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2005 YZ250F seized piston after 7 hours

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First post for me...great site! What a wealth of information! I'm trying to understand what caused it. I did the engine rebuild myself with the help of my local shop and the $100 repair manual. I've had many bikes in the past 30 years between myself and my son and can pretty much do it all. I'm 49 and not a fast rider anymore. LOL I had a new Vertex piston and my stock cylinder sent to CVTeck in Quebec for replating and matching. Everything was torn down and cleaned. Assembly of new crank, crank bearings, rod, piston, water pump seals, valve seals and magneto went well. New gaskets and many O rings were also replaced. I'm not using any trick parts. The carb has the Boyesen Quickshot and in the muffler a Ignition db Dawg to keep things quieter. I took the bike out on some back roads three times for 20 minutes each time to break it in, not taking it past half throttle and letting it get stone cold after each ride. First time at the track I just took my time. I also made a carb adjustment...the db Dawg causes the bike to run richer, for that they recommend that you raise the clip one notch to drop the needle to lean things out a bit and make the bike snapper again. Now I'm on the second from the top. Fast foreword- third time out and now at a practice track (Fourwinds MX Park...in Ontario) after three hours of riding and resting I start the bike and go back on the track again and it seemed to be acting like it was cold, not as strong as it was...I'm only in second gear 1/4 throttle and on the second jump it seized up. Couldn't kick start it. At home I drain the oil and measure 1.1 liters so it's up to capacity plus there is some still in there. (Spectro 4 SEA 10w40) The piston had siezed against the cylinder with two melting stripes from the top edge of the piston down to the bottom of the skirt but ONLY on the intake side. The cylinder wall had started to melt in two stripes also. The top of the piston is in perfect shape. Through the melted side of the piston I am still able to see light through the three lubricating pin holes in the piston ring groove. I had the piston and cylinder sent back to CVTeck to check clearances but they say it's a oil issue, nothing they've done. My shop now suggests replacing the oil pump. It is abit pitted (worn) from an earlier problem so a new one is on the way. But...my way of thinking is the pump would pressureize the oil to send it to the top end and the crank would splash the oil to oil the lower end of the cylinder and the bottom side of the piston. Is this right? The head, cams and valves all look to be in good shape...like they were getting enough oil. WHAT DO YOU THINK??? Any suggestions would be appreciated. Thanks

here's two links to the damage

http://i379.photobucket.com/albums/oo231/Courticerider/Copy1ofP1030153.jpg

http://i379.photobucket.com/albums/oo231/Courticerider/Copy1ofP1030161.jpg

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Not the oil pump. First thing to go with an oil pump is the cam journals. not having your piston and barrel (or even pictures) and forced to speculate, I say it was a piston/ring/barrel problem, caused by either a defect in them, improper fitting or debris sucked through the carb.

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I just posted two links to the damage.

Makes me wonder how something sucked in could do two stripes.

I just thought of something else but it might not be relevant...I tore down the carb and found the slow jet completely plugged and air pressuse would not clear it out. The smallest wire I could find was from a wire brush which did the trick. The slow jet is a 45... std -42.

Also coolent levels were right up. With the cap off it's 1/2 an inch down. Checked the impeller and it turns freely and is smooth. (new seals, both sides)

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that looks like a heat seizure or oil issue check your oil pump like noted above. also i it could have been from the pluged jet and the bike running way to lean and hard core over heating. I dont hear about it happening in 4 strokes much but in the land of 2 strokes it happens all the time.

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looks like the piston to wall clearance was too tight, proabably due to a poor replate job. id go with a new oem cylinder and piston.

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Now, the siezure was only on the intake side, correct?

I have to wonder why you did not stop riding sooner, that damage did not happen in a minute, that is like 20 or more minutes of the engine screaming in agony.

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That looks like it ran without oil, but Im no expert.

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That's right ,only on the intake side. The other is side shows no wear or colour change...nothing.

It all seemed to happen, at least noise wise at the run up to a jump that I made and then locked up after the landing. Not reving high at all... I was dogging it.

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