Hmmm thats an odd one I have only see this a couple times. Once was years ago with the 140L piston on an engine that had been detonating in the mid range. The guy just kept running it this way for a long time and eventually the top of the piston cracked but worse that this.
The other was a test piston that I ran a couple years ago. Contact Richard he should take care of it for you.
www.yspartsandservice.com
Troy Newman
Team YS