About Park Tool Derailleur Hanger Alignment Gauge - Park Tool #DAG-1: The Derailleur Hanger Alignment Gauge can help solve a multitude of derailleur shifting problems, most notably with index shifting systems. This tool is extremely steady in operation and is simple to use. The tool threads directly into the derailleur hanger. The gauge bar holds a gauge which marks the distance to the wheel. The gauge then is rotated with the wheel to the 3, 6, 9, and 12 o-clock positions, noting any differences in measurements. The alignment of the derailleur hanger is accomplished by apply.