Scooterotica

Just sounds like sloppy workmanship not bothering to Finnish the cylinder off by radiusing the ports....for such a little effort and relatively small amount of time, they could have saved someone some serious ball ache not to mention time and money not to mention their oun reputation. All things considered sloppy work plus folks moaning cuz they won't answer the phone don't think I'll be bothering with em any time soon!

The rebore seemed well done and cheap enough with the new piston but it let me down as we can see. I don't think they did anything maliciously, I think it was more a case of lack of knowledge on their part.
We trust all the dealers out there to check stuff like that for us, most of us aren't experts and rely on them to get stuff right. I know this sounds a bit naive on my my part but that's my take on this dealer and others.

Is it a Mahle thin ring piston (or similar?)

The reason I ask is I've had several go in stage four barrels in the exact same way as you described.
That's including spending a few hours before assembly smoothing the edges of all ports. Several people I have spoke to have suggested the rings on these cheaper pistons are quite brittle in the first place. I'm no metal expert, but I think they are made from chrome plated cast iron rather than a flexible alloy.

Jordan

Just rebuilt my 175 conversion following a blow up last week, was fitted with an Escorts 62.00mm piston with thin rings. Now using a Mahle 62.8mm but with much thicker 2.5mm Goetze rings, got it from scooter restorations Nottingham. Maybe worth a try?

Couldn't see the make, did say something like EPL.