Hi Folks - it's a brass propnut.. Now - for the other questions. This happened while I was attempting to break the engine in. I was running it at full throttle and transitioning it in and out of "4 cycle" mode as indicated in the instructions. So it was really rich down to a little leaner - then back out again. It was during this time that the thing flew off. It came off once before when I had started the process but was starting the engine with my electric starter.