Definition
The distance between corresponding points on adjacent threads of a screw, bolt, or other threaded fastener, measured parallel to the axis of the fastener. In the inch system, thread pitch is commonly expressed as the number of threads per inch (TPI). In the metric system, it is expressed directly as the distance in millimeters between adjacent thread crests.
Plain English
How close together the ridges are on a screw or bolt. On inch-sized hardware it's usually given as how many threads fit in one inch; on metric hardware it's the distance from one ridge to the next in millimeters.
Context Anchor
Seen in aircraft maintenance when matching bolts, screws, nuts, spark plugs, or threaded fittings.
Derivation
Pitch comes from Old English picchen, meaning to fix or set in place at regular intervals. The same idea appears in 'pitch of a roof' (slope) and 'pitch of a propeller' (advance per turn). For threads, it refers to the regular spacing set along the length of the fastener.
Why Pilots Care
Ensures fasteners engage properly and resist loosening under aircraft vibration.
Intuition Check
Thread pitch is not the angle of the screw thread. It is the spacing from one thread ridge to the next.
Example Sentence 1
The mechanic checked the thread pitch of the replacement bolt to make sure it matched the original before installation.
Example Sentence 2
Using the wrong thread pitch prevents proper engagement in the aircraft structure.