'03 YZ250 Piston Damage

[SM421]

I recently purchased a used 2003 YZ250. After a couple of outtings, I pulled the top end for an inspection. I discovered small craters in the crown of the piston as if the bike had broken a ring at one time and smashed it between the piston and head. Apparently the previously own replaced the rings and put it back together. What is really strange is that in addition to the craters, there is a 3mm band around the entire crown of the piston that looks as if the piston was melting. I've never seen this type of deterioration before. Anyone have an explaination for this kind of damage?

[FFRacing11x]

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[psycho393]

Looks to me like he lost rings, or maybe low end bearing races (experience here) and took the cheap way out by throwing enough in to keep it running and sell it. Nothing more. I would replaces the piston, rings and check my low end bearings and double check all tollerances on the rod bearings. Other then that I wouldn't worry to much if the cylinder looks good. Good luck!

[suzuki29]

i went through the same thing before and had to replace the lower end. to me with a few years of experience id say your crank bearing went out. did it get the cylinder too??

[Vintage Dave393]

Looks like damage caused by bearing failure to me. I would find the seller and after you finish pounding him ask him what maintenance he had to do to get it running again. You need to know if the problem was fixed or if you have more work to do (bottom end). I'm guessing he didn't say anything about the damage prior to you finding it?

[SM421]

Well I decided the safe thing to do was to disassemble everything. The rod and rod bearing were perfect. There was no damage to the main bearings or the cases. Since it was apart, I replaced the main bearing and seals any way.

The cylinder was damaged so everything from the cases up is new. To include a new cylinder.

I still believe the damage was from a broken ring or a top end bearing. But, since the previous owner replaced the damaged parts I'll never know for sure.

I actually bought the bike from a dealership where I have done business for years. I went back to them and explained what I found wrong with the motor. The bike was a trade in and I'm sure the shop had no clue what the inside of the motor was like. But the owner helped me out by selling me all the parts a cost.

[Aus_Rider_22]

Good to hear it's all good now.

[yzernie]

They didn't fix it for you but it is good to hear the dealer helped you out with the at cost parts. They could have said to go pound sand. There still are some decent dealerships out there.

[Oring]

Run the girl abit richer my piston looked the same but it was definately running lean.
ring damage and lots of it. The lighter break up is caused when your running lean, Look on the inside of the head! you,ll probably see something similar!
03 are good bikes. Mine was a gem. Had the suspenders redone to suit my fat guts and after that it was majic!
Got to get a new yz 250 alloy frame! (dreaming)
Check your barrel out , wack another slug in there and get out riding!
Good luck!

[mossman]

Make sure your jetting is spot on. It looks like there may have been some detonation around the perimeter of the dome. That is a sure sign of a lean condition. There is a chance that the damage was from debris, but based on your description (piston looked like it was melting), it sounds like detonation.

Good luck,
Joe