A robotic surgical instrument, comprising: a shaft; an articulation at a distal end of the shaft for articulating an end effector, the articulation being driveable by a pair of driving elements; and an instrument interface at a proximal end of the shaft, the instrument interface comprising: a chassis; an instrument interface element slideable along a guide bar for driving the pair of driving elements, wherein the pair of driving elements are fast with respect to the interface element so that a displacement of the instrument interface element with respect to the guide bar is transferred to the pair of driving elements; the chassis comprising a support element configured to interface the guide bar along at least a portion of its length; and a securing element for retaining the guide bar against the support element to thereby secure the guide bar to the chassis.