Hmmm, I fear the open ported cylinder is starting to take its toll on the piston skirt. Note how good the piston skirt looks below the port window in the skirt ... and how it is showing skuffing on either side of the clear part of the skirt? Like many engine designs these days, the piston is not well supported on that side. Once wear starts, it will only progress from there. I believe there are more open ported cylinders these days as a compromise between ease of production and desired performance. In the process, piston wear is accelerated.