A stir friction welding tool with a variable length pin relative to the shank of the tool is disclosed. The pin is moveable from a totally retracted position relative to the shank such that the shank may engage a workpiece without the pin engaging the workpiece. The pin then may be slowly extended into the work piece. The stir friction welding tool may also have a variable diameter shank to provide a variable volume cavity. Then as the pin is extended further into the workpiece the variable width shank can expand to increase the circumference of the shoulder and the volume of the cavity in the shank such that the pin length to cavity volume are within a range to insure a good surface material flow and a high weld quality using the stir friction welding process.