I got a 99 WR400F from a guy out of state, it had some knocking in the head that I thought was simple piston slap so I bought it for 1800. I tear into the motor and find the crank was bad. Got another crank for 278. Then I find out from a shop tech that the pockets in the head are worn out well past tolerance due to sand being in the motor at some point in time. So now the head is toast too. The only good things are the bottom end minus the crank (unless I hear from them even more stuff is toast, ugh) and the cylinder and piston. Yamaha wants around 700$ for a complete cylinder head. Not cheap, but not bad considering what my KLX parts cost me. What causes sand to get sucked into these bikes so I can avoid this happening under my ownership? Should I look into another motor? The oil tank in the frame was all full of black goop also, and several rinses with solvent and compressed air havent cleared it. Anyone have ideas on how to clean this thing out?