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Cracked piston

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Jun 28, 2001
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#1
ok so i was riding my bike pretty hard a lot of high revs. and i heard a funny noise come out of my engine so i backed off and grabbed the clutch, when i came to a stop and tried to start my bike up again. there was just nothing and there was very little effort required to kick it over. so i got my bike back to my shed and tore the engine down and discovered there was a peice of the piston missing. the piece was about 4-5 cm long, 1-2 cm wide and about 1cm deep, it went from the top of the piston down to the ring. it is a fairly sizable piece. oh and it is right in front of the exhaust port. any ideas as to why this happend? i run 98 octane premium unleaded, FMF fatty, power core II silencer and jetting is as per FMF's specs.
 
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#2
What kind of bike is it and what year? Also how many hours do you have on it since the last rebuilt.
I would guess that the piston was worn and it caught the edge of the exhaust port and cracked it. There has been some model that would do this when they were new because of the position of the rings and the ports.
 

Rcannon

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#3
I worked on a CR 80 that failed much like yours did. The reason was simply running the piston too long without a ring change.

You lucked out. This 80 dropped the piston chunk in the lower end and did minor damage. In both cases it could have been much worse.
 
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#4
it is a 2001 CR125, the piston and rings only had 15-20 hours on them. i ran a piston and rings for 80 hours in it and they still could have went much longer. i cannot find the metal chunk though and i cant find any trace of it. there are no scratches on or markes on any of the ports and i have no idea where it could have gone. what do you guys think i should do about the missing peice of piston?
 

Rcannon

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#5
15 to 20 hours is quite a bit for a 125. Just because you once ran one for 80 hours does not make it right.... (not trying to sound like an ass)

I watched my friend do almost the same thing on his CR 80. Yes, the original piston lasted much longer than his replacement. Why? Maybe the cylinder became bigger as it was worn in. The biggest difference is this young man became a much faster rider and was much harder on the engine.

 

I know you dont want to hear this, but you have to find that piece of piston if it made it in the bottom end . Hopefully, it went out the exhaust pipe.  I would always be paranoid of it finding its way into the lower end. I know its a stretch, but it could happen.
 
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Yogurt

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#6
Yes, better safe than sorry. If you can't find the missing or broken piece, always assume some of it went into your bottom end. I had a similar problem on my CR125, but instead it sucked the piston circlip out and some of it went down in the lower end. I unbolted the complete motor from out of the chassis, and with the top end removed, I poured parts cleaner into the engine cases and flushed out the lower end like that. Then tipping it upside down and waiting for the piece to fall out. It eventually did, but just make sure your crank rotates smoothly. If it doesn't roll smoothly, chances are a small piece could have gotten into the crank bearing.
Keep in mind this flushing trick is just a cheap trick. Someone else may recommend that you split the cases entirely and search and clean the lower end that way. But, I did like I said above, and have had no problems. Good luck.
 

rickyd

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#7
I would have the bore on the cylider checked, could be tapered, causing the piston too "wobble" and catch the ex port..
Rick
 
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#8
hey thanks for the fast response dudes. im taking the cylinder, head and chipped piston into the shop in about an hour to see what they think. the peice was small enough to blow right out of the exhaust is this likly or could it have happend?
 

rickyd

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#9
It could of blown out through the exhaust, but, best be sure it is not down in the cases.. Just too be safe..
Rick
 
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#11
spot on marcus, i took my piston, head and cylinder into my favorite local bike shop and the mechanic said that over time i had melted the piston in front of the exhaust port. and he said this because he suspects my jetting is way too lean. so i guess it turns out it was my own stupid fault. luckly my cylinder and head are in good condition. and because it was melted there is no peice in my bottem end.
thanks guys
 

Rcannon

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#12
Good news and don't feel too bad.

I am glad that I have never done anything stupid, but I have friends that have. Yea, sure. Dont ever bring up that incident when I cut my chain too short, ok????