Kt100 trashed piston skirt

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Kt100guy85

Member
Hello all, my name is Joe and thank you for having me on the forum. I acquired an invader kart with a kt100, I put about $800 into the kart rebuilding the top end, a new patriot 3 disc clutch and a multitude of other parts. So I'm riding the kart and hit 13,800 rpm's and the motor dies I thought maybe I stuck the piston and so I tried hand turning it and the motor would turn over some but not all the way. I tore the motor down to find the exhaust side piston skirt was DESTROYED! It pitted the head pretty badly but there isn't any cylinder damage amazingly. When I did the rebuild I didn't bother with the mains or seals. I ran 16:1 amsoil full synthetic race oil and if the motor had an hour on it since the top end rebuild that would be a lot. My crank does have a good amount of end play I can't measure it but I am certain its more than the .020 tolerance. My question is does this sound like the mains could have done this? Does it sound like I stuck the piston? I didn't see any lean scores on the remaining piston...... I'm placing an order with comet karts for new mains, seals, piston, ring, both rod bearings, a new rod just incase it was damaged, a new head, and other odds and ends. I'm rambling but I just need to know the causes of a broken skirt. Thank you all for any guidance and or help. Btw can anyone recommend a machine shop in central FL to press in the mains? Again thank you all.
 
twostroketoowoke said:
Sounds like the ring caught somewhere and all hell broke loose. Are the exhaust bridges still intact? I am genuinely curious.
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20201109_201659.jpg
 
This piece of the skirt struck the crank, you can see the milling marks of the crank on it.
20201109_203820.jpg
 
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I’m stumped for answers. It didn’t stick and didn’t mess up the cylinder and the only thing I can think of is the piston rocking at a weird angle, but I’m not smart enough to know how. Where’s @Pete_Muller to chime in!
 
I do appreciate your help. I'm stumped too, I wonder if it was just a bad piston from factory, it was a KSI. One thing I did notice was the KSI that was in the motor before I rebuilt it had a skirt that was just a few centimeters shorter than the one that shattered.
 
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Almost looks like collision damage .
The end grain on the piston ? Does it show a fresh break , nice and clean .
The one piston picture looks like it's not fresh .
Break the other skirt off and see if it looks similar at the fracture .
Excess clearance?
Is the piston bottom edge chamfered or square .
 
flattop1 said:
Almost looks like collision damage .
The end grain on the piston ? Does it show a fresh break , nice and clean .
The one piston picture looks like it's not fresh .
Break the other skirt off and see if it looks similar at the fracture .
Excess clearance?
Is the piston bottom edge chamfered or square .
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The piston is fresh, it was a ksi coated piston that's why its grey, the old KSI is chamfered. The new freshly destroyed one is square. What do you mean by collision? Pardon my ignorance. Thank you for any help.
 
flattop1 said:
Almost looks like collision damage .
The end grain on the piston ? Does it show a fresh break , nice and clean .
The one piston picture looks like it's not fresh .
Break the other skirt off and see if it looks similar at the fracture .
Excess clearance?
Is the piston bottom edge chamfered or square .
Click to expand...
20201109_214717.jpg
 
Did you possibly put the piston in backwards?. Piston pins are offset front to back on the Yamaha and will add a great deal of stress to the skirt if in wrong. Arrow should point to the ex. side.
 
bpr said:
Did you possibly put the piston in backwards?. Piston pins are offset front to back on the Yamaha and will add a great deal of stress to the skirt if in wrong. Arrow should point to the ex. side.
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Ya arrow towards exhaust for sure. I triple checked it when I was putting it together.
 
What kind of exhaust pipe? I would not rule out something that could have been in the pipe coming back into the exhaust port and breaking the piston skirt.

That's a VERY unusual break. Have you split the cases and looked for any other loose parts?

Also, why is there such a large gap between case halves in the one photo? Is there that much gasket thickness in there?

PM
 
Pete_Muller said:
What kind of exhaust pipe? I would not rule out something that could have been in the pipe coming back into the exhaust port and breaking the piston skirt.

That's a VERY unusual break. Have you split the cases and looked for any other loose parts?

Also, why is there such a large gap between case halves in the one photo? Is there that much gasket thickness in there?

PM
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Ya I split the cases already, that's why there's the massive gap. The pipe is a c4 I believe, almost comical since I blew the engine lol. I only found pieces of skirt in the case and accounted for about 90% of the skirt with the exception of the pieces that became powdered aluminum that destroyed the head. I didn't think about something coming back from the exhaust but that is a very valid point. Thank you!
 
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