Hmmmm I have never seen a cast piston do this inside 7-8 gallons unless engine was being broke in with the WOT ultra rich method or over reving of the engine occured before break in was complete or there was an upper rod bushing failure which is pretty unlikely with the R7 rod or wrist pin breaking in half very very very seldom have I seen this and is usually caused by the aforementioned rod bushing . I have seen and sold more than a thousand novarossi engines I am sure of that .

Btw , that piston looks very very burnt on the underside , thats interesting cause a)either you are using old nitrotane or b) theres much more than a quart through the engine

Curious as to what fuel you were using?