Abstract: The device includes a blade (1), one end of which carries a testing probe (2). The probe (2) includes a first ultrasonic transducer (23) constituting an emitter and a second ultrasonic transducer (24) constituting a receiver, which are pivotably mounted at the end of the blade (1) about two axes parallel to the longitudinal direction of the blade (1), arranged separated from one another in the transverse direction of the blade (1), so as to be placed in a narrow annular passage, in contact with a cylindrical wall to be tested, in two positions separated from one another in a circumferential direction of the annular passage.

Abstract: The process comprises the steps of calculating, by varying the opening angle of a cluster, a first group of reference points of x, z coordinates in an incidence plane of the beam, so that the times taken by the ultrasounds to pass through the reference points and through the focal point are identical, then calculating a second group of reference points of y, z coordinates in a plane perpendicular to the incidence plane, defining two polynomials of degree 4 by polynomial interpolation by the method of least squares of the two previously calculated groups of points of the following form: f(x)-ax.sup.4 +bx.sup.3 +cx.sup.2 +dx g(y)=a'y.sup.4 +b'y.sup.3 +c'y.sup.2 +d'y, determining the x, y and z coordinates of the points of the output surface by the relation z=f(x)+g(y), and producing the output surface by means of the coordinates.