A dual rod drilling system having a coupling system for a dual rod drilling system comprising: a coupler (118, 310, 618) comprising: an inner bore (133, 330) having a non-circular profile and a longitudinal axis; a cross aperture (135, 332) having an axis that is perpendicular to the longitudinal axis, the cross aperture having a first width; an inner member (116, 306, 312) comprising: a torque-carrying section (121, 326, 338) having a non-circular profile adapted to mate with the non-circular profile of the inner bore of the coupler, the torque-carrying section (121, 326, 338) having a maximum width; a non-torque carrying portion (125, 328, 340) having a maximum width less than the maximum width of the torque-carrying section (121, 326, 338); and a groove (123, 320, 318) positioned between the torque-carrying section and the non-torque carrying section, the groove having a width at least equal to the first width of the cross aperture of the coupler; and a pin (129, 316) positioned within the cross aperture of the coupler and within the groove of the inner member so as to secure the inner member at least partially within the coupler, wherein the coupler includes a main body having the inner bore and a sleeve (137, 637) positioned around an exterior surface of the main body, the sleeve having at least one drilling fluid flow passage. a coupler (118, 310, 618) comprising: an inner bore (133, 330) having a non-circular profile and a longitudinal axis; a cross aperture (135, 332) having an axis that is perpendicular to the longitudinal axis, the cross aperture having a first width; an inner bore (133, 330) having a non-circular profile and a longitudinal axis; a cross aperture (135, 332) having an axis that is perpendicular to the longitudinal axis, the cross aperture having a first width; an inner member (116, 306, 312) comprising: a torque-carrying section (121, 326, 338) having a non-circular profile adapted to mate with the non-circular profile of the inner bore of the coupler, the torque-carrying section (121, 326, 338) having a maximum width; a non-torque carrying portion (125, 328, 340) having a maximum width less than the maximum width of the torque-carrying section (121, 326, 338); and a groove (123, 320, 318) positioned between the torque-carrying section and the non-torque carrying section, the groove having a width at least equal to the first width of the cross aperture of the coupler; and a torque-carrying section (121, 326, 338) having a non-circular profile adapted to mate with the non-circular profile of the inner bore of the coupler, the torque-carrying section (121, 326, 338) having a maximum width; a non-torque carrying portion (125, 328, 340) having a maximum width less than the maximum width of the torque-carrying section (121, 326, 338); and a groove (123, 320, 318) positioned between the torque-carrying section and the non-torque carrying section, the groove having a width at least equal to the first width of the cross aperture of the coupler; and a pin (129, 316) positioned within the cross aperture of the coupler and within the groove of the inner member so as to secure the inner member at least partially within the coupler, wherein the coupler includes a main body having the inner bore and a sleeve (137, 637) positioned around an exterior surface of the main body, the sleeve having at least one drilling fluid flow passage.