Piston destruction issues

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12/5/2018 2:47 PM

Can anyone tell what is causing this? This has happened twice. I got the bike like such and then Replaced the top end for it to do it again 4 hours later. I was told it could be detonation and running lean. Bike is an 03 yz125. Photo
Photo
Photo

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12/5/2018 3:05 PM

What brand piston?

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
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12/5/2018 3:39 PM

mattyhamz2 wrote:

What brand piston?

both of them were ProX.

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12/5/2018 3:44 PM

What is piston to cylinder clearance? If cylinder getting worn and piston slapping/rocking I could see causing that type of damage.........

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12/5/2018 4:06 PM

the bike also ran hot so maybe that could add to the fact that maybe it ran lean or something?

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12/5/2018 4:41 PM

What gas is used? Premix ratio? Jetting? Sparkplug color? Also x2 on the above

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12/5/2018 6:24 PM

If as stated, it happen twice in 4 hours....
I would be looking at trash in the crank cavity.
Excess piston to cylinder clearance.
Old fuel mix.
Not allowing the engine to fully warm up prior to putting it under heavy load.
Did you check the squish?

Paw Paw

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12/5/2018 9:45 PM
Edited Date/Time: 12/5/2018 9:57 PM

m1ke20012 wrote:

the bike also ran hot so maybe that could add to the fact that maybe it ran lean or something?

it looks like it seized.

there are also a few threads on the net about pro x pistons breaking exactly like yours.

https://www.google.com/search?q=pro+x+piston+broke&ie=utf-8&oe=utf-8&client=firefox-b-1

exactly what do you mean by running hot?

exactly what was the piston to cylinder clearance?

exactly how did you measure the clearance?

can you post a photo of the end of the spark plug from a slight angle?

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12/6/2018 2:53 PM

kb228 wrote:

What gas is used? Premix ratio? Jetting? Sparkplug color? Also x2 on the above

93 octane 32:1, jetting unknown Photo
Photo

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12/6/2018 2:55 PM

m1ke20012 wrote:

the bike also ran hot so maybe that could add to the fact that maybe it ran lean or something?

barnett468 wrote:

it looks like it seized.

there are also a few threads on the net about pro x pistons breaking exactly like yours.

https://www.google.com/search?q=pro+x+piston+broke&ie=utf-8&oe=utf-8&client=firefox-b-1

exactly what do you mean by running hot?

exactly what was the piston to cylinder clearance?

exactly how did you measure the clearance?

can you post a photo of the end of the spark plug from a slight angle?

I don’t remember those specs and it just ran hot but here is the sparkplug Photo
Photo

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12/6/2018 2:58 PM
Edited Date/Time: 12/6/2018 3:15 PM

Paw Paw 271 wrote:

If as stated, it happen twice in 4 hours....
I would be looking at trash in the crank cavity.
Excess piston to cylinder clearance.
Old fuel mix.
Not allowing the engine to fully warm up prior to putting it under heavy load.
Did you check the squish?

Paw Paw

Thanks ill be checking all these when I reassemble do you have any suggestions for heat cycling the piston? Is there a way you had always done it? Do you think it would be wise to have the cylinder bored out or replated incase it isn’t the same clearance all the way through?

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12/6/2018 4:07 PM

The top of piston shows the squish band was kissin it. I check my bikes for squish clearance...if it is under .030" and the wrist pin gts a little sloppy the piston could be kissing the head,,crushing ring land tight,,,then the scuffing happens leading to the black residue on skirts as it scrapes and finally cracks Dirt entering up stream could lead to this too....kinda like what came first chicken or egg.
Check check double check all the details. Don't assume a shop bored it with proper clearance...check it yourself,,,don't assume piston top isn't too close...check it all the way around with clay...I have seen pistons tighter on one side...Don't assume main bearings are good cause they are new...check them up and down with rod.
.

