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power valve hit piston

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piston.jpg

So I'm pretty sure this is from the power valve hitting the piston, can any body tell me why this happened, and what parts am i gonna need to replace. I'm guessing that im going to need a new piston, rings, powervalve, and stopper. also im told the stock powervalve for the 98 yz 250 is terrible, is there an upgraded one i can buy and where could i get it?

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May be that it hit the opening into the power valve and not the power valve itself. Did you check the cylinder head? It may need to be sleeved. Try Q & E in Placentia; they do great work for reasonable cost.

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piston.jpg

So I'm pretty sure this is from the power valve hitting the piston, can any body tell me why this happened, and what parts am i gonna need to replace. I'm guessing that im going to need a new piston, rings, powervalve, and stopper. also im told the stock powervalve for the 98 yz 250 is terrible, is there an upgraded one i can buy and where could i get it?

Ignore the suggestion to get the cylinder re-sleeved unless you actually have a sleeve installed. The 1998 YZ250 did not come with one. It uses nikasil like every other mx'er and there's no need to sleeve the cylinder unless there is MAJOR gouging in it that cannot be fixed with plating.

The stock PV for the 98 is not terrible. It does its job just fine if it is properly maintained.

Just check all bore measurements. Work your power valve by hand to see if it sticks out into the bore.

You can see in the pic below (ignore the blue circled areas, they are for another post explaining something else) above the oval-ish blue highlight that the PV comes pretty close to coming into the cylinder area and, if you cylinder was bored out a lot without milling off a little of the PV, it may stick out far enough to contact the piston.

RIGHT-CLICK, VIEW IMAGE to see the high-resolution image

img0098zc9.jpg

Check your bore diameter. If the bike has been bored out a lot, and the power valve was not re-shaped to account for the larger diameter bore, the power valve could very well hit the piston.

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As to what you'll need to fix the problem...

Piston Kit (Piston, Rings, Circlips, Wristpin, Wristpin bearings, Gaskets)

Replate Cylinder (Depending on condition of cylinder)

Power Valve assembly (if it was hitting the piston, it's damaged too)

Gasket Kit (I ordered the complete gasket kit because it gives you ALL the gaskets you'll need in one kit, and you can store the ones that come with the piston kit for a later top-end job)

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piston.jpg

So I'm pretty sure this is from the power valve hitting the piston, can any body tell me why this happened, and what parts am i gonna need to replace. I'm guessing that im going to need a new piston, rings, powervalve, and stopper. also im told the stock powervalve for the 98 yz 250 is terrible, is there an upgraded one i can buy and where could i get it?

You may also want to correct the punctuation error in your Signature to use an apostrophe ' instead of a comma , in the contraction, "I,d" (sic)

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The powervalve in this bike rotates too far once the stops wear down. Its a good design, but needs to have the powervalve ground down where it hits before it can do any damage. You can also replace the parts on a regular basis, but the grinding accomplishes the same thing for cheap. I would agree with replacing yours now, but if this is caught early, grinding the pvv works just fine. Its aluminum so a person can use a hand file to open it up some.

Did you actually hurt your cylinder? Often times it will rub the piston without doing any cylinder damage.

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All that and a grammar lesson too...

The suggestion was to get a sleeve not re-sleeved & only if it hit it hard enough and chunked the nikasil coating (that is why I suggested to check the cylinder...) By the looks of the piston and rings it looks like it hit the wall of the cylinder pretty hard.

I,m sure if you look at the pic again it,d be hard for you to not concur.

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R-Cannon-Your the Man-This guy knows his stuff --helped me with my 94 yz 250.

Mine was a combination of Crap!!.#1Cylinder was wore alittle to far.My rings hit the top of the exhaust port.#2.The power-valve linkage was sloppy-and the screw that retains or steadys the pv was loose.#3.PV stop was bent This combined with the the rest of the slop let the pv open too far. Now far as the Cylinder-Since these are getting way hard to get -Take it in .I have heard pro's and con's about each method.I nikasil mine Sweet''.My KTM buddy at work sleeved his ride. He said #1.Std sleeve can rebuilt many times.#2.More clearance from PV -vs-Pistion. And last but least--The design in my opinion is crap! If i had it my way--I would re-design it.This seems to be one of the weakest links in YZ Top-end falure ---Good Luck.

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R-Cannon-Your the Man-This guy knows his stuff --helped me with my 94 yz 250.

Mine was a combination of Crap!!.#1Cylinder was wore alittle to far.My rings hit the top of the exhaust port.#2.The power-valve linkage was sloppy-and the screw that retains or steadys the pv was loose.#3.PV stop was bent This combined with the the rest of the slop let the pv open too far. Now far as the Cylinder-Since these are getting way hard to get -Take it in .I have heard pro's and con's about each method.I nikasil mine Sweet''.My KTM buddy at work sleeved his ride. He said #1.Std sleeve can rebuilt many times.#2.More clearance from PV -vs-Pistion. And last but least--The design in my opinion is crap! If i had it my way--I would re-design it.This seems to be one of the weakest links in YZ Top-end falure ---Good Luck.

I understand why you dont like it, and you are right, but I liked the design because it was so easy to deal with. The stopper on the left side of the cylinder and the pv were to soft, but other than that, the design rocked! Well, not for this poor guy, but still. I found out about this in Eric Gorrs book. I went out to the garage the day I got it and looked at mine. It had been rubbing, but only enough to leave a polished spot on the piston. I cut it back and its still running today. If a person knows about it and cuts the pv back 1-2mm, it will never be a problem.

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STD piston doesn't require reshaping of the power valve. Usually anything +1mm will require mod. So 4 oversizes above STD is where they say to reshape.

I bored my 87 out to 70mm and had to round the powervalve maybe 1mm. But if I hadn't it would have destroyed my piston/cylinder. -BIG DAN:thumbsup:

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It actually did no damage to the cylinder at all surprisingly, i found out what caused it and it wasn't a mechanical error, it was the mechanic (me). I learned the hard way to not tear stuff appart when your in a hurry and leaving for a trip, i left a bolt out of the power valve stopper and loosened up wile riding, the rest is obvious. im going to take the power valve out of the cylinder later on today hopefully and see if it is salvagable ill post some pictures as soon as i get time

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Hello there

I wonder if I still have to do a cut back the 1 to 2mm on the PV on a bike like mine YZ250 year 2009.

regards

arnego2

No, this was for the powervalve on the earlier engine. The newer engine uses something totally different.

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Post revisited.....that piston is almost exactly what my old piston looked like before I rebuilt the motor. Today I learned my powervalve is still having issues with hitting the piston rings...will probably have to put a new set of rings in????? Will need to tear apart (again) and see what can be saved.

Here is what the new sleeve and piston look like after about 10 minutes of running. I also am curious about those 2 identical score lines in vertical lines on opposite sides of the exhaust port....and the scorn area on top of the powervalve

http://i182.photobuc...e/cylinder2.jpg

Edited by e36graduate

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