GD&T Symbol Guide
Counterbore
Defines a flat-bottomed cylindrical recess coaxial with a smaller hole.
Definition
A counterbore is a larger, flat-bottomed cylindrical hole aligned with another cylindrical hole.
It provides a recessed flat mounting surface, commonly for socket head cap screws.
Application
List the counterbore symbol with the counterbore diameter and depth below the smaller hole size.
Use it when the fastener head must sit below or within the part surface.
Counterbore Drawing Use
Counterbore is read with the adjacent dimension, note, or feature control frame so inspection uses the intended feature definition.
3D Tolerance Zone
This symbol defines or modifies a size, shape, or manufacturing dimension rather than creating a GD&T tolerance zone by itself.
Inspection Method
Read the symbol with the attached dimension, note, or feature control frame before evaluating the feature.
Verify the controlled feature using the drawing's stated limits, datum setup, and any special notes tied to the symbol.
Worked Read: Counterbore
A drawing includes the Counterbore symbol with a related feature requirement.
Identify
⌴
Recognize the Counterbore symbol and the feature it modifies.
Read
Adjacent value
Use the attached dimension, note, or frame cells as the controlling requirement.
Inspect
Per drawing
Measure the named feature using the specified interpretation rather than assuming a generic size check.
Counterbore clarifies how the feature is defined so the drawing and inspection method stay aligned.
Comparison Table
| Item | Controls | Typical Placement |
|---|---|---|
| Counterbore | Dimension interpretation | Next to a dimension, note, or frame |
| GD&T control | Geometric variation | Feature control frame |
| Datum reference | Measurement setup | Datum feature or frame cell |
Notes
This page is part of the drawing-symbol reference set rather than a standalone geometric tolerance category.
When the symbol modifies a feature control frame, read it together with the tolerance value and datum references.