2003 KTM 250 EXC...Pulled the top end and piston. I hand checked the rod movement and it has about a half a millimeter of movement up and down...Is that normal or should their be zero movement up/down?

It rotates smoothly and other than that half a millimeter - it seems fine. I should mention I've never done this before

http://www.linearrow...412_171919.html

This is what I found on top of the piston....I've been told it's either detonation and/or the lower rod bearing is bad and letting the piston hit the head.

Thoughts?

Oh - the crank/bottom end has about 110 hours on it.

Something came loose, and did a nice tap dance on your piston. Seen that before on my old Bultaco, i would pull bottom and check it , if you do not find anything else.

That is not deto, that is something banging around. You lost something for sure. I would pull it apart down to the crank. Contact Andrew: http://www.cookseycrank.com/ This guy is the best!
Joe........

I've seen that when the upper or lower rod bearing fails and the needles literally get thrown up into the combustion chamber.

Usually when the bearings fail, you can hear some pretty bad noises coming from your motor. That motor looks like it was run pretty hard for quite a while without anyone being the wiser.

On 2 strokes, your best friend are your ears. I always listen to my motor for a few seconds before hitting the track to make sure nothing has failed overnight.

Now, the question is... how did it fail? High hours? or bad oil/fuel mixture?

Edited by tye1138, 14 April 2012 - 09:44 AM.

It was running Honda Synthetic (blue bottle) which when mixed with 4 gallons comes out to 32:1. I was blasting through the woods w/two buddies...we stopped at a crossing and stopped. When we started up again I heard an "odd" noise and killed it.

Then, thought I was hearing things and started it up...ran fine w/no odd noise I could hear, then I took off and ran about 50 yds before noticing it had lost power and turned it off.

The bike had 110 hours on the last total rebuild. I'm told thats about when the top end needs refreshing but the bottom end should have lasted lots longer.

Edited by nondem, 14 April 2012 - 09:24 PM.