counterbore

[koun-ter-bawr, -bohr; koun-ter-bawr, -bohr]
noun
1.
a tool for enlarging a drilled hole for a portion of its length, as to permit sinking a screw head.
2.
the portion of the hole so enlarged.
verb (used with object), counterbored, counterboring.
3.
to enlarge (a drilled hole) along part of the length of the hole by drilling with a bit having a greater diameter.

Origin:
1605–15; counter- + bore1

counterborer, noun
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Example sentences
The cage is positioned on a gasket in a close- clearance counterbore in the valve body.
The split shaft collar is recessed into a counterbore, which is cast into the bottom of the triangular plate.
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