A while back, I was doing a long-ish ride in the summer on a 100+ degree day, not pushing hard or climbing hills. Just after sunset on the way home, my 70cc DR kit made a weird plinking noise then died, not to start again.
Running stock carb with 82 jet, and super-stuffy race crankshaft, MLM people's sidebleed pipe. I have a pull start but it was stuck and the motor could not be turned, so I assumed I had seized it.
That day cylinder head temps had been high, but not especially hot, low 400's. The last reading on the Trailtech was 405F.
Finally pulled it apart to find the piston was solidly stuck in the cylinder, not due to melted aluminum as I had expected, but because one of the piston wrist-pin clips had come out and lodged itself between the piston and cylinder wall! Once I rigged a "press" of sorts to push the piston out of the cylinder, I found a little chunk of metal that had chipped off the base of the cylinder :-(
(please see photos)
Also noticed the top of the piston looked pretty well-baked with some signs of blow-by (but I'm no expert), the marks in the photo on the top of piston I made with my "press" while pushing the piston out. The piston rings were so brittle that when I went to remove them, they just cracked apart in pieces. The wrist pin was also so surprisingly stuck in the piston, that I had to rig another "press" with a c-clamp to push it out and separate the piston from it's beloved crank.
How the heck does that happen?! And hope i'm not being Mr. Obvious here, but my cylinder (and rest of the kit) is now toast, right?