What do you think caused this?

[Abracadaver]

Interesting. 

[Mech]

That roughness ? The piston nearly seized.

By the looks of it it was caused by overheating.

See just in front of your thumb, on the corner of the skirt, there's a dark spot.. Well that looks like the sort of hard surface pistons get when they overheat. The black bit will be hard and hard to scrape off if it's that.. otherwise it's just an oil spot.. haha. Try scratching the black bit with a scraper or screwdriver. It's some sort of aluminium anodising or something that happens and it's like that surface they put on aluminium doors and windows. Sometimes there can just be a tiny amount and it seems like the piston could be re-used, and they can, but they have a sharp tap when they would normally have piston slap noise, and they can tap/rattle a lot in use.. just because that little spot is too harder metal.

[Gwbarm]

Is that from 2 stroke or 4 stroke engine.

[Mech]

Ya see that bottom ring there Gw....

[Gwbarm]

I did see it i have just never seen a 4 stroke piston chewed up that bad it must have not been getting any oil at all.

[Mech]

I've seen worse. One thing that's a bit strange is that despite the skirt getting hot, the heat doesn't appear to have come from the top of the piston. The rings haven't seized/been jammed in the grooves. Just the skirt got hot. Water cooled engines that have the lower part of their cylinder cooling passages full of crud(used to be common), do that. Unusual these days to have a water jacket full of rusty deposit..

[Mech]

If it's air cooled, that local heat might have come from the exhaust's front pipe..

[Spiderosso]

Whitch brand and model is-it ?