A holder for a collar having an axially forwardly directed shoulder and axially rearward thereof a radially outwardly directed centering surface has a drive sleeve fitted over the collar and a plurality of angularly spaced jaws each formed with an axially rearwardly directed face and displaceable between an outer position with their faces bearing on the shoulder and an inner position with their faces out of engagement with the collar. An axially displaceable actuating member can move the jaws between their inner and outer positions. A plurality of retaining fingers each have an axially rearwardly directed face and axially rearward thereof a radially outwardly directed centering face. These fingers are each radially elastically deformable between an outer position with the front-end faces bearing axially rearward on the shoulder and the centering faces bearing radially outward on the collar centering surface and an inner position out of contact with the collar.