Two Stroke Motocross

Two Stroke Motocross Forum => Technical => Topic started by: MX_Peter on February 16, 2012, 11:11:04 PM

Title: First piston change on my RM250 2007
Post by: MX_Peter on February 16, 2012, 11:11:04 PM

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:

Title: First piston change on my RM250 2007
Post by: factoryX on February 17, 2012, 02:26:57 AM

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.

Title: First piston change on my RM250 2007
Post by: TMKIWI on February 17, 2012, 07:18:01 AM

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.?

Title: First piston change on my RM250 2007
Post by: MX_Peter on February 17, 2012, 10:35:13 AM

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

Title: First piston change on my RM250 2007
Post by: MX_Peter on February 17, 2012, 10:40:58 AM

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

Title: First piston change on my RM250 2007
Post by: SachsGS on February 17, 2012, 03:42:25 PM

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.

Title: First piston change on my RM250 2007
Post by: MX_Peter on February 17, 2012, 04:55:07 PM

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

Title: First piston change on my RM250 2007
Post by: Recovered on February 17, 2012, 06:04:43 PM

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

Title: First piston change on my RM250 2007
Post by: cnrcpla on February 17, 2012, 11:43:52 PM

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.

Title: First piston change on my RM250 2007
Post by: MetalMike1982 on February 22, 2012, 06:03:07 PM

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 .

Title: First piston change on my RM250 2007
Post by: downonmonday on February 22, 2012, 09:14:33 PM

Quote from: MetalMike1982 on February 23, 2012, 02:03:07 AM

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!!!!

Title: First piston change on my RM250 2007
Post by: MX_Peter on February 22, 2012, 09:35:08 PM

Quote from: oldermxr on February 18, 2012, 02:04:43 AM

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