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12/6/2018 4:27 PM

riv187 wrote:

The top of piston shows the squish band was kissin it. I check my bikes for squish clearance...if it is under .030" and the wrist pin gts a little sloppy the piston could be kissing the head,,crushing ring land tight,,,then the scuffing happens leading to the black residue on skirts as it scrapes and finally cracks Dirt entering up stream could lead to this too....kinda like what came first chicken or egg.
Check check double check all the details. Don't assume a shop bored it with proper clearance...check it yourself,,,don't assume piston top isn't too close...check it all the way around with clay...I have seen pistons tighter on one side...Don't assume main bearings are good cause they are new...check them up and down with rod.
.

interesting info, so you're saying the squish is wrong and its hitting the head? the ring is indeed squished on both and I cant even get it out on the one its so bad in there. Would that also account for the shirt breakage? I can show more pictures if you want.

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12/6/2018 4:34 PM
Edited Date/Time: 12/6/2018 4:34 PM

Loose bore=piston rocking / piston skirt fails. If you are going to try it again, put a "D" piston in it if available.

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12/6/2018 4:45 PM

project racer wrote:

Loose bore=piston rocking / piston skirt fails. If you are going to try it again, put a "D" piston in it if available.

So what you're saying is that the piston is too small and it has play?

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12/6/2018 5:07 PM
Edited Date/Time: 12/6/2018 5:08 PM

project racer wrote:

Loose bore=piston rocking / piston skirt fails. If you are going to try it again, put a "D" piston in it if available.

m1ke20012 wrote:

So what you're saying is that the piston is too small and it has play?

Look in the service manual for cylinder to bore clearance specs. If it's more than .003 - .004, it's loose. The D piston may make up a fraction of the clearance to get it closer. A big concern would be stray tramp metal littering the bottom end. There is aluminum specs everywhere on that head/plug. Also ,what is the condition of the plating on the cylinder?

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12/6/2018 5:44 PM
Edited Date/Time: 12/6/2018 5:51 PM

project racer wrote:

Loose bore=piston rocking / piston skirt fails. If you are going to try it again, put a "D" piston in it if available.

m1ke20012 wrote:

So what you're saying is that the piston is too small and it has play?

i seriously doubt it broke from too much play.

i would use a different brand of piston.

check the squish clearance and make sure the skirt is not hitting the crank.

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12/6/2018 5:46 PM

riv187 wrote:

The top of piston shows the squish band was kissin it. I check my bikes for squish clearance...if it is under .030" and the wrist pin gts a little sloppy the piston could be kissing the head,,crushing ring land tight,,,then the scuffing happens leading to the black residue on skirts as it scrapes and finally cracks Dirt entering up stream could lead to this too....kinda like what came first chicken or egg.
Check check double check all the details. Don't assume a shop bored it with proper clearance...check it yourself,,,don't assume piston top isn't too close...check it all the way around with clay...I have seen pistons tighter on one side...Don't assume main bearings are good cause they are new...check them up and down with rod.
.

"The top of piston shows the squish band was kissin it."

not necessarily.

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12/6/2018 5:50 PM

m1ke20012 wrote:

I don’t remember those specs and it just ran hot but here is the sparkplug Photo
Photo

"I don’t remember those specs and it just ran hot but here is the sparkplug"

ok, explain in detail exactly why you think it ran hot, ie, hotter than normal.

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12/6/2018 6:15 PM

My question is why does that base gasket look like it is either the wrong one or installed wrong. It doesn't appear to fit correctly which would lead me to believe a leak could have existed. I would also say most of the times I've seen broken piston is because of detonation or garbage left behind in the bottom end.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

12/6/2018 6:21 PM

I would also say I've been a big fan of Harry Klemm from way back and he has some interesting reads on his website. Although he does watercraft mostly now, he was once one of the top 2 stroke guys around. This link talks about piston seizures, might give some insight to some. http://www.groupk.com/tec-seizures.htm

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

12/6/2018 7:08 PM

dkurtd wrote:

I would also say I've been a big fan of Harry Klemm from way back and he has some interesting reads on his website. Although he does watercraft mostly now, he was once one of the top 2 stroke guys around. This link talks about piston seizures, might give some insight to some. http://www.groupk.com/tec-seizures.htm

Harry was the tuner for the mini program when i worked in the r and d department of kawasaki. He tried to blow up the rental car when we went to loretta lynns. He ported my tecate cylinder and i got a pipe from him for it. I beat jimmy white and the honda factory riders with it when i won the last 3 wheeler race at saddleback.

