Hi, riding my 2006 YZ450 Thursday, my bike siezed up. My friend who was riding behind me said he smelled something. This bike has 40hrs, is stock, (except for jetting). I changed the oil every 3-4 hrs. I drained the engine oil and measured it and it was exactly 1 qt. I also drained drained the coolant and it was just a tad short of 1 qt, which is the capacity of the system. The track was hardpacked and it was a cool day. Probably the about the smallest load on the engine of any riding conditiions. I have not tore the bike down yet, but I cannot understand what would cause such a dramatic failure with all fluids good, and easy conditions. Maybe the waterpump failed? Seems unlikely. I have not tore down the engine yet. This will be my first 4 stroke rebuild. It is my hope that only the top end is damaged. Anyway, I don't know the cause, and was hoping someone out there could help. Thanks friends!!