I was dumb enough to state recently on a thread about drive chains that I had no trouble using an O-ring chain on my 550, despite dire warnings from members about this wider chain chewing up the case, seals, etc. Good thing for me I was just smart enough to realize that SOHC gurus like TwoTired know what they are talking about. I resolved to go back to a standard chain, even though I was sure I had no damage to that point.
Tonight I removed the drive sprocket to check, and lo and behold there is a score mark on the case. Fortunately it measures only 0.2 mm. on a depth gauge, but no doubt it would have gotten much worse as this relatively new chain wore and was free to move more side to side. I'm more worried about the seal to the left, which no doubt thinner and more vulnerable than the case.
In any case I am enlightened, and my deepest gratitude to this board for hopefully saving me from catastrophic damage to my machine.
