Had the same thing happen to my me a couple of months back.
Got to the field and had a flight with my raptor 90 when the engine cut mid flight. I then spent the rest of the day trying to sort out the problem but kept getting lots of engine cuts.
Wasnt until I was getting ready to kick the hell out of the heli and it was almost dark that a friend stopped me, until I'd at least taken the engine apart for a look. Found exactly the same thing in mine plus lots of tiny pieces of silicone where the piston had been slicing through the tube on each stroke!
One things for sure, i've never heard an engine sound as sick as that one
. Still, will know what it is if I ever hear it again!