I have a 2003 YZ450 and was out riding this past weekend. I just bought the bike and the guy that I bought it from said that it was all set up and ready to ride so I didnt do any thorough checking of it. I rode it pretty hard on Sat and the gearing was all wrong so it staying in the high RPMs way more than it should. I decided to quit for the day and slowed to about 5-10mph at my pit and put the bike in 1st and the bike just seized. I tried to put it in neutral and kick it, but the kickstarter wouldnt budge. It also wouldnt budge the wheel when i tried to bump start it and dragged real hard with the clutch pulled while in gear. Done for the day, I decided to look at it when i got home. Last night I checked it out...nothing wrong with the plug (first sign of a blown motor many times). I opened the cover and turned the motor with a wrench and it felt real tight (you could feel the friction. It loosened up a bit on the down stroke and then would get real "frictiony" and tight on the compression stroke. I checked the clearances and the exhaust valve was tight by .05mm. I did an adjustment and brought it within specs. I put it back together and it fired right up. I let it idle for a few mins, but only rode it for about 30 seconds since it was dark out. Does an exhaust valve .05mm out of specs seem like it would cause this problem or is my bike a ticking time bomb? As a side note - I thought for sure the engine was seized and needed at least a resleeving and I brought it into the local shop yesterday to have them do the work for me since I am very time strapped. They wanted $500 just to strip the motor down to diagnose the problem before they did any work! I laughed at him and loaded the bike back up to strip the motor down myself.