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Big time cylinder issues

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My 2006 yz250f needs a new cylinder and i need it to be cheap. I put new circlips in yesterday and out everything back together and started it up. within 2 minutes of running the engine, it locked up. I tore it down today and the piston broke off of the rod and went up cockeyed into the cylinder. There are now several different gashes in it. So now I'm looking into a new cylinder and piston. I think I can find a piston for a cheap enough price, but id really like to bore the bike out while i have this oppertunity. I know that i can buy a piston to fit a 6mm big bore which would make it a 300cc vs. a 250cc. So what I'm really wondering is if i can take my stock cylinder into a machine shop and have them bore it out for me. Then if i bought a new piston i'd be set. Could i do this? and how to i get the shards of piston out of the bottom end of the engine? thanks for any help

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Your better off buying a big bore kit. 300cc is pushing it in my opinion for a 250F and no a local machine shop cannot just bore it out. It has to be plated (http://www.mt-llc.com/) or sleeved. Cylinder Works sells a couple different size big bore kits and so do a couple of other companys. I am wondering why you were replacing the circlips. Was it a brand new piston?

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I bought the bike used about a year ago and i recently had to replace a valve that broke. So i decided to look at the cylinder and it was a JE piston with crappy snap rings instead of circlips. So i replaced them while i was getting the new valve.

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You can't just throw in a circlip into a piston designed for snap rings. The piston was designed specifically to use the snap rings. That's what caused your problem. In the future use only the clips that are supplied with the piston, NEVER use different hardware.

Another question is, how on earth did a valve break and not damage the piston in the first place?

Sounds like a bomb waiting to go off if you ask me. If it were my bike, the valve seats would be cut, all 5 valves replaced, cam chain replaced, along with new piston/rings, and spec the crank for tolerance.

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how would the snap rings cause the problem? that just holds the pin in.

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how would the snap rings cause the problem? that just holds the pin in.

Its not that simple. The physics going on inside that engine while at full throttle wont let you get away with just slapping any old circlip in there. The circlips that come with the piston are designed for that piston only. They are not interchangable. Stock Yamaha pistons use snap rings instead of circlips and they will not work on an aftermarket piston/pin. Also you never reuse a snap ring. I also would never install a used piston. If you drop a valve you should automatically replace the piston, and crank. I have seen people spec cranks after dropping a valve and the crank spec'ed out fine only to have it let go 1/2 and hour into running it again. Not worth the risk.

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would that cause the rod to rip to pin out of the piston?

It could but remember you also said the piston was used. There is no way to tell the condition of a used piston unless you are going to have it magnafluxed (x-rayed) which wouldnt really be worth the money. Your better off buying a brand new piston. Most times trying to save a buck will cost you hundreds on these bikes. Better to buy new.

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would that cause the rod to rip to pin out of the piston?

inertia of the crank rotating after the piston locks up in the bore

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Let me explain the whole story. Last summer i was having some clutch Slippage issues, So i took fixing that on as my winter project. I took the engine out and Tried to fix that. I figured out halfway through that i had been running to much oil and it wasnt oil that was approved for wet clutches. So i put the bike together (breaking and having to replace the the water pump in the process). I decided to check the valves while i was there. the valve broke because of my mistake. I managed to time the bike backwards. All i did was like up the dots. I didn't think about the direction on the intake cam which was wrong. So when i put the bike back together and kicked it over, the left intake valve smashed into the piston. I quickly realized it was timed wrong and fixed it. The bike then ran for 2 or 3 hours before it died. When i got the bike back to the house, i decided to take a break from the bike before i ruined something else. Coreyl offered to take my bike and fix it. So i decided to let him because he seemed to think he could do it. So he tore the top end down and told me to buy a top end gasket set and a valve. I thought i was going to sell the bike soon so i bought a stainless steel valve. He put it in and went to our local shop and got a new valve retainer and circlips for the piston because he told me that i needed new ones. So he got everything in and tried it. It started first kick and sounded great. then it started to make a knocking noise and locked up after that. And now I've found that coreyl's decision to replace the snap rings has caused this.

I'm really wanting to get this bike fixed as cheaply as possible. I'm saving up to by my first truck within the next six months, So if i can keep this under 300 id be happy. I found a big bore kit from cylinder works for $240 on oncycles.com but that is like half of what it costs on cylinder works site. So it sounds a bit strange to me.

Can i keep the stainless valve? or does the have to be replaced?

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You only replaced one valve and you replaced it with a stainless valve? Is that correct? I would be happy to help you out but I wont hodge podge something together like that. What exactly is broken/damaged in the engine right now? In my opinion to do it right you need to first off match all of the valves. Make them all ti or make them all stainless. I would have the cylinder replated by millenium technologies back in Wisconsin. They specialize in doing this kind of work and are the best at it. They can fix the gouges in the cylinder too as long as you didnt crack the cylinder. Get a new oem piston kit, an new timing chain, and a new crank and bearings. Without doing all of that you may end up right back here with a broken engine again. You really need to do all of this if you are serious about fixing the bike properly.

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was it really the snap rings that caused this or could the piston had damage from smashing into the valve while it was broken? there was a flat area on the piston that you could see from it making contact with the valve. wouldnt that weaken the piston?

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I'll go ahead and buy the ti valve. I understand that one completly. The timing chain ill probably replace it too seeing as coreyl put the tensioner in wrong so it is more than likely stretched. Ill also look into a stock piston. How much do you think millenium would charge me? It almost seems like i'd be better off buying a new cylinder. And Why the crank?

as for the piston and the valve making contact, it took two small chips out of it so i doubt that itd weaken it.

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I'll go ahead and buy the ti valve. I understand that one completly. The timing chain ill probably replace it too seeing as coreyl put the tensioner in wrong so it is more than likely stretched. Ill also look into a stock piston. How much do you think millenium would charge me? It almost seems like i'd be better off buying a new cylinder. And Why the crank?

as for the piston and the valve making contact, it took two small chips out of it so i doubt that itd weaken it.

I think its about $200.00 but you would have to contact them. If you can find a brand new one for around that price get a new one. Millenium's coatings are far superior to the oem cylinder coating and will last a lot longer. As for the crank its a gamble putting the old one back in. You had piston to valve contact and a piston seize in the cylinder these both put undue stress on the crank. You could spec it and it checks out fine and still have it grenade 30 minutes after putting it all back together again. Is it worth the risk to you? Sounds like you've already had enough problems from not doing it correctly. Just my .02

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I'll take a look at my crank later this week and try to determine if i need one. So ill only keep it if it is perfect. What about getting that 269 bore kit for 240? that would give me a new cylinder, piston, rings, and gaskets. Seems like a good deal providing me with more power which i truely wouldnt mind.

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the one for 240 is just a cylinder it does not include the piston

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If you after economy skip the big bore, stock bore pistons and gaskets are cheaper.

I know, i just found what i thought to be a big bore kit for 240. Turns out its just a cylinder. Back to having nothing i guess

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