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Piston pin tight in piston

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Hey guys. I have a 2005 yz250 and I have had a lot of trouble keeping it running. 3 cylinders, 3 oem top ends, and 2 different heads and still won't live. The top ring keeps catching the exhaust port and destroys the piston and takes the cylinder with it. This occurs after less than 10 hours of run time.

When I bought it from the previous owner it had a wiseco top end, dep pipe, the cylinder head measured .045-.048 on squish, 180 main jet, 50 pilot, n3ew at 2 (from top). I ran super M at 32:1 and ran 1 gallon c12 to 2 gallons pump premium. The bike ran incredibly good for about 6 hours before it hit the port and blew up.

Now the second rebuild I bought a brand new B cylinder and B piston kit from Yamaha, measured all parts and assembled. Continued running the same fuel and roughly the same jetting making changes as needed and still after about 7 hours run time it had the same failure.

Currently I have 1.4 hours on a millennium cylinder, I purchased a different stock head off of ebay (thinking this could be the problem) C sized piston (had cylinder micd at local Yamaha dealer and they suggested and sold me the C sized piston) I measured everything as always, assembled and runs great. However the piston is already showing wear in the same spot under the top ring just like the previous pistons did (looking up in the exhaust port the piston is getting shiny under the top piston ring) and I can tell its ready to blow up again.

Now the crank has always been in good shape. No up and down and the main bearings are smooth. The crank seals are new on this last rebuild and timing was stock for the first rebuilds and I recently backed off because of slight detonation on account I am no longer running a blend of race fuel on this last rebuild.

Now I guess I would like to hear suggestions on what could be causing this but also I would like to mention that each one of these rebuilds I had a very difficult time getting the piston pin OUT of the piston. It was very tight (swollen from getting to hot?). Is anyone else experiencing this or could this be my issue? I've had a handful of other bikes and I remember the pin sliding out.

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Mine caught the exhaust too after about the same time frame. I 295'd it. Interesting u had the same issue.

 

Piston pin tapped right out though. My 3/8 extension fits perfect to just tapper out

Edited by jeffdanger

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Mine caught the exhaust too after about the same time frame. I 295'd it. Interesting u had the same issue.

Piston pin tapped right out though. My 3/8 extension fits perfect to just tapper out

Yeah I'm just at a loss. I'm usually pretty good at diagnosing a problem but this ones got me. I too use my extension to "tap" the pin out. So is the 295 kit working out for you? I recessed the power valve wing also because after the first failure it to had a shiny spot where it made contact with the piston.

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Are you using the same power valve main valve? Is it possibly hitting the piston?

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Are you using the same power valve main valve? Is it possibly hitting the piston?

I am, but I did grind it down slightly and it doesn't hang into the bore in the fully closed position. Do those wear out and get closer to the bore? Because I originally thought that is what was causing my problems.

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I am, but I did grind it down slightly and it doesn't hang into the bore in the fully closed position. Do those wear out and get closer to the bore? Because I originally thought that is what was causing my problems.

Mine didnt hit my powervalve. Looked like my ring broke. Probably because im so damn fast.

295 is ok on the mx track, i think she had plenty of power as a 250...i have the feeling i will notice it more in the desert and dunes.

 

There IS a little stopper piece that can break off on top of the powervalve...theres threads on it here...however it wasnt broken on mine...and if i remember correctly it still isnt a part that would cause this sort of issue.

 

Theres also a detonation theory...i had some detonation on my intake but none on my exhaust.

Someone else had a factory port chamfer theory...id be interested to inspect a factory cylinder for that.

 

I cant find any useful links as to why these rings are breaking. Id like to know just to know.

Edited by jeffdanger

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Mine didnt hit my powervalve. Looked like my ring broke. Probably because im so damn fast.

295 is ok on the mx track, i think she had plenty of power as a 250...i have the feeling i will notice it more in the desert and dunes.

There IS a little stopper piece that can break off on top of the powervalve...theres threads on it here...however it wasnt broken on mine...and if i remember correctly it still isnt a part that would cause this sort of issue.

I cant find any useful links as to why these rings are breaking. Id like to know just to know.

Yeah I have seen the threads on the tip of the valve breaking off. Mine is whole. I too would like to know what is causing the rings to break. I am about to sell the bike because I feel like I've tried everything. I have owned several 2 strokes and have never had problems like this.

Also how can a power valve be installed backwards? I have seen and heard of this happening. Is that referring to the cams that open the 2 side power valves being installed opposite?

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If the power valve is sinking into the cylinder due to wear the first thing it will contact is the piston ring, as it "pushes out" into the exhaust port slightly as it passes.  That's why my thought went to power valve after reading that this is happening on multiple pistons and cylinders. 

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If the power valve is sinking into the cylinder due to wear the first thing it will contact is the piston ring, as it "pushes out" into the exhaust port slightly as it passes. That's why my thought went to power valve after reading that this is happening on multiple pistons and cylinders.

I thought the same thing so on my first actual rebuild (with the oem B piston and cylinder) I took a dermal to the curve of the valve where it "would" make contact. I then slid the valve in position and made sure it didn't poke into the bore. But really the only items that have been reused is the crank and the power valve system. ImageUploadedByThumper Talk1401880699.991963.jpg

Here is a picture showing the wear on the piston. This is a few hours before it would catch the ring.

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So...i called forward motion just to see if they had any input...

They havent seen this problem as a common issue...and they said they get about 3 yz cylinders a day.

So i suppose its not as common as we think.

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So...i called forward motion just to see if they had any input...

They havent seen this problem as a common issue...and they said they get about 3 yz cylinders a day.

So i suppose its not as common as we think.

Well thank you for digging deeper. I have spoke with a handful of really reputable Yamaha tuners in my area as well as the dealership and my mechanic friends and nobody has any ideas. One of the tuners mentioned I should find an oem expansion chamber. He was saying maybe that dep pipe has too radical of a return cone and could be holding heat in but I'm not buying that. I mean I'm not the only one to own that pipe.

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Any other suggestions out there as to why my bike is eating pistons after about 6-8 hours run time? Gonna sell it as is and get something different if I can't figure it out soon.

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