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12/6/2018 7:57 PM

dkurtd wrote:

I would also say I've been a big fan of Harry Klemm from way back and he has some interesting reads on his website. Although he does watercraft mostly now, he was once one of the top 2 stroke guys around. This link talks about piston seizures, might give some insight to some. http://www.groupk.com/tec-seizures.htm

barnett468 wrote:

Harry was the tuner for the mini program when i worked in the r and d department of kawasaki. He tried to blow up the rental car when we went to loretta lynns. He ported my tecate cylinder and i got a pipe from him for it. I beat jimmy white and the honda factory riders with it when i won the last 3 wheeler race at saddleback.

He ported my 1984 KX80 and 1985 KX125 when he ran Klemm Research. Both ran very well after he was done.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

12/6/2018 9:35 PM

Does that YZ125 engine have a spacer plate.... It looks like there is a spacer plate under the base gasket in the one picture.
Photo

STOCK YZ125
Photo
Photo

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12/7/2018 5:42 AM

riv187 wrote:

The top of piston shows the squish band was kissin it. I check my bikes for squish clearance...if it is under .030" and the wrist pin gts a little sloppy the piston could be kissing the head,,crushing ring land tight,,,then the scuffing happens leading to the black residue on skirts as it scrapes and finally cracks Dirt entering up stream could lead to this too....kinda like what came first chicken or egg.
Check check double check all the details. Don't assume a shop bored it with proper clearance...check it yourself,,,don't assume piston top isn't too close...check it all the way around with clay...I have seen pistons tighter on one side...Don't assume main bearings are good cause they are new...check them up and down with rod.
.

m1ke20012 wrote:

interesting info, so you're saying the squish is wrong and its hitting the head? the ring is indeed squished on both and I cant even get it out on the one its so bad in there. Would that also account for the shirt breakage? I can show more pictures if you want.

Yes ,I believe the shadow ring around piston crown is from it kissing the heads squish band. That will collapse ring land,,,cause piston to bang back forth without the ring as a guide. Heat on piston now couldn't transfer from dome properly(the ring not making full contact with cylinder) and all broke loose in final nano second. The question is why did it kiss the head? To tight clearance up top and wide on bore? Wore main rod or wrist pin bearing alowing extra movement,,,dirt sucked in wore out bearing and let piston fly up higher? You said this happened 4 hours earlier to last guy. Was the bearings replaced main/rod ? or just just new piston? Possibly the head was cut before,,,the gasket kit had thin ones,,and the piston was a little prouder than stock (stuck up higher in bore) . All the little tollerances stack up and led to this. I love dirtbikes,,but I've built many race engines for circle track and dragsters too. I've learned early on about tolerance stack up leading to catastrophic failure ...when a shop does anything- install rod,mains,bore or hone,end gap on ring,,-trust but verify!

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12/7/2018 6:24 AM

riv187 wrote:

The top of piston shows the squish band was kissin it. I check my bikes for squish clearance...if it is under .030" and the wrist pin gts a little sloppy the piston could be kissing the head,,crushing ring land tight,,,then the scuffing happens leading to the black residue on skirts as it scrapes and finally cracks Dirt entering up stream could lead to this too....kinda like what came first chicken or egg.
Check check double check all the details. Don't assume a shop bored it with proper clearance...check it yourself,,,don't assume piston top isn't too close...check it all the way around with clay...I have seen pistons tighter on one side...Don't assume main bearings are good cause they are new...check them up and down with rod.
.

