I wouldnt even want to guess the amount of hours Ive got on my 08 yz450. Ive had it for a year, and I ride 2-3 days a week, I would think a safe bet would be to say I put 4-6 hours a week on it. I do change the oil every time I ride however. So, prob 200 plus hours on my motor, I havent even lifted the valve cover, I know i know, but it still starts fine. Im only pulling it apart to put in the piston I got from Ron Hamp and to check it out. So my valves were still in spec, every single one of them. Ti valves look brand new after getting blasted, seats are perfect. Only weird thing, I always use a hammer and a small socket to get the cotter out of the retainer to get the valves out, and when I did this, two of the valve springs broke, they were very very very brittle. I must have somehow side loaded them a bit, but careful inspection shows I didnt even nip the side of the machined spring cup in the head. Kinda worried me as I would think that any kind of operation would be more pressure on the spring than what I did to remove the valve cotters. No matter, new springs from yamaha and its back together, with lots of compression and a bark like a factory yamaha....Just doing some heat cycles and then I'll put her on the dyno. Changed the clutch and frequent oil changes (mobil1 15-50) obviously help, Im an abuser and a deep sand railer, and the clutch was still decent after a year, threw in a new one for the freshened motor. After going through this bike, I am realizing how nicely and carefully designed this chassis and engine is. There are reasons its so smooth and quiet, and there was much thought that went into every aspect of this motor. I am very impressed with yamaha quality and engineering.