Something's gone terribly wrong... (hole in piston)
So, I recently finished reassembling my engine. After some cranking (to be expected), it finally started. It seemed to run alright, but was making a significant rattling/ticking noise. The ticking was very consistent -- sped up and slowed down along with revs. I wasn't sure what to make of it. I know it wasn't the cam chain because I just replaced the tensioner. Anyway, after a few minutes of running, I heard a louder crack and it died. Pics below show what I found when I investigated. Note that the damage is only to cylinder 2, all of the others are completely fine.
What happened? Did I replace the cams incorrectly and cause a timing issue? Was there debris inside the cylinder? The hole in the piston doesn't appear to be burnt or melted...just punched through. Same goes for the spark plug. I'm just not sure what I did wrong.
Thanks for any insight!