I was riding my yz450 a couple days ago in the sand. It was soft sand dunes and the bike was running warm (not enough to boil out my antifreeze though). Anyways wide open in 3rd gear bike sputters hard then locks up. I am a little concerned that it locked up at such a speed thinking maybe seized. At first kick start is hard and rocking in gear seems to confirm a locked motor. A little more fiddling and i frees but does not feel real smooth through the stroke. Well I let it cool a minute and it fires up but runs rough and bam stops dead. Well we are in the middle of no where so I let it cool for about half hour push it over a sand hill (real fun even with two of us) but being that it was pretty much flat and downhill I decided to ride it out if it would go. Fired up running a little rough got about a quarter mile bogs and dies. Fire up again had to feather a higher rpm start and cruised back in 3rd gear about half throttle. Felt like it had some power but I did not push it so I dont really know. Ran a little better at the end but kick over still felt a little rough and bike was noisy and clanky sounding. So I started tearing down today thinking maybe a partial seize of a ring or crank bearing. Exhaust valves were both tight (.13 and .15) intakes were all in spec. There was no galling in the cam journals. Removed the head dont see any galling of the cylinder wall. Turning over with head removed by wrench on flywheel or by kicstart is smooth and easy. So now I am kind of at a loss. Could it still be a crank bearing issue. Possible somthing broke in the clutch of tranny area. Any advice on specifics to look for would be appreciated thanks. My next step will be draining the oil checking for chunks of metal.