I am sorry but didn't collect pictures. I have decarbon'd my 503 many times, and only my 582 once. The first time I did the used 503 it was above 200 hours and it took considerable time and effort. The rings were not stuck but sludge was plentiful. I have no idea what kind of oil the previous owner used. The crown of the pistons were black but not much buildup. The combustion chamber was black with wet-looking buildup several thou' thick.
Underside of the crown was a baked on dark brown buildup that was dense and heavy and probably affected the balance of things. I used a dental excavator to slowly remove buildup everywhere except ring grooves, rings, and any surface that slides up and down the cylinder wall. The contact edge of the rings were not touched at all. The sides of them were hard to clean and I wore a 10x loop and DID NOT use sandpaper. As I said I used a shoestring with polishing compound to clean the grooves. The first time took almost 2 days. Then the 50 hour cleanings took almost 1/2 day. That includes everything from previous flight (dirty) to first flight (clean). I always used the same rings and pistons. The pistons and rings got replaced once when the wear limits were exceeded. On that occasion I did the Rotax break-in procedure like the engine was new. If you are a wrench-head already, you will know this but ALWAYS catalog parts at disassembly and put them back exactly as removed and in the same orientation.
