KX85 cracked piston

[Bidule]

Hi guys

I bought a used 2005 KX 85 for my son. There was a lot of spooge dripping on the ground from the exhaust. I checked the compression and it was at 100, manual says 119 to 180. So i decided to replace the Top end.  I Noticed the piston was cracked at several places and i am just curious about what caused this ?
I removed a thick layer of carbon on top of the piston for clarity before taking the picture. Now should i order a 1 ring or 2 ring piston ? My son wants to do some racing nest summer. 

  


Thanks

[Foxx4Beaver]

how's the cylinder look?
typically a cast piston cracking like that is caused by excessive piston to cylinder wall clearances...
unless you plan on doing top ends frequently, I myself would stick with the dual ring...and forged piston.
consider yourself very lucky you found that!!...before this happened ...

[Bidule]

The cylinder looks pretty good to me, no scratch, but i don't have a bore gauge to measure if it's within limit.

[Foxx4Beaver]

if you have a local dealer, or even machine shop, they can measure it for you...
I wouldn't buy anything yet until you find out what the bore measures....
I'm not suggesting that you need to or have to, but I myself would probably send the cylinder off to get re-plated and fitted for a forged piston...especially if you plan on using it for racing....then you'll know it's all good and ready to go, providing the bottom end is in good condition.

but again, find out what the bore measures 1st.

[KXDINO]

I had a bit to do with kx85 owned 3 of them in the past , when you get the clearance checked and if all is well you need a hr meter to keep track of engine rebuilds , the harder it get ridden more often you need to do a top end , as well as keeping air cleaner maintain and good 2 stroke oil will help engine life , better to replace top end before it blows , i cant remember how many hrs i use to do the 85 top end but i know on our kx125 08 i change the single ring at 9 hrs and the piston at 18 hrs thats with a pro rider on it , lucky the parts are fairly cheap, hope this helps.