Definition
On a threaded fastener such as a bolt or screw, the distance from one thread crest to the next, measured parallel to the axis of the fastener. In the U.S. system, pitch is commonly expressed indirectly as threads per inch (TPI); in metric fasteners, it is given directly in millimeters (for example, 1.25 mm).
Plain English
How far apart the threads are on a bolt or screw. Closer-together threads mean a finer pitch; farther-apart threads mean a coarser pitch.
Context Anchor
Seen in aircraft maintenance when identifying screws, bolts, nuts, and threaded fittings before installation or replacement.
Derivation
From the older mechanical sense of 'pitch' meaning the spacing or interval between repeated features. In aviation hardware it specifically refers to the spacing between threads, distinct from the everyday meaning of 'pitch' as throwing or as nose-up/nose-down attitude.
Why Pilots Care
Using a fastener with the wrong thread pitch — even if the diameter looks right — can strip threads, fail to tighten properly, or back out in service. Correct pitch is essential for a safe, airworthy installation.
Analogy
Thread pitch is like the spacing between teeth on a comb: close teeth mean small spacing, and wider teeth mean larger spacing.
Intuition Check
Do not read pitch here as aircraft nose attitude or sound tone. In this maintenance context, pitch means the spacing between screw threads.
Example Sentence 1
The technician checked the bolt's diameter and pitch against the parts manual before installing it.
Example Sentence 2
A mismatch in pitch prevented the nut from seating properly on the stud.