👉 A counterbore is a type of bore that has a small diameter, typically between 0.25 and 1 inch (64 to 3.8 mm), and is used in drilling applications where the size of the hole must be controlled for safety reasons or because it is difficult or impractical to use larger holes. Counterbores are often used in hydraulic or pneumatic systems, where the size of the bore is critical for proper operation and maintenance.