125-200cc Just rebuild my '11 WR300 and my son's '13 WR144 flames out after 4.1 hours...

UGGGGGG.....
We got the bike used with the never use 144 Kit installed fresh. Plug looked good, gas was 32:1 and fresh. Have not drained the trans yet to see if it passed coolant and overheated, will be doing that tonight....

Use PB Blaster and Scotch Brite to clean the aluminum off the cylinder then check it with a dial bore guage, I bet it will be fine. It looks to me like it was either too hot from being lean or the ring caught the exhaust port or debris and stuck causing blow by to heat up the front of the piston. Once it's scuffed up like that it's hard to tell what happened first.

Ahh man that sucks, had a 360 been over bored seize on me at 17.5 hours purely my fault didnt allow it enough time to warm up before i went full tilt.

Damn, Sorry. It sure looks like a piece of nikasil lining on the piston. I hate to say it but I think you are looking at a re-nikasil. Scotch bright or use some muriatic acid and a Q-Tip to remove the piston aluminum from the cylinder wall. But don't get carried away with the acid because it will dissolve the cylinder base material if the liner is missing. You should quickly figure out what is going on as grooves/missing lining are below the surface versus piston transfer which is above the cylinder surface.

I can get a pretty good deal for a re-lining and it's local to work for me. Still unwanted money being spent.

I'll try the PB blaster/scotch brite to remove some to get a better idea whats going on wise with the condition before I start with the acid in there. close inspection has not really happened yet, I only get late nights and some weekends to work on the bikes because of my schedule and other commitments. Like helping work another clubs enduro this weekend, we had ours the weekend before easter. Then starting next weekend our Hare Scrambles series kicks off. I am part of the traveling committee that works the whole series along with the sponsoring clubs. I'll traveling the northeast a lot till November.

if the money is unwanted spending it bike stuff is the best way to get ride of it IMHO

WHAT?

After I got the new piston and time allowed, I brought the jug and other parts to PowerSeal. Jamie looked at it (and the old piston and plug that was in it) and feels it could be 2 things, either bad gas with moisture build up in it or something was in the bottom end and made it way into the cylinder. I eliminated the possibility of it being the gas dumping and cleaning out the tank so far.