What tool is used to modify a pre-drilled hole for a fluid-tight fastener?

The tool used to modify a pre-drilled hole for a fluid-tight fastener is a counterbore and countersink. This tool allows for the creation of recesses that enable the heads of screws or other fasteners to sit flush with or below the surface of the material.

When installing fluid-tight fasteners, it is crucial to ensure that the fastener is properly seated to maintain a seal that prevents fluid leakage. The counterbore is used to enlarge the hole, while the countersink creates a conical shape at the top of the hole, allowing for a smooth transition to the fastener head. This ensures that the fastener can be torqued correctly and provides an optimal sealing surface.

The other tools mentioned do not serve this specific function. A drill bit is primarily used for creating holes rather than modifying them for fluid-tight applications. A rivet puller is designed for the installation of rivets, which is not the same as modifying a hole for fasteners needing a tight seal. A chisel is used for cutting or shaping materials but would not provide the precise modifications necessary for fluid-tight fasteners. Thus, understanding the role of the counterbore and countersink is essential for ensuring proper fastener installation in applications where