A thermal printer mechanism includes a print head assembly for locating a print head at a predetermined optimum position for printing or at a cleaning position and an adjustable guide path assembly for maintaining the center line of print stock aligned with the centerline of the print head for different widths of stock. The print head assembly includes a print head supported by a mounting bracket so that the head is maintained in a predetermined fixed position with respect to the platen shaft. A head force bracket sums the force from a pair of springs and applies it via a pivot shaft to the midpoint of the line of tangency of the print head with the platen roller so that the print head "floats" on the roller. The print head assembly can be pivoted from the printing position to a cleaning position by changing the positions of pins in a pair of camming slots. The print stock guide path assembly includes left and right guide racks engaged by a common pinion so that the width of the guide path may be easily adjusted while maintaining the center line of the guide path aligned with the center line of the print head.