Abstract: A bolt tensioner has a cylinder and piston assembly whose piston is connected to a puller bar on which is mounted a socket formed with segments for connecting the puller bar to a bolt to be tensioned, and the inner surface of a sleeve reciprocable on said puller bar and the outer surfaces of said segments have coactive circumferential ridges and grooves with beveled sides to actuate and hold the segments into cooperative relation to the bolt and means to actuate the segments out of said cooperative relation upon movement of the sleeve into lower and upper positions, respectively. A reciprocable power operated apparatus connected to the upper end of the sleeve reciprocates the sleeve.