Abstract: The present invention is a method of detecting and sizing of a small crack in a root between two crests in stud bolt threads. The key idea is from the fact that the Rayleigh wave is detected between large regularly spaced pulses from the thread. The delay time is the same as the propagation delay time of the slow Rayleigh wave and is proportional to the size of the crack. To efficiently detect the slow Rayleigh wave, three methods based on digital signal processing are proposed; modified wave shaping, dynamic predictive deconvolution, and dynamic predictive deconvolution, and dynamic predictive deconvolution combined with wave shaping.