It's a bit difficult to tell from the photo, as it's quite dark, but it looks like you may have excessive wear on the piston teeth. If so, it may be that the gearbox isn't shimmed properly, or the holes in the bushing for the Sector gear are worn, allowing the gear to wobble and not make clean contact with the piston. It could also be that the Sector gear isn't meeting the first (back) tooth correctly, as it looks like the back tooth is worn away, in which case you need to set the length properly by adding a spacer between the piston head and piston, or adding one to the inside of the cylinder head. Either way, if the piston teeth are badly worn you'll need to replace the piston.
