Counterbore
Glossary · Part Features
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A counterbore is a flat-bottomed enlargement at the top of a hole that lets a bolt head or washer sit flush with or below the surface.
In practice
Cut with a piloted tool so the enlarged section stays concentric with the hole beneath it. The flat bottom gives the fastener head a square seat.
Why it matters
A counterbore whose bottom is not square to the hole loads the fastener sideways. On a bolted joint that sees vibration, that is where the bolt eventually fails.
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