Symbol: 

Definition:

The counterbore symbol is used on a drawing to indicate that a counterbore hole feature is required. A counterbore is a flat-bottomed cylindrical hole that is larger than and coaxial to another cylindrical hole. It is used to provide a flat and recessed mounting surface for mating parts. The hardware most commonly used with a counterbore is a socket head cap screw.

Application:

Counterbore dimensions are indicated on the drawing directly below the dimensions of the smaller coaxial hole, as shown in the model below. The counterbore dimension contains the counterbore symbol, the diameter of the counterbore, and the depth of the counterbore.

In the example below, the drawing calls out five thru holes of a diameter between 0.531 and 0.546, each with a counterbore having a diameter of 0.813 to 0.828 and a depth of 0.520 to 0.540.