Definition
A bolt with a cylindrical head containing a recessed internal hex (or sometimes star-shaped) drive that is turned with an Allen key or hex wrench rather than an external wrench. Socket-head bolts are typically machined from high-strength alloy steel and are used where a strong, compact fastener is needed in a confined space or where a flush or counterbored installation is desired.
Plain English
A bolt with a hole shaped like a hex in the top of its head. You tighten it using an Allen key that fits into that hole, instead of putting a wrench around the outside of the head.
Context Anchor
Seen during aircraft maintenance, inspections, and parts identification, especially where there is not much room around the bolt head for a regular wrench.
Derivation
‘Socket’ here means a recess or hollow shaped to receive a tool. The bolt’s head holds the socket (the hex-shaped hole) instead of the tool holding the bolt. That flips the usual picture: the wrench goes inside the bolt, not around it.
Why Pilots Care
Provides a low-profile head that reduces drag and prevents interference with nearby components or airflow.
Intuition Check
Do not read socket here as an electrical socket. In a socket-head bolt, the socket is the recessed tool opening in the bolt head.
Example Sentence 1
The mechanic used an Allen key to back out the socket-head bolts securing the magneto cover.
Example Sentence 2
Socket-head bolts were chosen for the avionics tray because their heads sit flush inside the cockpit.