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Broken Piston Skirt

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My sons KX 65 locked up one day and I took it apart and found that the piston skirt was broken. The bore was also damaged so I sent it out to be welded, plated, honed and purchased a new piston. I put the bike back together and it ran great for a month before it locked up again. I took it apart and found the piston skirt is broken again. Does any have some information on what I should be looking for that would cause this? I have attached some photos. Any help would be greatly appreciated!

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It looks like the piston wasn't happy in the bore before the skirt broke?

Change your ignition side seal as it may have let air in and leaned the mixture up then caused damage.

Send the cylinder and a new piston to a very reputable facility or buy new parts.

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You should have purchased the piston and sent it in with the cylinder to get the clearances/tolerances correct.

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How many hours did you get on it before it blew up? Cast pistons don't like lots of hours and they show the stress by breaking the skirt. It's very common actually if ridden hard even 50 hours is enough to cause this. You need to set a better top end maintenance scheduled.

 

And J that looks like just some rust on the rod, would need better pictures to be sure.

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What brand of piston is it? I'd probably run a forged piston to avoid that sort of problem in the future

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I use Amsoil Interceptor and I mix it at 32:1...8 oz. in my 2 gallon can. Would running lean cause this problem? I posted pictures of the plug and it it grey color. Sorry I'm new to this 2 stroke, just trying to learn.

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First, That sucks man, Second, cast do not hold up well if they are run are really hard.  Amsoil Interceptor at 32:1 I would say is a bit rich.  Detonation will cause what u got going on, so too lean on jetting or air leak on the ignition side seal.  It could have been minor detonation but enough to trash the piston.  This is a Pro-x piston out of a kx500 with <6hrs.  A little detonation can make a mess of things.photo11_zps6kzvaglj.jpg

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Broken skirts are usually due to stress cycling causing a fatigue failure.  At the kind of hours this occurred at this is definitely not the case.  I don't believe jetting or premix ratio had anything to do with it either. 

 

Theres definitely something screwy going on in there.  Do you have any pics of the cylinder?  I don't know if this engine was a case reed or not, but if its not a case reed and its been over-ported to the extent that there isn't enough material left on the intake side to fully support the piston that could do it.  This type of failure is very unusual except for very high hour motors that have been run too long without service.  For an engine like this you should be getting 60-80 hours out of a top end before servicing anything.

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your rod is f-ed. see the discoloration near the big end? prob piston to crank contact.

You could check the lower rod bearing by lifting up and down on the rod. If you feel any up and down play, the bearing needs to be replaced. Side to side play is normal.

Edited by 351mustanger

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I use Amsoil Interceptor and I mix it at 32:1...8 oz. in my 2 gallon can. Would running lean cause this problem? I posted pictures of the plug and it it grey color. Sorry I'm new to this 2 stroke, just trying to learn.

yes, lean will melt/kill things. this attached picture is pretty good for a two stroke, white usually means lean, but it can also mean you're getting closer to rich, and its actually the beginning of gray. gray with black will usually mean rich or cold.

looking at your plug after the catastrophic failure isn't going to be truthful, as aluminum and steel can be welded to it from the come apart.

what you want to do is a leak down test to make sure the crank seals are good. if they leak, the transmission can add oil to the mixture, leaning out the air fuel mixture slightly and causing more smoke. if the stator side leaks, it makes the air fuel mixture very lean, and usually melts/breaks pistons.

keep in mind air fuel mixture ratio (your jetting/needle/air screw) and lubrication ratio (the oil you mix) are completely different things that happen to be in the same engine.

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also, changing the lubrication mixture (oil in gas) will change the air fuel mixture, but only slightly compared to jetting. going from 32:1 to 80:1 will richen the air fuel, because less lubricating oil is present.

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And J that looks like just some rust on the rod, would need better pictures to be sure.

 

The upper 2/3 of the rod in pic #2 looks normal.  The bottom 1/3 near the big end with the gray/blue color got hot.  The pistons (not sure about ALL engines) will hit the crank halves if either rod bearing is bad. I rebuilt a 2000 KX250 with that issue.  Here's the piston from that engine with notches cut from the skirt where it hit the crank.

 

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Thanks for all the advise, I will get back to the bike tomorrow and finish tearing it down. I will take some more pics and post them. I think the color on the crank is rust...though I'm not sure how that would get there. I replaced the crank with a Hot Rod crank and rod last summer. Thanks again for the help, I appreciate it!!

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the crank discoloration more likely is from when they assemble it they heat that part really hot to aid in assembly, I have seen brand new crank assy look like that.  But, it is alarming when you do see it.

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the crank discoloration more likely is from when they assemble it they heat that part really hot to aid in assembly, I have seen brand new crank assy look like that.  But, it is alarming when you do see it.

 

The rod is not an interference fit.  The crank itself is pressed together but I have not seen one yet that was heated to do so.  I would not use a crank that showed that kind of bluing on the rod.  That coloration indicates that the rod has gotten hot enough to affect the metallurgic properties of the material. I would expect that rod to snap off in short order if you continued to use it.

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the crank discoloration more likely is from when they assemble it they heat that part really hot to aid in assembly, I have seen brand new crank assy look like that.  But, it is alarming when you do see it.

No. You see bluing on the crank halves where you press in the pin. Theres no reason to heat the connecting rod itself.

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