What is the special cutter called that is used to create holes for fluid-tight fasteners?

The term for the special cutter used to create holes for fluid-tight fasteners is indeed the counterbore and countersink cutter. This tool is specifically designed to facilitate the installation of fasteners that require a precise fit to ensure fluid-tight seals.

The counterbore feature creates a larger diameter hole that allows the head of the fastener to sit below the surface of the material. This is essential in applications where a smooth surface is necessary to prevent fluid leakage. Additionally, the countersink aspect of the cutter adds a bevel around the top of the hole, allowing the fastener to fit snugly and securely. This dual functionality is critical in aerospace applications where precision and reliability are paramount.

In comparison, other tools mentioned serve different purposes. A countersink cutter only provides the bevel and doesn't expand the diameter of the hole, while a reamer is used for finishing drilled holes to achieve a specific diameter and improve surface finish but doesn't create a dual feature for fasteners. A pilot drill is primarily for creating a guide hole for larger drill bits and does not contribute to the fluid-tight fitting needed for fasteners in aerospace structures.