OK. I'll start out this thread with a disclosure: This is actually a snowmobile piston failure. While I know that this may be politically wrong, this is costing me much frustration and $$$ (this is a special order piston and it has wrecked the plating...for the third time) and I can't think of a better place to get help. Besides, this will translate over to failures that we experience in our bikes as well. That said, this is a piston from a 2003 Arctic Cat Mod 800 motor. This piston has about 10 hours of run time. Judging from what I could gather, the culprits are either: (1) Lean jetting. (2) Improper warm up. (3) Too hot of spark plug heat range. 2 and 3 are out of the question because I am running the same plugs as always (I can also provides provide pictures if needed, but they looked fine to me) and the sled had been running for more than an hour before it melted down. This is the third time now that this exact same thing has happened. It runs great for weeks and even months before this happens. I can't figure it out. There were no air leaks that I could tell. The temperature and altitude and oil were all the same as usual. There are no signs of detonation. The other piston and cylinder are fine. The thing is that I have been running the sled for ten hours on this piston. When I rebuilt it this time I was extra careful to check for air leaks. I jetted up the pilot and the main jets two sizes. After one ride I made sure that everything (intake, carbs, base nuts, etc.) were still torqued correctly. The carbs are clean. This thing shows no signs of running lean and has performed great every time until it squeaks the piston. What is going on here? It has the four corner seizure pattern. These photos were taken after I cleaned the smeared aluminum off of the cylinder using acid.