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Broken stock piston skirt in '06 125

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My son went over to practice at a buddies house a week or so ago while my wife and I were at our twin girls' cross country meet. I get a frantic call on the way home that the bike locked up and he's going home to get his race bike. "Well don't leave the broke one there!" Were my exact words. Kinda.

Finally got a chance to tear it apart today to find one bottom side of the piston missing. I couldn't see anything through the exhaust hole. It was locked up at BDC. Pulled the plug and there was part of the piston up in the electrode.

Now for the absolutely crazy part. No other damage at all. None. He said after he launched off a jump it made a horrible rattle. When he landed it died. He tried to kick it but it was locked solid.

Cylinder and head were perfect no imperfections at all. I completely flushed the bottom end with premix fuel about 5 times. No play at all in the crank. I probably spun the crank when I had it to the point of ready to split the cases for an hour or more. I couldn't believe it.

So after 5 (or more) years of YZ 2 strokes starting with 85s and using stock pistons only (except one Namura) it happened and I'm eating my crow. The top end had right around 20 hours on it. I was actually going to do a top end on it as soon as I had time.

Today's my 45th birthday. That's about a $600 present. Somebody was looking out for me today!

My wife is gone with the kids and has the camera. I might post some pictures of the piston tomorrow.

Oh, and btw, a Wiseco went in. I already had the kit and was planning on giving them a try anyway. It's been through 2 warm ups and sounds ready to rip.

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Looks like the cause was a head gasket failure. For some reason I didn't pay much attention to the top of the piston when I pulled it out. I was looking more at the broken skirt. I noticed the top of the piston had some damage when I pulled the head but didn't look at it closer after I pulled the cylinder because I was more focused on the skirt. I should of realized it wasn't from stuff banging around in there when I saw the head wasn't damaged. I'll post pics later.

Major head gasket fail that looks like it went on for a while. Funny thing is the bike never spewed coolant or gave any sign of a head gasket leak. I pulled the head back off and am going to take it to a machine shop tomorrow to see if it's warped. Hopefully it was just a failed inner oring. If it is warped I'll have them shave some I guess.

Here's the ugliness.

2igiqt1.jpg

64qqs7.jpg

30j1yew.jpg

1oss1s.jpg

2itsm0y.jpg

Edited by Mxracer49

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A buddy and I both blew our top ends on the same day. His is a CR, mine a YZ. His piston looked as if a chunk just broke off of one side above the ring. Mine just melted the very top front edge off. His cylinder was in perfect condition and it cost him $150 and less then a week and he was up and going. My cylinder was jacked and cost me something like $315 and 2 weeks to be up and going. I guess when a piston goes it's pretty much luck as to what is going to happen in there.

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Where'd all the rest of the piston go? In your pipe? Were you able to fish them out above the crank?

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Where'd all the rest of the piston go? In your pipe? Were you able to fish them out above the crank?

There was only one piece in the pipe. The other 4 or 5 chunks were in the bottom end. Like I said, I cannot believe it didn't tear up the crank or cylinder. Luckily it died in mid air and locked up the kick starter so he wasn't able to completely trash it.

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There was only one piece in the pipe. The other 4 or 5 chunks were in the bottom end. Like I said, I cannot believe it didn't tear up the crank or cylinder. Luckily it died in mid air and locked up the kick starter so he wasn't able to completely trash it.

So you fished pieces out by flooding crank with gas?

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So you fished pieces out by flooding crank with gas?

After I took the rest of the motor out of the frame and broke everything down on the clutch side getting ready to split the cases I flipped the motor upside down. 2 or 3 chunks came out. I slowly rotated the crank, flipped it over again and a couple more came out. The crank was freed up at that point. That's when I filled it with premix fuel about 5 times. Sloshed it around a bunch and dumped it out on a big piece of white cardboard so I could see what, if anything was coming out. I got 2 or 3 small pieces out the first flush. The rest of the flushes were clean.

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After I took the rest of the motor out of the frame and broke everything down on the clutch side getting ready to split the cases I flipped the motor upside down. 2 or 3 chunks came out. I slowly rotated the crank, flipped it over again and a couple more came out. The crank was freed up at that point. That's when I filled it with premix fuel about 5 times. Sloshed it around a bunch and dumped it out on a big piece of white cardboard so I could see what, if anything was coming out. I got 2 or 3 small pieces out the first flush. The rest of the flushes were clean.

OK cool. Please update once running. Have a bottom end here which is suspect. Will try this.

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I'd tear the engine apart. Every minute running at 3000rpm won't go unnoted.

