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Piston hit head?

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97 kx125.

Bike was running fine on Sunday and all the sudden stalled out. Lost all compression. I did a leaks down test and it seemed fine, compression was gone though. Pulled the top end off tonight and found this.

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The cylinder looks perfectly fine... It is bored .050. There's maybe a 1/16" total play side to side in the rod, and there seems to be no up and down. It cranks buttery smooth, and I drained the oil and it looks pretty fresh still.

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Draining the oil tells you nothing about the crankcase. 2 separate areas. Something got in there and was bouncing around for a while. You didn't say if you're the last builder of it or bought it like this and not know it. That looks exactly like a broken sparkplug ground strap. I've seen it many times, even on my own bikes. Even a few times a broken powervalve.

Even seen where a guy lost a bolt that fell in a reed cage, finished the build and  BOOM on first kick.

 

But nothing to do but replace and search for parts n pieces in the case to make sure it's all out. Double check the crank and make sure it wasn't a needle bearing from big end of rod.

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I was the last builder. It ran fine for 11 trips to the track. I'll check the plug I remember looking and seeing it was rich but didn't inspect real close.

The small end bearing looks good, so I was thinking big end. Only thing I Didn't rebuild was the crank so I guess it may have came back to bite me in the $$$.

Can the big end bearing seem to operate great even if it has some damage? I feel no play in it.

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If that was hitting the head, I don't see how you couldn't have heard that right away - or feel it when cranking it over.  Have to agree with SAPPERS.  Looks pretty evident that something was in there making small hits all over the top of the piston around the edges where the clearance is tightest.  Sorry about your bad luck there, man.  Hopefully that didn't transmit into the rod bearings and cause premature wear on them.  The mashing of the piston edges may have pinched the piston ring to a position where it lost a good seal against the cylinder wall and that's when you lost compression.   Good luck!

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Been there, Done that. Both from broken plugs. I don't have pics of the pistons on my PB, but they were as bad as yours too. These are from 125 & 250...Both in the same week. Phathead racing made money off me that week too.

 

 

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Edited by SAPPERS

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Spark plug will be the first thing I check tonight when I get home...  Thanks for the advice, I'll post my find.

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image.jpg

Looks like whatever was in there dinged it up as well.

When cylinders are bored does there need to be head work as well? Receipts from a previous owner wen cylinder was bored show nothing for the head.

Ring is complete, small end bearing looks great. Bike still cranks great and smooth even before I took it apart.

Maybe something fell in when I assembled and it took this long to do enough damage?

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Has the ring keeper pin fallen out of the piston? Something was defiantly floating around in there,

As for the play in the crank you want to look for up and down play not side to side. 

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Locating pin is still in the piston.

 

Those needle bearing looking things on top of the piston... are magnetic.  Theres no up and down play in the crank and it operates smoothly.

 

I would think if the big end bearing was shot the crank would grind some, and there would be lots of damage up the skirt and sides of the piston as well as the top?

 

Maybe I made a huge mistake and let something fall on top during assembly...

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I also seen where you said it was bored oversize. Check your powervalves. See if maybe they are chipped or broken. Overboring requires the clearancing of the PVs and the PO might not have done it correctly.

Check to make sure all the screws holding in the reeds are there, look at carb real close. If your carb has the little screen on the air bleed on the inlet side and it popped out, that may have been it. Not all carbs have that though..

 

Other than that, it may have just been a simple mistake of possibly something falling in the airbox during a filter cleaning or something. Hard to say at this point. But either way, it needs to be checked over very closely and rebuilt.

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It's the rod bearing.  He doesn't know what he's looking at.  It's the only place to get needle bearings mushed into your piston.  You can see them in the 3rd pic at 4:00.

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It's the rod bearing.  He doesn't know what he's looking at.  It's the only place to get needle bearings mushed into your piston.  You can see them in the 3rd pic at 4:00.

 

I think this is the case too after going over it all again last night...  I just assumed the crank would be sloppy if a needle worked its way out.

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Rotate the crank so the bottom rod bearing it on top and post a pic of that area, lower rod.

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And the third picture looks like a missing needle

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All that bluing on the bottom of the rod is a tell tale sign the bearing is shot and heated up.

 

KXPics012.jpg

Edited by Jeekinz

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makes sense, thank you.

 

Do you think my head looks savable if I sent it somewhere, or am I better off finding one on ebay?

 

Gonna split the cases this weekend and get that crank sent out somewhere on Monday.

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If you can get it to clean up like Sappers did then you'd be ok.  But I'm sure you could find a clean one on Ebay for next to nothing.  Use the parts diagrams to see what years used the same head then search Ebay.

 

What's the cylinder look like though?

Edited by Jeekinz

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