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The distance between the crest and the pitch line in an external thread is called the thread addendum. It is designated with has. (Internal threads don’t have an addendum).

The best method to measure the addendum is with a dedicated addendum gauge. The gauge is composed of a flat surface that sits on the thread’s crest and a ball-tipped dial indicator. The ball diameter should be as close as possible to the ideal wire diameter used for the Over-Wire measurements. You can check out the ideal diameter in our threading calculator (Select the thread and to the over-wire tab).
If the dedicated gauge is not available, you should measure the pitch and major diameters and calculate the addendum according to the below formula.

• These formulas are valid for 60° symmetrical straight threads (ISO metric and Unified Inch standards)
$$\large h_{as} = \frac {\left (Major Diameter\,-\,Pitch Diameter\right )}{2} = \frac {\left (d – d_2\right )}{2}$$
$$\large d_2 = d – \frac{3}{8}\times\sqrt{3} \times P$$
$$\large h_{as} = 0.5\,\times\,\left (d – \frac{3}{8}\times\sqrt{3} \times P \right )= 0.3247595\times P$$
$$\large h_{as} = \frac {\left (Major Diam.\,-\,Pitch Diam. \right )}{2}$$
$$\large = \frac {\left (d – d_2\right )}{2} = \normalsize 0.3247595\times P$$

As can be seen from the formula, the thread Addendum depends solely on the pitch.

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