inc the width of the transducers 24 are preferably maximized to fit into the bolt holes 20 with minimal separation between elements the height of each transducer 24 is preferably equal to or greater than its width in order to promote the signal to noise response from a defect reflector in the configuration shown in fig2 and 3 the transducers 24 are arranged on the transducer unit 22 as a linear array and as such can share a single pulser the transducers 24 produce a longitudinal wave at a fixed angle of zero degrees normal to the surface of the wedge 26 on which the transducers 24 are mounted groups of the transducers 24 can be pulsed simultaneously or multiplexed to simulate movement in the axial direction the entire group of transducers 24 can be rotated to provide radial coverage along a loci 40 of points 36 fig5 according to one aspect of the invention the position of the transducer unit 22 for proper orientation of the transducers 24 is determined so that the ultrasonic signals generated by the transducers 24 intersect axial radial type defects throughout the volume of the wheel 10 between the bolt holes 20 and the hub bore 16 as shown in fig5 and 6 in fig5 the center to center distance between each bolt hole 20 and the hub.