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Just got around to picking up a new cylinder and pro-x piston for my 92 kdx 200. I checked the ring gap and it was within tolerance. So I went ahead and installed the piston. Put the bike together and ran it for ten minutes. After letting it cool completely, I started it back up and in two minutes of slow revving I heard a sharp crack and the bike felt a loss of power, So i killed it and it would not start again. What I found was the dome of the piston had cracked near the top ring gap and destroyed the side of the cylinder. I'm not sure what could have caused this as the ring is still seated in the piston, just the side of the piston is melted after the top cracked. All parts were brand new and it was a B piston. I will put up pictures shortly.

That really sucks.

I am just trying to figure out what caused this, I wasn't runnning lean and checked for leaks already, never had anything like this happen on any 2t ive built before.

Did you measure ring gap end?

How did you warm it up?

Yeah I measured the ring gap, and I did what the manual said run at low throttle/ idle for ten minutes at temp, then let fully cool down, I was just starting it up for the twenty minute cycle when this happend, The top end maybe had all of 12 minutes on it, The strange part is the top ring is fully intact, slightly bent but fully intact.

I have gone through my share of bad tuning/ soft seizing on various 2t and never had this happen so soon on a rebuild. Sure if I rode like and ass on a lean bike then yes but I was not showing any signs of running to hot or lean, checked the plug after the first ten minutes and it was black and oily. It looks Like it seized on the ring gap? but I have never scene a bike crack the dome/ have the ring still completely intact.

Is it a shadow or maybe the camera angle, but thats looks like a large gap between the piston and the cylinder at about the 10 - 11 position in the picture with the piston all the way up.

Somewhat a gap from the piston shredding the cylinder, but i think most is a shadow.

Sorry to read of your troubles, by the way!

Is there a port where that area of the piston moved up and down? Did it catch on something?

Yeah im thinking the ring may have caught part if the intake port, I did chamfer them the best i could maybe it wasn't enough.

On a side note it was the fastest ive ever spent $400 bucks

One of these days it will run.... Got unload some other projects before I buy another top end unfortunately....

Looks like it caught on the exhaust port.

I'm wondering if the piston's ring stop peg is off location or maybe that transfer port was opened up too wide toward the rear. Either of these could cause the end of the ring to snag in the port. Can we see what the cylinder bore looks like?

Wow. I have never seen anything like that in all of the top ends I have ever done. I feel for you, Buddy. :-(

That is an odd failure. It might be worth a call to some one (Pro X or where ever you bought the parts) and see if there is anything they can or will do for you.

Cylinder pic

http://s1193.photobucket.com/albums/aa3 ... CF0169.jpg

Something looks wrong around that intake port??

Yeah I will have to compare it to the other bad cylinder I have. Kinda a shifty situation, as that was the nos cylinder that was on eBay for a while so I am not sure what to do. I know my ring gap was right.

New Cylinder on left, Old on right



old piston on left, new on right.




Could the new piston being taller have anything to do with it?