Breaking in a new 90 sized ABC engine, about one gallon through, all on test stand, I am at WOT and hear a clattering racket in the engine. Shut it down, remove from stand, remove muffler, look in the exhaust port and see a lot of aluminum debris. Remove the cylinder head and look down into something like a gravel pit. A piece of aluminum has broken off from somewhere internally and gone into the upper chamber, where it broke up and partially melted around the jagged edges, sort of like river stone. Remove backplate, see nothing amiss in the lower end, no aluminum appears to be missing from the piston skirt. Sent it back to service department for them to figure out. Total mass of the debris amounts to maybe a pea-sized piece of metal.

I don't want to mention manufacturer, because I am not inclined to think it is a problem with this particular brand. Just curious whether anyone else has had something like this, and did you ever figure out what it was?