Length of Engagement

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Length of Engagement sketch

What is the Length of Engagement?

Length of Engagement (LE) is the axial distance of full (360°) threads in contact between the male and female parts of a threading assembly. It is usually measured in multiples of the pitch. For example, 5P means a length of engagement of five times the pitch.

Each threading standard takes a different approach to account for the length of engagement:

  • Unified Inch Threads per ASME B1.1: The values listed in the charts for UNC/UNF are valid between 5P (five times the pitch) to 1.5D (1.5 times the nominal diameter) engagement length. However, the standard provides formulas that take it into account. If your length of engagement is outside of this range, you should make manual calculations. (Or use the calculator)
  • Metric Threads according to ISO 965: The standard divides the length of engagement into 3 groups. Normal, Short, and Long (designates N, S & L). It provides charts and formulas to determine the LE range for each diameter and pitch combination. Following that, it recommends which classes to use for each LE range. However, the designer of a part may choose a class combination outside of this list. The ThreaDoctor Calculator provides the Normal LE range in the tolerances tab. Anything outside the normal range is considered low or high.

 To get more precise tolerances for specific figures of LE outside the range, you should make manual calculations or better use a tool such as the ThreaDoctor.

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