Editing Screw thread (section)

====Pitch diameter====
[[File:Diagram of thread combinations of snug fit.jpg|thumb|Variants of snug fit. Only threads with matched PDs are truly snug, axially as well as radially.]]

The pitch diameter (PD, or ''D''<sub>2</sub>) of a particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points. When viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance. Equivalently, a line running parallel to the axis and a distance ''D''<sub>2</sub> away from it, the "PD line," slices the ''sharp-V'' form of the thread, having flanks coincident with the flanks of the thread under test, at exactly 50% of its height. We have assumed that the flanks have the proper shape, angle, and pitch for the specified thread standard. It is generally unrelated to the major (''D'') and minor (''D''<sub>1</sub>) diameters, especially if the crest and root truncations of the sharp-V form at these diameters are unknown. Everything else being ideal, ''D''<sub>2</sub>, ''D'', & ''D''<sub>1</sub>, together, would fully describe the thread form. Knowledge of PD determines the position of the sharp-V thread form, the sides of which coincide with the straight sides of the thread flanks: e.g., the crest of the external thread would truncate these sides a radial displacement ''D''&nbsp;−&nbsp;''D''<sub>2</sub> away from the position of the PD line.

Provided that there are moderate non-negative clearances between the root and crest of the opposing threads, and everything else is ideal, if the pitch diameters of a screw and nut are exactly matched, there should be no play at all between the two as assembled, even in the presence of positive root-crest clearances. This is the case when the flanks of the threads come into intimate contact with one another, before the roots and crests do, if at all.

However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing the galling of the threads. For this reason, some ''allowance'', or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing ''interference'' and to expedite hand assembly up to the length of engagement. Such allowances, or ''fundamental deviations'', as ISO standards call them, are provided for in various degrees in corresponding ''classes'' of fit for ranges of thread sizes. At one extreme, no allowance is provided by a class, but the maximum PD of the external thread is specified to be the same as the minimum PD of the internal thread, within specified tolerances, ensuring that the two can be assembled, with some looseness of fit still possible due to the margin of tolerance. A class called ''interference fit'' may even provide for negative allowances, where the PD of the screw is greater than the PD of the nut by at least the amount of the allowance.

The pitch diameter of external threads is measured by various methods: 
* A dedicated type of [[Micrometer (device)|micrometer]], called a thread mic or pitch mic, which has a V-anvil and a conical spindle tip, contacts the thread flanks for a direct reading.
* A general-purpose micrometer (flat anvil and spindle) is used over a set of three wires that rest on the thread flanks, and a known constant is subtracted from the reading. (The wires are truly gauge pins, being ground to precise size, although "wires" is their common name.) This method is called the 3-wire method. Sometimes grease is used to hold the wires in place, helping the user to juggle the part, mic, and wires into position.
* An [[optical comparator]] may also be used to determine PD graphically.