Scooterotica

Looks like it caught the the lip of port, don't know how. Exactly the same problem with the first piston that popped in the same way, both after about 1200 miles. Im going to ask the question to the dealer what exactly has caused it to happen twice in the same place.
Looks like I might be getting a Muggy 186 but I like the 20mm Dellorto carbs

Is the piston fitted the wrong way around ?

No definitely fitted the right way. It was running very well. I'm thinking something isn't lined up correctly withe exhaust port. Getting it rebored again could be another waste if money. Anyway, I've just ordered a Mugello, see how that goes with a 20mm Dellorto

Wrong size rings .

looking at the picture the ring has snagged on the way down, so i would think its caught on the edge of the exhaust port, if the port has a sharp edge it can cause this problem, all ports on 2 strokes should be radiused to avoid snagging, larger ports equals more of a problem as a general rule, what you find is the ring will expand into the port slightly as it goes past the exhaust port, then gets squeezed back into the groove on the piston as the piston closes the port, thats why a radius helps, a sharp edge and it can catch and break, a little work with a small file would get rid of the edge and give the radius needed to avoid snagging and broken rings.
holty

Looks like after hitting bdc on the way back up the ring has moved / come out into the exhaust port catching the top edge .
just put my glasses on and i think Holtys right its done it on way down.

is it the original piston or correct piston for the barrel, some alternatives have ring pegs in different places that can allow the rings to open, this will allow them to open into the ports

It's annoying that good shop didn't take more care instead of just reboring it in the first place but I guess it's a cheap kit. The kit was good when it worked but ran out of steam at 60 MPH/ GPS but it got there as quick as most other scoots. Hoping for more success with the Muggy.

Holty has got it right, I think. Hope others can learn and check the AF 175 kits, if they buy one, they can be good but they also originate from India etc.

I've got a 175 af barrell hope mine stay fine