Mountings for boring tool bits

A mounting for a pair of boring tool cutting bits, the mounting being such that the two cutting bits, when present, project beyond a forward end of a mounting member and are displaced through 180.degree. relative to each other, each being mounted on a respective cutting bit carrier which is radially adjustable relative to and guided on a toothing on the mounting member, the said toothing extending in the direction of adjustment at right angles to the rotary axis of the mounting member and having parallel teeth, by means of a similar toothing and which can be fixed relative to the mounting member by means of a clamping screw passing through an oblong hole in the cutting bit carrier and engaging with the mounting member, there also being provided for the radial adjustment of the cutting bit carrier, in each case, an adjusting screw engaging with the carrier and extending in the direction of the toothing, and wherein the toothing for the two cutting bit carriers is provided on the forward face of the mounting member in a common radial plane with continuous teeth running across the whole breadth of the forward face, and the cutting bit carriers have a similar toothing on their rear ends facing towards the said forward face, and that the clamping screws are arranged parallel to the said rotary axis.