Abstract: The boring bar of a boring plate lathe is provided with a device for maintaining the concentricity of the boring bar within the workpiece. This stabilizer assembly consists of a guide mounted on a piston shaft which is resiliently mounted in a cylindrical housing. The exterior surface of the housing is threaded and is provided with an adjustment nut. The entire stabilizer assembly is fitted into a cavity in the tool head to a depth limited by engagement with the adjustment nut. The stabilizer assembly is positioned on the tool head to enable the guide to extend radially outward to engage the work surface. The resilient force which is exerted by the guide on the work surface can be set by rotation of the adjustment nut to extend or retract the housing in the tool head cavity.