A boring bar and drill stem insert for boring tools which are positioned at opposite ends thereof and has a transverse recess with a rectangular cross section opening toward the front face of the boring bar and extending at right angles to the axis of the boring bar to accept a drill stem insert of identical cross section. A clamping screw is provided at right angles to the transverse recess and is adapted to clamp the insert in the boring bar. A centering body, for centering the insert, engages a side face of the insert. A centering wedge is provided having a trapezoidal cross section and constitutes the centering body and is received partly in a groove in the front face of the boring bar insert. Both grooves are of a trapezoidal cross section and the wedge can be forced into the grooves by means of a screw extending in the axial direction of the boring bar.