What does counterbore mean?
A counterbore is a cylindrical, flat-bottomed enlargement machined into the upper end of a hole so that the head of a bolt, cap screw, or similar fastener can sit flush with, or slightly below, the surrounding surface. The term refers both to the recess itself and to the specialised cutting tool that produces it, and it also functions as a verb: a machinist may counterbore a hole after drilling it. The word is at home in technical settings — mechanical engineering, metalworking, and woodworking — and frequently appears on engineering drawings alongside dimensions for diameter and depth. Its closest companion terms are the countersink, which cuts a cone-shaped recess for angled screw heads, and the spotface, a shallow flat seating for irregular surfaces. Though obscure outside these trades, counterbore exemplifies how precise vocabulary encodes precise geometry, distinguishing a flat-bottomed cylinder from its conical cousin by a single syllable.
A cylindrical, flat-bottomed enlargement at the end of a hole, cut so that a fastener head can sit flush with or below the surface.
"The flange required a counterbore deep enough to hide the socket-head cap screw entirely."
Also denotes the cutting tool that produces this recess.
To enlarge the end of a drilled hole with a cylindrical, flat-bottomed recess.
"Counterbore each mounting hole before inserting the bolts."
Regular plural; used when referring to multiple recesses or multiple cutting tools.
"All four counterbores were machined to the specified depth."
The difference between a counterbore and a countersink is pure geometry — one is flat-bottomed and cylindrical, the other cone-shaped — and getting it wrong means your bolt heads will never sit flush.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of counterbore
Counterbore is a transparent English compound formed from the prefix 'counter-' (meaning opposite or complementary, from Latin 'contra') and 'bore', from Old English 'borian', meaning to pierce or drill a hole. The compound arose in the context of machining and metalwork, likely in the nineteenth century as precision engineering practices developed, though its exact first attestation is uncertain. It shares its root with everyday words such as 'bore', 'burrow', and compounds like 'borehole', all tracing back to the Germanic drilling verb at the heart of 'bore'.
Related word forms
How counterbore is actually used
Counterbore belongs firmly to technical registers — engineering drawings, machining, woodworking, and metal fabrication. It appears both as a noun (the recess or the tool) and as a verb (to counterbore a hole), with verb forms 'counterbored' and 'counterboring'. On engineering drawings it is often abbreviated '⌴' or written as 'C'BORE'. It carries no figurative or informal usage.
Easily confused with counterbore
A countersink is a conical (cone-shaped) recess for flat-head screws, whereas a counterbore is a cylindrical, flat-bottomed recess for cylindrical fastener heads like bolts and socket screws.
A spotface is a shallow flat facing cut on an uneven surface to provide a bearing seat, while a counterbore is a deeper cylindrical enlargement of the hole itself.