Hi all,
got a bit shocked yesterday, removing the freshly bored top end from my "75 DT 250. About 100Km done with new Wiseco piston and now i see slightly worn cylinder alredy. Its just the area around the inlet. Cross pattern is gone and surface looks like polished. The pictures maybe are a bit misleading, i can´t feel any scratches, but i think, something bad would have happened, driving further. On the piston, absolutely nothing is visible.
Any ideas, how this could happen? 
The oil pump is giving the right amount of oil with open and with closed throttle.
Spark plug looks nicely brown
Is there any top side or bottom side on the piston rings maybe, what i missed ?
Hope for some good ideas.
Regards Guido
Hi all,
got a bit shocked yesterday, removing the freshly bored top end from my "75 DT 250. About 100Km done with new Wiseco piston and now i see slightly worn cylinder alredy. Its just the area around the inlet. Cross pattern is gone and surface looks like polished. The pictures maybe are a bit misleading, i can´t feel any scratches, but i think, something bad would have happened, driving further. On the piston, absolutely nothing is visible.
Any ideas, how this could happen? 
The oil pump is giving the right amount of oil with open and with closed throttle.
Spark plug looks nicely brown
Is there any top side or bottom side on the piston rings maybe, what i missed ?
Hope for some good ideas.
Regards Guido
  

Hi - why did you remove it all again?

Check and double check all your intake system from filter to carb.

Yes, often that kind of wear is from dirt ingestion...  also many shops do not clean the cylinders well after boring and grit can hide in ports if not thoroughly cleaned.

How's the piston look. Does it look like it's been trying to seize--lightly seize if there is such a thing. Best bet is stop looking. As JammyYammy asked in #2--why did you go looking. Was it getting noisy compared to first startup 100km ago. Are there similar marks on the exhaust side. Tis worrying though i can understand.. As the crank goes past bdc it pushes the piston hard against the inlet. Pistons have offset on the pin to counteract that as much as pos. Bit of useless info i read once but that's not helping sorry. If that's wrong info someone put me right please.

Yes, in the Yamaha Service manual it says install the piston rings with the number upwards.
I am not sure if yours rings were genuine Yamaha, Wiseco or some other brand but incorrectly installed rings could definitely do that.
 

Thank you all for your input.
Cylinder was bored by an experienced company. They stated tt was cleaned carefully, ready to install, but nevertheless, i cleaned it myself again.
I removed the motor again to split the cases. I have doubts about the main bearings, as small parts from the oil pump disappeared somewhere, i never found them.
The piston is a Wiseco, honestly i didn´t check, if the piston rings have to be installed with a certain side up. Thhe ring gas were both ok..
As written abolutely no marks to feel on the barrel, just visually defect, as the cross pattern has gon. I hope to find the problem and re-honig would be sufficient. Not to bore again.
Meanwhile, i completely lost enthusiasm for this bike. Never run good within the last 2 years.
Found also the clutch basket Bronce bearing has a lot of play.
This bikes are not forseen for a lifetime more than 40000Km

It would be great to see pictures of your piston...  especially close up of piston intake skirt.

Hi Mark, thank you. Pictures of the piston attached.
The rings just have the szie engraved , 1,5 mm over.
The upper ring, i installed with engraved number up, the lower one with number down, but couldn´t imagine it plays a role. Visually, the piston looks brand new, but now on the pictures, i see small marks, starting at the inlet side about middle to bottom of the piston. Strange. If dirt would be the reason for the wear, it would appear less homogenous surface, where the wear is, i think.
I will bring back the parts to the shop and ask about his opinion.
Hi Mark, thank you. Pictures of the piston attached.
The rings just have the szie engraved , 1,5 mm over.
The upper ring, i installed with engraved number up, the lower one with number down, but couldn´t imagine it plays a role. Visually, the piston looks brand new, but now on the pictures, i see small marks, starting at the inlet side about middle to bottom of the piston. Strange. If dirt would be the reason for the wear, it would appear less homogenous surface, where the wear is, i think.
I will bring back the parts to the shop and ask about his opinion.