Looking for an explaination of what may have happened. I have a brand new YS FZ110. This is my first ever 4 stroke. Yesterday I took it to the field to break it in. I am turning a 14 - 8 APC prop. A 3 inch spinner with an aluminum back plate and plastic cone. Everything was tight with lock nut in place. High speed needle was set at 1 1/2 turns out to start the motor and then opened another 1/4 turn to richen up further once the motor started. I am using Magnum Fuels fuel that is 20% Nitro.

I don't have a safe way to bench mount this motor to do the break in, so I mounted to the plane and went to the field. This allowed me to use their plane stands and also have some more experienced people around in case I had questions. Everything started and ran REALLY well. I can't believe how strong this engine is just in the break in stage. Anyways, the instructions say to run it for 20 minutes. I was at 19 minutes and I was talking to one of the more experienced guys at the field when the motor ran out of fuel. You could hear the motor lean out quickly. I chopped the throttle to kill the engine. When I did this you heard a kind of pop. Everything seemed ok, I refueled and went to start up again. Using my starter something seemed to "grind" upon trying to start.

Upon investigation it seems that the knurled drive washer on the 110 and the knurled backplate of the spinner have spun. Both are worn down pretty good. Everything was tight upon start up including the lock nut. When we dissassembled everything the lock nuts were still locked tight. It seems that the drive washer may have not been seated fully onto the drive washer retainer and during the breakin, everything seated and caused just enough room for everything to slip. The guy I was with said he thought the engine backfired when I chopped the throttle. This is when the slip happened.

Question is, is there something I did wrong here? Will YS warranty the drive washer or am I going to have to suck it up and buy a new part and a new spinner?