1998 kx 125 strange piston wear

[Owenfoye15]

I went to do I top end on my bike because the compression felt low and it is now winter so there is not much riding to do anyway. When I took the piston out I noticed a lip had formed on the exhaust port side of the piston. There was no wear or scoring on the cylinder which I found strange. I put a new piston in warmed up the bike at idle with a few blips on the throttle. During this time when ever I would blip the throttle there would be a low quite sound like sand was in my expansion chamber. I blew out my pipe but the noise was still there. I pull the head of to just make sure everything look right after being ran and the piston had a small lip on it again. I've link a picture of the piston that I replaced with the new one. 

[harleydgriffith]

Makes me wonder if the jug is out of round. Is there matching deformation on the cylinder wall?

[Owenfoye15]

Cylinder is brand new 30 hours ago. I had a friend measured it and it was all fine. Only thing I can think is a problem with the connecting rod 

[Owenfoye15]

The thing I'm finding strange is the wear looks like it's only happening when the piston is going down because it is lipped up

[Greenwrench]

is the power valve hitting the piston ?

I've seen that kinda thing a few times but there's usually little marks on the piston too I'm having trouble downloading pics through mobile app

[Owenfoye15]

that could definitely be a possibility. Do you happen to know what that problem is fixed?

[DylDog]

the powervalve needs to be trimmed so it doesnt protrude into the cylinder during operation.