Abstract: Improved back reamers (20, 60, 80, 100) are disclosed for use in back reaming a pilot bore in a trenchless boring operation. In three of the heads (20, 60, 100), truncated cylinders are mounted on a body (22) of the head with the truncated cylinders each defining a convex truncated surface facing the walls of the bore. Rotation of the head while moving the head in the direction to be reamed, the convex truncated surface will compact the soil into a wall of the bore of desired final diameter and stabilize the wall. A back reamer (80) is provided with a plurality of directional cams (84) which permit the head to be oscillated in either direction about the drill string axis. The heads can be reversed to act as drill bits as well.