i have had my 660 for about a year and a half, have done about 12,000 kms on it, around 300-350 hours and have had no trouble at all. I race it off and on and ride fairly hard on the road so it gets a hard time and cant complane at all. There are other things that break and vibrate off but the motor has been sweet. I pulled down the top end just to inspect the wear on the piston and bore and it was all good, may throw a tensioner and chain at it soon but just as a preventative not because it needs it, big singles tend to be be quite hard on chains etc...