Abstract: A thrust system for a horizontal directional drill is provided. The thrust system includes a drill boom having a drive sprocket positioned at the first end thereof and a driven sprocket positioned at the second end thereof. A carriage for supporting the horizontal drill is slidably received on the drill boom. The carriage includes idler sprockets positioned between the drive sprocket and driven sprocket in the drill boom. A serpentine chain extends about the drive sprocket, the driven sprocket and the idler sprockets in order to provide greater thrust and pullback of the drill at given torques. Rotation of the drive sprocket in a first direction draws the carriage towards the first end of the drill boom, and rotation of the drive sprocket in a second direction draws the carriage towards the second end of the drill boom.