• synonyms


[koun-ter-bawr, -bohr; koun-ter-bawr, -bohr]
  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.
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verb (used with object), coun·ter·bored, coun·ter·bor·ing.
  1. to enlarge (a drilled hole) along part of the length of the hole by drilling with a bit having a greater diameter.
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Origin of counterbore

First recorded in 1605–15; counter- + bore2
Related formscoun·ter·bor·er, noun
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for counterbore

Historical Examples

  • If taken off at all it must be pulled, for the dowel-pins, key and counterbore are close fits.

    Steam Turbines

    Hubert E. Collins

  • For enlarging a hole for a certain distance the counterbore is employed.

  • A counterbore in the arbor hub provides clearance for the hub of the pulley which projects beyond the rim on one side.

    Turning and Boring

    Franklin D. Jones

  • For counterboring small holes or for facing the metal around their ends, the form of counterbore shown in Fig. 1746 is employed.

  • That it may always do this and yet leave sufficient clearance, the counterbore has been devised.