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2008 yz250f problems

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So I bought this bike and the next day it the crank seized. I rebuilt the whole bike but all I replaced was the crank. The bike ran fine for almost three months. When I was riding it one day (I was going hard through the gears) it stalled just like it did the first time when the crank seized, and when it stalled, I knew it blew up again so I pulled in the clutch and went on the side of the road. The kickstart was stuck and I thought for sure the crank seized again. Thankfully I wasn't far from home so I pushed it home and when I got home the kickstart wasnt stuck anymore and the bike kicked over freely without any weird noises but It wouldn't  start so I started to take it apart to see what happend and I took off the head so far and I can't find anything wrong. No metal shavings in the oil. Does anyone have any ideas what could have gone wrong and how I can fix it. Please help I'm only 15. 

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"......so I pulled in the clutch and went on the side of the road........"

a YZ250F is not a street bike.  Don't treat it like one, and it'll hang together better.  Riding one of these on the street is especially stressful on the engine.  The YZ is geared stock to run 45-50 wide open, so if you were running down the road wide open, well it will eventually pop.

When it gets hot and seizes the piston, the cylinder gets scuffed, the coating gets ruined, and the piston swells up.  Once it cools, the ALUMINUM piston then shrinks back down smaller than it was, so it needs replaced, along with the cylinder.  Now, that's assuming it was the piston that seized.  It could have been the cam caps, or some other cause.  

These are close tolerance engines, and as such, it takes precise measurements to be able to tell if something is out of tolerance.  You don't mention what tools you used to determine your decision, what parts you measured and checked, how often you change your oil.  

Don't take my post bad, just an observation, not a lot of information to go on from you.  We here on the 250F side, for the most part, do our maintenance, and run the bikes off-road, 5 hrs oil change intervals.  The rev limiter is designed to help save the engine during an accidental over-rev, but not sustained life at 12,500 rpm.

The 08 is a nice bike, worked on one for a friend, plenty of power, light weight.  If you decide to keep it, then I'd tear down the engine, measure the piston/cylinder clearance, probably replace both at this point, replace the cam chain and sliders, inspect the crank and rod, transmission shift forks, clutch, oil pump, clutch basket, clean the flywheel (because that's where ferrous metal will stick, bearings and such), blow out all the oil passages...... you get the idea.  Do it all right, work slowly, work clean, stop when you get tired, take your time, do you due diligence and you will have a great running bike again.  Save your money, do the work yourself, and learn.  Its worth the effort..... if you want a good bike.

Edited by ronbuell
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@ronbuell thanks for your advise. You were right. I took of the cylinder and the piston was chipped and my cylinder was scored. Thank god that was the only thing. Still costed me $360. Lesson learned. Parts coming tomorrow. 

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Yeah, just glad it wasn't the crank.  Just be sure you go thru it thoroughly and clean it, and look at the gear/rotor oil pump, if it scored, replace those 2 parts as well.

Good luck with it.

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