First piston change on my RM250 2007

[MX_Peter]

Today I decided to change piston on my RM250. I bought it from first owner and piston has never been changed, so this is first top end  change. From your experience can someone tell me how my cylinder looks like. Bike has aprox. 40h total ride. I will install Mitaka piston with RIK rings. My crankshaft also has some small rust spots, why is that?

Here are the pics:

[factoryX]

Clean up the cylinder walls with a scotch bright pad, and go from there. If looks bad have it replated, if not throw another piston in. Do not hone the bore.

[TMKIWI]

The marks on the bore were caused by ring blowby.
The rings were in for too long.
Scotch pad the bore and as long as there is no scoring/grooves you will be fine.
Clean the power valves and check the jetting.
Where abouts has your crank got rust spots.?

[MX_Peter]

It looks similar to this pic, maybe even little bit more:

[MX_Peter]

Do I clean it with water or dry...don't know what product is similar to scotch bright in europe...any thoughts?

[SachsGS]

It looks like she got hot and squeaked the piston a bit.If you look at the top images it almost looks like there is some leakage around the inner O-ring head gasket.Maybe check the head for flatness to be careful.

[MX_Peter]

I will thank you. I used something similar to scotch bright pad and polished it it's smooth but it can be still seen vertical lines. I noticed something in cylinder where the smaller two power valves are. It was really small brake (you can compare it to the right valve). I piled it and smooth out with the sand paper #1200

For larger pic see the link:
http://shrani.si/f/M/wA/isUj65O/dscn9969-01.jpg

[Recovered]

after you scotch brite the cylinder put the new piston ring in the cylinder and use the piston to tap it in straight and then look all around between the ring and cylinder walls for daylight do that every half inch or so till you reach the bottom of the cylinder if you see any daylight between the ring and cylinder walls along the way you'll need to get it replated

[cnrcpla]

My cylinder was a little worse then yours and I had to get it re-platted. If you run your finger around the areas that look worn and feel it dip down or feel uneven, then its time to re-plate it. Also, the method that oldermxr said would be good to do if you can't feel any uneven-ness with your finger, and it will give you the peace of mind knowing its all set. Good luck.

[MetalMike1982]

Definatly looks like that topend saw some heat at some point . I would have that crack in the cylinder you speak of checked out by a machine shop or something , you dont want to snag a ring and grenade the motor 4 stroke style .

[downonmonday]

Definatly looks like that topend saw some heat at some point . I would have that crack in the cylinder you speak of checked out by a machine shop or something , you dont want to snag a ring and grenade the motor 4 stroke style .

Now that is bad if you grenade like a 4T!!!!

[MX_Peter]

after you scotch brite the cylinder put the new piston ring in the cylinder and use the piston to tap it in straight and then look all around between the ring and cylinder walls for daylight do that every half inch or so till you reach the bottom of the cylinder if you see any daylight between the ring and cylinder walls along the way you'll need to get it replated

I did that...it looked fine no light got throe.
@MetalMike1982  I piled that little crack with THIS by my self and polished it with sand paper #1200. Tomorrow I will go to mechanic, to see if it's OK