Definition
A threaded fastener with a finished head, designed to be screwed directly into a tapped (threaded) hole in a part rather than being held by a separate nut. Cap screws are made to closer tolerances and have a more refined finish than ordinary bolts, and the bearing surface under the head is machined square to the threads.
Plain English
A high-quality screw with a neat, finished head that threads into a hole in the part itself, instead of needing a nut on the other side.
Context Anchor
Seen in aircraft maintenance, engine accessory mounting, component attachment, and parts manuals that specify the exact fastener to use.
Derivation
From 'cap' (the finished head that 'caps' the fastener) and 'screw' (a threaded fastener). The word 'cap' here refers to the well-formed head, which distinguishes a cap screw from a rougher bolt.
Why Pilots Care
Proper use prevents vibration-induced loosening in critical flight components such as cylinder heads or control surfaces.
Intuition Check
A cap screw is not a screw with a protective cap on it. Here, “cap” refers to the head of the fastener, and the screw normally tightens into a threaded hole.
Example Sentence 1
The mechanic removed four cap screws to lift the valve cover off the cylinder head.
Example Sentence 2
Inspect all cap screws on the landing gear for signs of wear or improper torque before flight.