When inspecting the hole, what should fit completely through the assembly?

The correct answer is a go pin. In assembly mechanics, especially when verifying the quality of a hole in a component, a go pin is used as a standard for determining whether the hole meets the necessary specifications. The go pin is designed to have a specific diameter that exactly matches the minimum allowable size for the hole.

When the go pin fits completely through the assembly, it indicates that the hole is adequately sized and free of obstructions, which is crucial for ensuring proper assembly and function of the aircraft component. This test demonstrates that the parts can be aligned correctly without any interference.

Other options do not serve the same purpose in assessing hole size and quality. A test block might be used for various quality checks but does not specifically indicate hole fitment like a go pin. A measurement rod typically measures distances but would not verify the pass/fail criteria for hole dimensions. A drill bit is primarily a tool for creating holes, not for checking their dimensions after they have been made. Thus, a go pin is the appropriate choice for ensuring that a hole is correctly sized for assembly operations.