Grumpy,
What I find interesting is the PATTERN of scoring/damage of the piston. On the intake side, you only see two major scoring streaks right along where the little pins that keep the rings from rotating, and the scoring goes right to the skirt of the piston. The center is pristine clear with no marks.
At the other end (exhaust side) however, it is scored throughout (the exhaust port had a bit of carbon deposits which might have fallen inside and caused damage).
The piston rings are definitely damaged on the exhaust side; I will replace them.
The cylinder score would better be described as a barely noticeable scratch/roughness felt with your fingernail. It does reach the bottom of the cylinder skirt. However, I don't see any blow-by below the top piston ring...
I use this trimmer 3 times a month for 15mins at home. So I guess I'll go with your advice and polish the piston, but I wonder what product should I polish it with?
