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2000 yz125 has scratches on piston by the exhaust port

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Lots of possibilities, but the most obvious answer would be dirt getting past your air filter.  Another possibility is that whoever did the work did not have things clean when the top-end was assembled.  Carefully examine the carb intake boot to see if there is any evidence of dirt getting by.  You could also pull the head and see if there are any scratches on the cylinder that you can fell with a fingernail.  

I would address any intake contamination issues and keep riding it.  If it was an assembly issue, the best way to address that is to learn to do your own work.  Smokers are easy.

Edited by rpt50
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That appears to be a PV sliding in past the stopper, no oil relief holes in piston at bridge, and/or dirt. At 3 hours, and "IF" you let the air filter get dirty, and dirt passed by it is owner error, not shop. 

Let us know what you find. 

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I will take a look at my carb too see if any dirt passed by, the shop said they did modify the PV so it wouldn't come into contact with the piston so most likely a dirt issue is what I'm thinking, I'll look into it and keep you posted. Also the pipe is missing a spring, could that be an issue if it's not sealed completely?

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1 hour ago, Herman Bueckert said:

I will take a look at my carb too see if any dirt passed by, the shop said they did modify the PV so it wouldn't come into contact with the piston so most likely a dirt issue is what I'm thinking, I'll look into it and keep you posted. Also the pipe is missing a spring, could that be an issue if it's not sealed completely?

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No spring isn't the issue, beyond springs there's orings that seal or should seal and once running its an output hole.That pic is confusing me

 

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That doesn't look like dirt damage to me...too rough. Hard to tell from that pic though to be honest. What type of initial start-up/run in did you do? I'll second the guess that maybe the PV is making contact.

 

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2 hours ago, Herman Bueckert said:

I will take a look at my carb too see if any dirt passed by, the shop said they did modify the PV so it wouldn't come into contact with the piston so most likely a dirt issue is what I'm thinking, I'll look into it and keep you posted. Also the pipe is missing a spring, could that be an issue if it's not sealed completely?

Sent from my SM-G920W8 using ThumperTalk mobile app
 

It is very unlikely for the exhaust O-rings to allow dirt past them. There is slight suction and reburn on a two stroke engine (but very little to cause vacuum). So, if shop relieved PV from piston, they didn't get it right. If you run silty/sandy dirt, and didn't oil the filter, or run the wrong one, than its dirt scuffs. But they do look to big for that.  

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Looks like it just barely hits powervalve.  Whatever they did to it wasn't good :naughty:

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Was that a piston with a coating on it? If so, it's just running in. Remember you are using a flashlight which casts weird shadows and uneven light. I've never liked what I saw looking in exhaust port, so I'd rip it down and a paper towel wipes oil off and it doesn't have a scratch..... how's compression? I would tear it down, chances are it's not what you saw....if it is, it could be carbon from power valve scuffing skirt or any number of things. If the power valve is tapping the piston, you would hear it when the rings went past and it would be a loud tick almost like the sound a kick start makes if you keep it half down when engine fires, you would know it wasn't right

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