An ultrasound probe assembly comprises a housing and a wedge, wherein wedges configured for pipes of different diameter may be easily interchanged in the assembly. Four wheels are attached to the housing, there being a front wheel pair and a rear wheel pair. Wheels of each pair are positioned on either side of a linear probe array, wherein the distance between wheels in each pair in a direction perpendicular to the array length is as small as possible. A position encoder monitors the position of the assembly during scanning, and a push lock switch is used to disable the encoder and the data acquisition while indexing to a new scan position on the pipe.