A robotic surgical instrument comprising: an end effector articulation at a distal end of a shaft, the articulator being driveable by a pair of driving elements； and at the shafts proximal end an instrument interface. The instrument-interface comprising: an interface-element that is slideable along a guide bar 930； a chassis comprising a support that interfaces the guide-bar 930 along a portion of its length； and a securing element 902A,1403 for retaining the guide bar 930 against the support to secure the guide-bar 930 to the chassis. The driving-elements are fast with the interface-element, so that displacement of the interface-element with respect to guide-bar 930 is transferred to the driving-elements. The support may comprise a curved surface. The support may be a corner 903A, where securing element 902A retains guide-bar 930 in the corner 903A. Corner 903A may be defined by two surfaces 1001,1003. Surfaces 1001,1003 may: be transverse to one another； have an angle between them of less than 180 degrees. Securing element 902A may comprise a shaft portion 1007 and a conical-shaped head 1009, the shaft 1007 being inserted into the chassis to secure guide-bar 930 to the chassis. Securing element 902A is preferably a countersunk screw.
