What tool is primarily used to measure the ring end gap?

The feeler gauge is the correct tool used to measure the ring end gap in engine cylinders because it consists of a set of thin, flat strips of metal with precise thicknesses. To measure the end gap, the feeler gauge is inserted between the ends of the piston ring when it is placed in a cylinder, allowing the technician to determine the gap by selecting the appropriate thickness that fits snugly within the gap. This measurement is crucial for ensuring proper engine performance, as the end gap affects the sealing of the combustion chamber and the efficiency of the engine.

Other tools, while useful for different measurements, are not intended for this specific task. A plug gauge is typically used to inspect the diameter of holes or the sizes of various components. A wire gauge is primarily meant for measuring the diameter of wires, and a micrometer, although precise for measuring thickness or dimensions, is not ideal for measuring the gap between the ends of a piston ring. The feeler gauge stands out because it allows for the flexibility and variety in thickness necessary to accurately assess the end gap in various engine configurations.