m1ke20012 wrote:

interesting info, so you're saying the squish is wrong and its hitting the head? the ring is indeed squished on both and I cant even get it out on the one its so bad in there. Would that also account for the shirt breakage? I can show more pictures if you want.

riv187 wrote:

Yes ,I believe the shadow ring around piston crown is from it kissing the heads squish band. That will collapse ring land,,,cause piston to bang back forth without the ring as a guide. Heat on piston now couldn't transfer from dome properly(the ring not making full contact with cylinder) and all broke loose in final nano second. The question is why did it kiss the head? To tight clearance up top and wide on bore? Wore main rod or wrist pin bearing alowing extra movement,,,dirt sucked in wore out bearing and let piston fly up higher? You said this happened 4 hours earlier to last guy. Was the bearings replaced main/rod ? or just just new piston? Possibly the head was cut before,,,the gasket kit had thin ones,,and the piston was a little prouder than stock (stuck up higher in bore) . All the little tollerances stack up and led to this. I love dirtbikes,,but I've built many race engines for circle track and dragsters too. I've learned early on about tolerance stack up leading to catastrophic failure ...when a shop does anything- install rod,mains,bore or hone,end gap on ring,,-trust but verify!

You’d be exactly right about all that, I had my sister and her boyfriend come over and they looked at it and we realized it was indeed hitting the head just barely especially with the way that black spot is, I held it up to the head and they match up. I guess I need to check the gapping this time and make sure the piston/cylinder clearance is good, I think im gonna get it honed out this time.

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12/7/2018 6:26 AM

project racer wrote:

Loose bore=piston rocking / piston skirt fails. If you are going to try it again, put a "D" piston in it if available.

m1ke20012 wrote:

So what you're saying is that the piston is too small and it has play?

barnett468 wrote:

i seriously doubt it broke from too much play.

i would use a different brand of piston.

check the squish clearance and make sure the skirt is not hitting the crank.

Thats what im doing this go around. Different piston and checking the gap.

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12/7/2018 6:28 AM

barnett468 wrote:

it looks like it seized.

there are also a few threads on the net about pro x pistons breaking exactly like yours.

https://www.google.com/search?q=pro+x+piston+broke&ie=utf-8&oe=utf-8&client=firefox-b-1

exactly what do you mean by running hot?

exactly what was the piston to cylinder clearance?

exactly how did you measure the clearance?

can you post a photo of the end of the spark plug from a slight angle?

m1ke20012 wrote:

I don’t remember those specs and it just ran hot but here is the sparkplug Photo
Photo

barnett468 wrote:

"I don’t remember those specs and it just ran hot but here is the sparkplug"

ok, explain in detail exactly why you think it ran hot, ie, hotter than normal.

Yes hotter than normal in the top end.

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12/7/2018 7:52 AM

m1ke20012 wrote:

I don’t remember those specs and it just ran hot but here is the sparkplug Photo
Photo

barnett468 wrote:

"I don’t remember those specs and it just ran hot but here is the sparkplug"

ok, explain in detail exactly why you think it ran hot, ie, hotter than normal.

m1ke20012 wrote:

Yes hotter than normal in the top end.

Yes but my question is exactly how do you know it was running hotter than normal?

Do you have a temperature gauge?

Were you going full throttle up a steep hill?

.

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12/7/2018 10:29 AM

The ring is the conductor of heat from piston top out to cylinder,,,when it stuck and jambed into ring land,,heat spiked on top leading to the specs in chamber. Then the piston snagged the exhaust port as it was able to rock more and the intake skirt slammed other side and broke simultanously. This is what I think happened in a less than a secound or 2. It was a chain reation.You may find .003" play in lower rod bearing,,.005" in main,,.005 in wrist pin,,Gasket was a little thinner,piston stuck up a little more than stock,,head surface before.010 and bingo it all stacked up to kiss the head.

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