I have nearly two years racked my head, where the grooves in the piston shirt and the lower cylinder come from. Even after meticulous cleaning of the entire intake area, the situation was the
same. I always polished the grooves with 600 emery cloth to see how the cylinder looks like the next time. Always the same sad picture.
The decisive tip came from Marco Böhmer: Oil carbon in the exhaust pipe is the reason. It is pushed back by the reflection wave is the cylinder.
I put a pound of M6 and M8 screws and shacked it a while. A hand full oil carbon came out of the pipe. Finally the exhaust pipe was blown out with compressed air. After that the groove issued has gone.