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Just did my first piston/cam chain replacement on the 250F today. I've had the bike 2 seasons, and I don't know how many hours were on it when I bought it, so better safe than sorry.
It was a LOT of work compared to what I'm used to with the 2 strokes. It wasn't hard, just involved many steps including removing the flywheel. I don't have those tools, so I had to make 2 trips to my local dealership to borrow the correct stuff.
The old piston was a wiseco (same as the one I bought) and it's in fantastic shape. Probably useable for a bunch more hours, but that's why they call it "preventative" maintenance. The cam chain was pretty stretched and is the real reason I did this job. The bike was running a little rough and it was down on power because of it.
Don't have time to start it up today, but there are no bolts leftover, so I know I did it right