Definition
A flat-bottomed enlargement at the top of a drilled hole, sized to allow the head of a bolt or socket-head screw to sit flush with or below the surface of the surrounding material. The term also refers to the cutting tool used to produce this enlargement.
Plain English
A wider, flat-bottomed pocket cut at the top of a hole so a bolt head can sit level with the surface instead of sticking up above it.
Context Anchor
Seen in aircraft maintenance, machining, and structural repair instructions when a fastener needs a flat seating area or a flush fit.
Derivation
From 'counter' (against, opposite) and 'bore' (a drilled hole). The tool bores a second, larger hole that works against the first to recess the fastener head.
Why Pilots Care
A bolt seated in a counterbore sits flush, which matters for clearance, aerodynamic smoothness, and preventing snagging or wear on adjacent parts.
Intuition Check
A counterbore is not a sloped recess. It has straight sides and a flat bottom, like a small shelf around the hole.
Example Sentence 1
The mechanic used a counterbore to recess the bolt head so it would sit flush with the panel surface.
Example Sentence 2
After drilling the pilot holes, the mechanic used a counterbore cutter to recess each bolt head below the rib surface.