What tool should be used to clean threads filled with mortar in the header?

Using a tap is the appropriate method for cleaning threads filled with mortar in the header. A tap is designed to create or restore threads in a material, allowing for proper engagement of screws or bolts. When threads are obstructed by mortar or debris, using a tap can effectively remove the obstruction and re-establish clean, functional threads.

In this context, the other tools mentioned are not suitable for thread cleaning. A chisel is primarily used for cutting or shaping materials rather than cleaning fine threads. A screwdriver is intended for turning screws and would not interact with threads in a way that cleans them. Pliers, on the other hand, are used for gripping and manipulating objects and do not have the precise functionality needed to clear threads accurately. Thus, the tap's specific design makes it the best tool for this task.