Abstract: The invention relates to an extrusion installation for elongate products, more particularly plastics tubes, having an extruder 1, followed by a water-cooled calibration sleeve 8 which can be acted upon by negative pressure and a wall thickness measuring device having an ultrasonic sensor 13. The characterizing feature of the invention is that the calibration sleeve 8 is preceded by a retaining device 10 by which the at least single ultrasonic sensor 13 is borne fixed or rotatable concentrically in relation to the elongate product with a predeterminable distance between its measuring head and the external surface of the elongate product 23, and the retaining device 10 has a releasable attachment sleeve 12 whose diameter so corresponds to the inlet side internal diameter of the calibration sleeve 8 that the attachment sleeve 12 performs a precalibration.