2T rebuild, failure, what went wrong??

Bloody hell, 6 weeks after doing a top end rebuild on my young blokes KX85,it has experienced a major failure. This bike has been super reliable and after the rebuild it would start first kick, idle like a beauty and go like the clappers. Last Wednesday arvo we had it on the grass track at Mooses and it was going great, so well in fact my 8 year old daughter could start it. It was flying right up to when we loaded it onto the trailer.

Yesterday morning I went to start it and it took about 8 kicks to start it (highly unusual) then once it started it sounded fine although it wouldn’t idle. It then stalled and wouldn’t start so I clutch started it and it started again and would rev out OK yet still wouldn’t idle, so I used the throttle to hold it just above idle and it started to make a fast ticking noise and then stopped abruptly and wouldn’t start again.

A serious lack of compression told me something serious had gone wrong, so off with the head. As soon as the head was off I could see trouble, there was a piece of piston ring embedded into the crown of the piston and there was a scarp on the barrel above the exhaust port . So I then started to remove the barrel, which proved to be a bitch as it didn’t want to come off like it was supposed too. I eventually got it off with much twisting and pulling, all the time I could feel that what was left of the rings was scoring the crap out of the bore. Once the barrel was off, I inspected it and the bore is cactus and the piston looked like this;

6 weeks ago that was a brand new piston and rings and it would have about 6hrs of hard riding on it since the rebuild.

My 1st question is what has gone wrong to cause this? It was running sweet since the rebuild and all of a sudden it failed at almost idle speed when cold.

I dunno if you can tell from the photo but it appears that the ring has failed on the end near the gap. The only thing that I can think of is that I made the mistake of thinking because they where genuine parts that there would be no need to check the ring gap and due to insufficient gap, the piston ring has” bunched up” so to speak and caught on the exhaust port.

My 2nd question is what to do with the stuffed barrel? Get it replated with Nikosil, bored out and have a cast iron liner put in or do I spend mega bucks and buy a whole new barrel and top end?