piston diagnostic pictures: 2000 KX65

[MXTex]

Included herein is a picture of the piston out of my son's 2000 KX65. I just bought this bike used and pulled the top end for a maintenance check. Needless to say, I'm going to be installing a new piston kit. I'd appreciate any feedback on these exhaust port side scuff marks on the piston. The cylinder looks fine. Could this be indicative of another problem or is this normal wear? I believe this is the original piston and the bike seemed to run fine; although we're coming off an XR50. Thanks in advance.

 

[Matt_H]

That is alot od blow by on that piston it should have been replaced awhile ago.

 

[125 rider]

That looks like some heavy exhaust side scuffing.

 

[MikeyG]

Does this bike have a Powervalve?

 

[MXTex]

no powervalve

 

[Jaybird]

How does the jug look in the area where the blow-by is?

 

[MikeT]

I was going to say that you need to drill those holes in the piston behind the exhaust port bridge, but it looks like you did. It might be a sign of a cold seizure early on in the pistons life and it just broke free.

 

[KenR]

Looks like a cold seize to me. Make sure junior warms that bike up real well before putting a bunch of load on the engine. We used to replace the ring on our 65SX after every few races. You'll get a lot more life out of the piston by frequent ring changes.