I bought a 2005 YZ250 about 6 mos ago, the bike needed quite a bit of work. After one ride with what seemed like not full power, I found excess crankshaft play on the flywheel side and pulled it apart. I put in a new Hot Rods crank, and found to governor (for power valve broke) and fixed it. The cylinder measured big, just out of specs. I just went with a new ring. After about 6 rides the piston failed (exhaust side missing piece above top ring & ring broke). The cylinder was not damaged so I replaced piston & rings. About another 6 - 8 rides almost exact same piston failure. These two failures happened at a very sandy track. This time I sent the cylinder to US Chrome, had it repaired & re-coated. I did notice a slight sticky-ness with the power valve moving in the cylinder, but I thought the spring forces were strong enough to overcome it. 1st ride at a hard rough surfaced track. Bike ran well. I checked the plug afterwards - half of insulator was reddish brown, the other half a little lighter. I talked to a person at the track about this (who I trusted). He thought it sounded like pre-detonation and reccommened using some higher octan gas. Before my next ride I mixed my normal 93 octan gas 3 gallons with 1 gallon of 110 octan race gas. I thought this would give me some cushion in case it was pre-detonation. 2nd ride at my favorite sandy track. Bike ran strong until about 3 laps I thought I noticed a flattening of power so I came in. I pulled the plug, it was white with a few shinny dots on the electrode. I did not have a bigger main jet w/ me so I lowered the clip on the neddle two knotches and went back out. It seemed like it wanted to load up, but then by the end of the first lap just ran worse, I pulled in and l went home. The piston & cylinder are damaged. The piston has the start of two small holes in the crown just over the ring on the exhaut side. I think it is the same failure I had before, just I caught it sooner (but not soon enought). Also the power valve was stuck in the 7/8's closed position, this is new and might have to do w/ the re-plating. Any thoughts ideas would be appreciated.