Piston and exhaust port became friendly

[MONKEYMOUSE]

Its a 2001 CR125, my friends bike. It was about 30 out side and we were ridding I was on his fathers bike, when his bike died for some god known reason. He kicked it about 15 times when it then fired it ran for about 30 seconds and you heard a nastyyy sound from the motor. The piston kissed the exhaust port and cracked a good pice of the dome off, pinched the ring tight in there and put a hair line crack in the piston down the side. Now what confused me is it is a fresh boor fresh top end all new stuff, motor only had a little over a tank through it when this happened. Good news is the bore is not even touched, no nicks no scrapes even the exhaust port if completely fine, heads fine as well and the pice was found in the exhaust. What I was scratching me head at is how?? why?? there seems to be no reason why this happened. I tried to upload a picture but I doesn't seem to be working for me. :moon:

 

[ScottS]

Ring gap wasn't in the right place and got snagged in the port ?

 

[marcusgunby]

Ive seen a few do this-never knew why-one owner ended up buying a new cylinder.

 

[Radiator Ron]

How did it get a fresh bore, that cylinder should be plated. If it was replated or a sleeve installed ensure all the ports are chamfured. This helps eliminate snagging the rings.

 

[Dirtbikehunter7]

Either the cylinder was plated and was bored which is a no no or the ring was installed incorrectly. Majority of the time this happens is from rings put in wrong. when the ring slides around and catches a port then yeah you are going to have some trouble. My money is on the ring being installed wrong.

dirtbikehunter7

 

[MONKEYMOUSE]

Yes the bore was done correctly and plated correctly. Also we know the rings were in the correct spot as when it broke it bent the piston crown down pinching the ring tight in, couldn't pull it out with a pair of pliers and the gap was in the correct spot. Just a fluke I guess.

 

[Studboy]

Make sure that the powervalve has enough clearance to the piston. If it doesn't have enough clearance it will run for a while and then snag.

 

[Dirtbikehunter7]

Well if that is the case then another thing I could think of off the top of my head is improper startup. If you happened to go WFO on the bike without giving the coolant time to heat up to normal operating temp then the pistol would expand rapidly but cylinder stay real cold thus expanding piston and not the cylinder. It is real common actually :( Thats why its so important to let your engine warm up especially in cold temperatures. Did he crank it up and go or did he does he practice a startup drill?

dirtbikehunter7

 

[Dirtbikehunter7]

Or after it was rebored the clearences just werent right for the bike. then theres also a chance if he was running WFO and let off throttle instantly he could have siezed up the piston that way.

 

[Micahdawg]

Putting a piston in backwards can also place the ring gaps right over the exhaust port. If it snags it will pop a chunk of the piston dome off too. The bike will run with piston backwards, but will probably be boggy on the top end hard to idle.

I don't see how you could install the piston into the cylinder with the ring not in the proper position. The alignment pin would bug the ring out so bad that I would think you would need to hammer the piston in and fold part of the ring in order to fit.

Micah

 

[MONKEYMOUSE]

The bike was properly warmed up, and the bike wasn't being ridden hard as the top was barly broken in. Power valve was all in spec. as well. who knows :coocoo:.

 

[Dirtbikehunter7]

since it was a fresh bore and the cylinder caught as much as you say it did maybe it was bored incorrectly. traditionally plated aluminum cylinders are not supposed to be bored. its possible that whoever redid the bore did not get the correct clearences for that particular piston. If it was too loose maybe the bike was getting too much piston slap. either way I suggest the bike be taken in and looked at by a tech because things dont generally go boom for no reason so it may do the same thing again if its not fixed. have it looked at.

 

[MONKEYMOUSE]

Well it wasn't the first piston in that bore, it was the second but I'de still consider that a freshbore. And the first top after it had just been done was fine it was raced the end of the season with it nothing happend. Then did a top after the last race and boom.

 

[bikepilot]

I have actually seen the aftermath of a ring installed unligned with the pin (I do know know if a hammer was involved in the installation or not), but the bike ran for a few hours then the ring caught on a port and snagged up locking the pistion to the cylinder. I had quite a time getting the top end apart!

 

[dirtbikr99]

hey, i kinda had the same thing happen....i have a 02 cr125. I got it bored 2 mm by fuel tech usa, they did not put enough clearence in the power valve. the motor started up ran good for 2 hours then started not to idle then was hard to start...took it apart the pistion dome had two parts indented on the pistion dome and scratches along the side. I was unsure oh why it happened...rebuilt it....same thing happened. then i checked the clearence with a feeler gage...it was to tight so i grided it down a little...put ti back together now it runs great. I would post a pic of the pistion so u could see if this is what your pistion looks like but i am at collage right now....good luck