Which I was going to do but there is no play in the crank. No catches at all the crank bearings when spun by hand. I ran the bike again yesterday. Did one more warm up, cool down then let it rip. Went through all the gears. Did a couple starts wide open through 3rd gear.

If there was a bad crank bearing I'd be able to hear it. The motor is tight. So far lol

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the problem is not noticable in the very beginning, I had a burst piston too, all moved nicely and when opening the engine I had no doubt to exchange all bearings in the crankcase, including the needle bearing. That apart a new crank or all new bearings are great, one does not really notice the decay when riding. But the differnce alone is worth replacing crank and bearings. Nothing beats a fresh engine :smashpc:

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Guys, keep in mind a piston top losing chunks can be because the bike is running lean. VERY common this time of year with dropping temps.

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This is a common problem for the yz125 with cast/oem pistons.

I've said it before somewhere, but I'll say it again - in a 125 if you run the calcs - piston life SHOULDN'T go much over 15 hours BY DESIGN.

What i mean is if ridden hard - any engineering consulting firm would tell you it should fail in that manner at 15 hours.

The skirt just takes a beating under heavy g force loading...wanting to collapse on way up and expand on way down. Couple that with all the port windows not providing optimum skirt contact situation (trade off to making good power - more window area)...and the piston will eventually break.

The damage to the top IMHO looks like pieces of the failed piston.

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This is a common problem for the yz125 with cast/oem pistons.

I've said it before somewhere, but I'll say it again - in a 125 if you run the calcs - piston life SHOULDN'T go much over 15 hours BY DESIGN.

What i mean is if ridden hard - any engineering consulting firm would tell you it should fail in that manner at 15 hours.

The skirt just takes a beating under heavy g force loading...wanting to collapse on way up and expand on way down. Couple that with all the port windows not providing optimum skirt contact situation (trade off to making good power - more window area)...and the piston will eventually break.

The damage to the top IMHO looks like pieces of the failed piston.

I am an engineering consultant and I say you are wrong.

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I am an engineering consultant and I say you are wrong.

Want me to post up the FEA I've run, actually I probably don't have it on this computer, but i'll dig around maybe i have some images still.

Want to discuss high cycle fatigue life for aluminum, in particular castings?

Lets remember that a yz125 piston see's a peak load of approximently 5000G's at peak RPM.

What about the tremendous point contact loading the skirt on the exhaust side sees due to the "T" port bridge? This is a HUGE contributing factor the the failures.

How about the material changes due to heat?

All these are relatively easy to simulate and make accurate assumptions for.

The rest...lubrication consistency and quality, proper warm up procedure, lack of overheating, even if briefly, riding style, time spent at high rpm, piston to bore clearance in this situation, are not as clear.

But I too, do consulting work, and this is one of the first projects I undertook for personal use - the failure and reasons for failure on yz125's. Why? Like clock work I would fail skirts at 15-17 hours on my personal yz125 with OEM pistons.. New cylinders, replated cylinders etc would never solve it.

When I finally went through all the effort to run "the numbers" it was extremely clear, 15 hours was all I was gonna get, "by the numbers".

If you want, email me and I can send over the data I used to come to that conclusion.

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Do forged pistons last longer Harris?

I always run wiseco and have never had any problems like this. I have takes a a old stock piston and an old wiseco and hit them with a hammer. The oem one exploded with a light hit from a little hammer while the wiseco took a sledge hammer and lots of hits and all it did was flaten.

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This is a common problem for the yz125 with cast/oem pistons.

The damage to the top IMHO looks like pieces of the failed piston.

But those pieces had to travel up there. It's possible each piece could of went through the ports but unlikely. As I said the cylinder is perfect. There was no damage at all. None. You would think with the amount of damage on top of the piston (if caused by a slow coolant leak) the top edge of the cylinder would have been damaged, too.

My theory is it had a very slight seepage of coolant since I did the last top end. Not enough to make the bike run noticeably hot, spew coolant or blow white out the exhaust. But enough to start eroding the top of the piston. I believe for whatever reason, the oring failed/leaked more that day causing the already brittle piston to break under the extreme heat. The piston broke in mid air and locked the bike up immediately. Whether something was damaged from that rotating force being stopped so suddenly is still to be seen. Take this for what it's worth but I've seen and worked on many motors and these crank bearings and crank felt as solid and turned as smooth as any new ones I've just swapped out.

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Do forged pistons last longer Harris?

I'm wouldn't say forged pistons last any longer as far as performance. The benefit of forged is that all you start losing is the performance/compression. With a cast piston there is the chance of breakage if ran past it's service life.

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