Abstract: Apparatus is disclosed for use in detecting flaws in the bolt hole area of railroad rails and the like, and is adapted to detect and record the location of an oversized hole indicating a crack or the like. The apparatus utilizes an ultrasonic transducer detector which is actuated by a free running pulse generator, and includes a counter and associated circuitry which compares the time required for the pulse to travel through the rail with a preset time and, if the rail has no flaw, causes an output to be produced that is used to reset the counter. The counter is also triggered by a speed related pulse generator that effectively measures the distance traveled by the car along the rail and thereby measures the size of the flaw. A flaw exceeding a predetermined size which is approximately the diameter of bolt holes, will trigger an alarm and cause a recorder to be activated.