The diameter at which the width of the thread ridge equals the width of the groove.
The pitch diameter is the dimension that actually controls whether two threaded parts assemble correctly, and it is the one a thread micrometer or three-wire measurement checks. The major diameter is easier to measure but says much less — a thread can be within tolerance on major diameter and still not fit.
For a 60° thread, d₂ = d − 0.64952 × P, where d is the major diameter and P the pitch. This is why a finer pitch gives a larger pitch diameter and therefore a stronger bolt.
Believing the major diameter defines a thread. Two threads with identical major diameters can differ in pitch, pitch diameter and therefore in whether a nut will go on.
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