Counterbore Data & Size Chart

Carr Lane Mfg. Co. is an industry-leading provider of counterbores for drill bushings and many other vital manufacturing products. Since 1952. Carr Lane has provided innovated engineering products and solutions to a large variety of industries.


What is a Counterbore?

Counterbores are cylindrical flat-bottomed holes that enlarge another coaxial hole. A counterbore can also refer to the tool used to create these holes (given the term counterbore cutter). The hole is typically used when a fastener is required to sit flush with or below the level of a workpiece's surface. It is also used to create a perpendicular surface for a fastener head on a surface that isn't.


What is the Counterboring Process?

The process creates a cylindrical cavity with a larger diameter at the head of a pre-drilled conical hole. The pre-drilled hole is meant to contain the fastener and prevent the two work pieces from separating when their two flat surfaces are joined. The larger cavity matches at least the width and depth of the head of the fastener.

This procedure most commonly uses a flat-bottomed-head screw to hold the object together. The screw will have a head that is below the surface of the piece being fastened. There are two cutting radii on the tool. One radius creates the counterbore hole, and the other radius creates the recessed cavity inside the material.

The fastener can be placed anywhere between flush with the surface of the material and below the surface so that the head will not interfere with the design.


How is a Counterbore Measured?

Measurements can include counterbores that contain the diameter of the hole and material surrounding it. This is measured by either how deep the counterbore is or how thick the remaining material is.

The counterbore can be measured with different types of gauges, the simplest being a caliper. Other types of measuring equipment, such as pocket comparators, gage pins, and depth micrometers, are also used in measurement.


Counterbore vs Countersink/Spotface

While they are similar, a countersink has an angled bottom, whereas a counterbore has a flat one. This angled bottom on countersinks can vary slightly.

In addition, a counterbore and spotface have many similarities. The former is used to recess a fastener while the latter is used to create a flat surface located allow a fastener to be used. In essence, a spotface is a shallow counterbore.


Which Type Of Counterbore Do I Need?

There are several types to choose from, depending on your type and length of your drill bushings. The below chart illustrates counterbore compatibility with Carr Lane drill bushings of various sizes.