresponding to standard screw pitches, which you can place screws against.
  • There are also two older, less common callout systems, Whitworth and British Association. These will mainly be found in older equipment or specialty applications.
  • For numbered Unified threads, the actual diameter of the screw corresponds to the following formula: actual diameter = (screw number x 0.13 inches) + 0.60 inches.
  • The pitch is the distance between threads on the screw shaft, that is, how tightly the threads are spaced.
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