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This paper presents the application of a new hydroelectric generator air gap measurement system. It is a continuous on-line system using optical triangulation, fiber-optics, a microprocessor-based monitor, specially developed instrumentation and power supplies. This system is non-invasive, insensitive to EMI and has 0.2% accuracy over a range of 2 to 40 mm. The paper presents air gap data obtained from B.C. Hydro's Mica Creek Unit G2 (457 MVA) during March 1986. Readings were taken from stator-to-rotor and from rotor-to-stator to determine the unit's characteristics prior to rotor rim shrink modifications. This data assisted in determining the individual contributions of the rotor and the stator to the generator's behavior and provides a comparison for post-modification testing.