first hand knowledge, i'm not exactly sure why the 03 and 04's were such a problem, but keeping belts on is like pullin teeth, mostly because the tensioners won't stay true no matter what, after 04 they were beefed up somewhat, but not fixed completely, chopper belt still comes off, o and i forgot to mention if it has chaff spreaders on it along with a chopper it WIll NOT spread corn chaff, those are only for small grain, challenger_ massey can not make them spread corn chaff. we had 3 only 3 and now none, they seem very well built, solid, easy to work on machines. But the problem is they need to be easy to work on because you do it so dammed often. Im not trying to put these machines down as they will alllll break down with time but this is my personal experience with 660_670 combines. Hope i didn't sound too pessimistic, and best of luck with what ever you decide to do!!!