Hi Bellhopper, I wonder if there might be some rust developing in the muffler as the saw sits and then it's being sucked into the cylinder when running the saw thus scoring the piston . I have also seen mufflers that are opened up like that one is with the short straight forward ports , in dusty storage environments (work shops, open storage shed's) can collect particles in the muffler and over time that gets ingested into the cylinder.