Abstract: Apparatus for insertion into a pipe adapted for mounting an ultrasonic flow measurement transducer, the apparatus comprising, in a preferred embodiment two flanges adapted to connect into a pipe, a center section connected between the two flanges, the two flanges having central openings therein for fluid flowing through the pipe, the center section being hollow for the flow of fluid from the pipe therethrough and communicating with the pipe through the openings in the flanges, the center section having a substantially square or rectangular cross-section, the center section having an outer substantially flat surface adapted for mounting of at least one pair of ultrasonic transmitting and receiving flow measurement transducers thereon.