Hey,

I have 97 Indy trail touring with 7000km (5000miles).
Last week, it blew a piston





I am not sure why, or what to do. A friend was using the machine, and I understand he was trying to pull a heavy load at he time. I had replaced the piston two years ago when it broke its skirt. That makes me think there is some piston slop, but I don't know if that caused this.

As you can see, there is some plating on the cylinder, and that makes me wonder if I need to replace the cylinder.
I can get a replacement cylinder (1) with about 2700KM on it for around $100.

As well, I am wondering if I should rebore the cylinders and go with oversized pistons.

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Personally...it looks to me like that's a slow melt that's happened over time. If it was due to too heavy of a load (overheating), the piston would have just expanded too much and seized. Check to make sure your jets are the right size; especially if you have any engine modifications done, you'll need to put bigger jets in. 5000 miles is kind of getting up there, could possibly be dirty carbs. Either way it looks like a lean condition that over time ate away at the piston. Check all your intake boots and fittings to make sure theres no air leaks as well. I'm sure other people will expand on the topic. Good luck with the rebuild, and as far as boring and putting bigger pistons in, it wont do a darn thing to fix the problem. Find the cause of this problem, then mod from there.

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I had cleaned the carbs just a few days before it happened. Would an air leak in the crackcase cause the same problem?

What about the build-up on the cylinder (2nd photo). Do you think that is removable?

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you could probbably clean the cylinder up with a hone

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Yea I scored up a cylinder once and honed it back to shape without any problems, just make sure there's not slices through the plating. You said you cleaned the carbs out a few days prior? Make sure, in that case, that you didn't rip any dry rotted intake boots or anything. Mine are starting to crack but that's from me taking my carbs on and off a million times, maybe your's are just from age and wear. I wish I had more experience with this type of problem, I don't know if the picture you showed us could determine if this happened over days or over month's...maybe even years. The way the piston is worn away so smoothly it looks to be like a long term lean condition that caused it, but then again maybe a few days putting on a few hundred miles could have caused this.

BTW, a leak in the crankcase could definatly cause this, because it would be drawing in unmetered air (metered by the carbs), allowing more air than usual to be combusted...aka lean condition. And, once again, good luck man.

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The intake boots are two years old. We did do an 8 hour run a few days before.

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BY THE WAY, it's on the outtake (muffler side)side.

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