Piston crown erosion question

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2/6/2021 11:53 AM

My son and I picked up a 1996 WR250ZH and are in the process of doing a partial restoration on it. It ran before tearing it down, but take a look at the exhaust side of this piston crown. It’s eroded and curved. I’ll be having the cylinder bored and will be putting in a new piston kit. Does anyone know what could have caused the piston crown erosion? Photo


2/6/2021 6:51 PM

There are others on here that are far more knowledgeable than me, but I would say that is a result of a lean condition and detonation. I would think about splitting cases for crank inspection and at least replace mains & seals. Check your carb settings for an overly lean setup. I would also inspect the head for warpage as it’s possible a leaking head gasket/orings could have contributed to the piston condition. If the cylinder is a plated cylinder, you will want to have it replated vs bored out. There is also something going on with the intake side of that piston...


2/6/2021 6:59 PM

Yeah, I just figured out that it’s a plated cylinder. It does need replacing, so I’m having that done, and am replacing all seals and bearings and putting in a new top end kit. I’ll check out possible causes for a lean condition, as well.


2/7/2021 5:39 AM

Show underside of piston, could have had a mag side main seal leak also... you want to make sure you find the cause of that piston condition before you rebuild everything.


2/7/2021 6:46 AM

I’ll pull it off and look under it. I’m thinking you’re right, as heat is the only thing that would cause it to melt like that.


2/7/2021 8:07 AM
Edited Date/Time: 2/7/2021 8:09 AM

That piston crown was pitted away by detonation (pre-ignition). You got very lucky that you didn't scatter that motor. The causes of detonation are:

-low quality fuel
-incorrect ignition timing (follow service manual instructions to verify correct ignition timing)
-excessive combustion temperature (caused by lean condition)
-excessive compression (probably not the culprit as the head is most likely stock. A thinner base gasket can cause this)
-incorrect heat range of spark plug.

You will have to do some CSI work to determine the root cause of the detonation. Air leaks can cause a lean condition, incorrect ignition timing etc....

One bit of advice it to make sure the powervalve stop (left side cap on cylinder) is not worn. This will allow the powervalve to over-rotate and encroach into the cylinder and snag the piston ring. This is a well known failure point of that motor. Good luck!


2/7/2021 11:23 AM

Pressure test it before you run it again.


04 KX125
94 KX250SM
75 Z1-900

2/7/2021 1:38 PM

Yeah, we just picked this up from a guy, so no telling of the abuse, and neglect it suffered. In the process of the tear down and inspection, now. I’ve never seen a piston in this condition before. I’m sure it will be all good once we get done with it. Thanks guys!