A number of nuclear power plant steam generators have been troubled with corrosion buildup between the tubes and supporting plates. A vibration type probe which traverses the interior of the tubes during plant shutdown has been developed to inspect the tube and support crevices. The probe incorporates a miniature electric vibrator and accelerometers to induce and measure tube motion at the support plate junctions. Laboratory investigation indicates that clearance gaps ranging from 1 to 30 mils can be accurately and rapidly measured even in the presence of tube misalignment and varied tube support conditions. The probe system is also suited to in-situ measurements of heat exchanger tube vibration characteristics.