Piston problem

[kevinkdx]

Every year I make a major comittment to tearing down my bike and rebuilding it. And it seems that every year that I do the rings are seized up in my piston. Now I run Wiseco Pro-Lite pistons every time I rebuild it and I take all the care in the world when breaking it in. I'm not sure what could be causeing the rings to seize up like this in the ring grooves. Maybe getting too hot? I did have a cracked head and all the previous pistons including the current have been run with the cracked head in place although i now have a new head. maybe the piston expanded too far and locked the rings in the grooves. I think that the rings should still be loose in the grooves when you tear it down after a year of riding? right? I ride a lot during the summer, maybe i'm not replacing rings often enough? I don't know but this is starting to make be wonder about my motor. I have put a lot of money into it over the years. new cylinder, new head, new crank, all new gaskets and seals. just don't understand it.

 

[kevinkdx]

There is also what appears to be carbon build up on the underside of the piston top. I was wondering if that was normal, because in most other motors that i have ever taken apart they may be brown but not with carbon built up on the underside.

 

[Rich Rohrich]

Excessive heat in the piston crown is the usual cause of the black carbon under the dome. The cracked head could easily cause overheating issues and excessive heat in the crown area. If you run the crown hot enough for extended periods of time it will tend to collapse the upper ring land and seize the ring in the groove.
Even if the cooling system isn't malfunctioning too hot a plug, excessive caron buildup on the crown or overly advanced ignition timing can cause preignition and have these same types of piston crown heat issues, with the same end results.

If you measure the ring land area on your piston and compare it to a new one you should be able to get an idea of this is the cause of your ring sticking problem.

 

[levert]

Adding to what Rich said, have you checked the cylinder bore.?You might have worn the chrome down and there is to much clearance even with a new piston.Usually you'll hear the knocking.
Rings are seated from the gases pushing them out against the cylinder, to much clearance and they leak,you must have some wicked blow by marks down the side of the piston?

 

[canyncarvr]

Some oils leave more carbon deposits than others.

That wouldn't have anything to do with the carbon on the underside of the piston. That cannot get there without a lot of heat!

You're referring to your 250?...or your 200? If it's the 200, try a pro-x piston next time.

Not for any particular reason. I'm just not a wiseco fan.

 

[Rich Rohrich]

Originally posted by canyncarvr
Some oils leave more carbon deposits than others.

That wouldn't have anything to do with the carbon on the underside of the piston. That cannot get there without a lot of heat!

That isn't entirely true. If there is a build up of carbon on the piston crown or combustion chamber it can often times stay red hot between cycles and cause the mixture to ignite ahead of the normal spark , in essence advancing the timing. With the power valve closed and the compression high this surface ignition can easily lead to further pre-ignition symptoms and overheat the crown to the point that carbon burns show up on the underside of the piston along with other heat related issues. Carbon tends to cause a lot of problems in a two-cycle engine.

 

[canyncarvr]

Well, pick pick pick!!

I wasn't referring to carbon buildup in the chamber or on the piston crown, but on the piston side/skirt that could be contributing to das-ringeth-sticketh.

I was unclear about that. But then, I didn't say, 'This is entirely true!' either. ;)

The fact that such carbon buildup wasn't mentioned notwithstanding; I was..uh..preceptive.


Taking into account what I know to be so even if I didn't say it then I'm sure what I said is true.

Can't find a problem with that 'eh?

Yeah. Carbon surely is a problem.

Cheers!

 

[kevinkdx]

I am refering to my KDX 250 in this case, I think that with the cracked head it just got too hot and bad things, carbon build up under the piston head and ring locking, was the outcome. Thanks for all of the replies and for the help.