I've just totalled the top end of my '02 WR426 and am hoping to get the bottom of what caused the problem so I don't rebuild it and it just happens again. I went for a ride last weekend and after about five minutes it started feeling a bit gutless. At first I thought the clutch might have been slipping but it was in spec. I turned around and went back to camp to investigate further when it locked up, coupled with a high pitch howling noise. I've just pulled the head and cylinder off and its dropped three valves - all inlets and made an absolute mess of the head and piston. A piece of valve must have gone down the cylinder as well, as there are a couple of very small gouges in the bore. In all three valves, the heads snapped off at roughly the same position. The top end appears to have been getting enough oil as there was plenty up there. In terms of background, the bike's done around 11,000 kms, has oil changes every five hours, a clean air filter every rider and valve clearance checks every 10 hours. About three months ago I pulled the gearbox down after worn dogs on 5th gear caused it to keep slipping out. At the time I put new rings in it and checked the piston tolerances and it was all within spec. I adjusted the valves at the time as well, while I had the head off, as they were close to the minimum clearance. Apart from this it's been a bulletproof bike. Since the rebuild I've done about 15 hours and three oil changes and it hasn't missed a beat. I had done an oil change the day before going out on last weekend's ride. Any suggestions as to what would cause this? And why the inlets only? After doing a search it doesn't appear to be a common problem.