A portable rock drill rig includes a pair of telescoping struts for supporting and positioning a mast or feed beam. The struts are connected at their upper ends to a support bracket which is connected to an elongated mast support member by a hydraulic cylinder to provide relative sliding movement between the bracket and support member. The drill rig includes a crawler undercarriage having spaced apart and pivotally mounted crawler assemblies. The lower ends of the struts are each connected to an arm which is fixed to the main frame of the undercarriage. The arms are also connected to respective track oscillation cylinders. The mounting arrangement of the struts provides for improved positioning of the mast for drilling, tramming and transport operations.