Time To Tear It Appart

[Just an old rider]

   Well I finally tore into the Kx500 today after it froze up on me and this is what I found  ...
       Broken rings , welded piston to cylinder and some misc.. scratches ...   

[BDI]

sure looks like it was run on strait gas.

[BDI]

On second thought looking at the picture it looks like the piston skirt broke off. Thats what happens when you run a cast piston to long. No Good  a lot of people think cast pistons will last for ever, they think theres no need to change em out thats a waste of money.

[Just an old rider]

  Yea im not sure what happened only that I get to rebuild it ...  I am going to change oils just in case it had something to do with it ... at any rate I am going to order a piston with window ... I guess it will help... I might have to bore out the cylinder where the piston welded itself to it ...    Im not sure yet ... the windowed pistons are only one size though from my understanding ...   I have some more checking out to do    thanks .. BDI

[BDI]

You know the way it is scored like that, I would say it was a lack of oil and the skirt busted of because it got welded to the wall. more pistures of the piston would help.

[Just an old rider]

 Here is some more pics of pistion .. I don't see where it is broken   

[Friar-Tuck]

  Yikes..  
 
  Like BDI has eluded to, if you check the recommended maintenance schedule on motocross bikes, I don't think 80% of us could afford to run them..
   Pistons and rings according to the owners manual of my 2000 is inspect every 3 races and replace every 5 races..  how many hours is that 15-20 including practice?   
 
      There is a thread on the piston window mod by Stewart you could apply to another or overbore piston if need be.
   There are several options at this point and maybe some of the other guys will chime in.

 If you are considering re-plating, you may be able to go back to stock, however there is a limit to how much plating can be applied.     Oversize, re-plate or oversize sleeve,
  or stock dia. and sleeve.   
      Well, Things can only get better now!
    Best of Luck...
Tuck\o/

[Just an old rider]

   Isnt't that how it works ... Ride it till it blows then do it again ....     

[Larry Wiechman]

  Isnt't that how it works ... Ride it till it blows then do it again ....     

 The classic 2-Stroke rebuild, EVERY MOVING PART!
 
Larry

[DoldGuy]

Old Rider,

If you go for an oversize piston be cautious of using the window mod on the piston as I know on the 88.5 piston the inside of the piston has been lightned and the walls of the piston are quite thinner & will not support the window mod, don't know the thickness of the in between over sized pistons (anybody?). The plating companies can work wonders & don't think yours would be a problem to have replated on a standard bore.

I was riding with a buddy years ago & had a similar problem, what had happened was the head gasket started to leak into the bore washing the cylinder down & leaving no fuel between the piston & bore and left the piston like yours. Check your head & cylinder surface to confirm a good sealing surface. Just something else to check to be safe.

Tuck, after I purcashed my 250F Yammie in 01, I was going over the "Recommended" Maint Schedule, and if memory serves me, it said to replace the valves every 20 hours. OUCH! Back then the Ti valves were almost $700 /set.

DoldGuy

[Just an old rider]

  Ok guys here is the new twist .. I went riding my wifes 125 kx  and it had a total of 3 hrs on it all by me... Well to make a long story short the thing seized up and I made sure the oil mixture was good ... so now I have two kx's down again and getting a little depressed.and befuddled.... I can not be that bad a rider ... I guess im just venting... and I contacted the builder of the motor and waiting back to see if he will look at it...

[KXcam22]

Old rider,
  My first guess was also straight gas.  It washes any residual lube off the intake side of the piston and it seizes there before the exhaust side of the piston has enough time to heat up and seize.  However, after the second bike I would guess that you may have got a batch of bad gas.  A similar thing happened to me and some friends at an MX race.  We all filled up at the same pump and every bike that ran that gas all had HUGE seizures right away.  None of us made it past practise. It also looked similar with the whole intake side of the piston seized.  A normal heat or lube seizure is usually a much smaller area. Yikes.  Hope this helps. Cam.

ps. I also once had a similar seizure on breaking in a new motor but eventually it turned out to be the CDI going south.  I racked my brains on that one since it appeared like I was running straight gas, although I knew that I wasn't.  Only way I found out was that on thorough analysis, I discovered I had no spark. Not your case since a 2nd motor has taken ill.