We bought a 2014 YZ250F and at 6 hours it just died and would not start back up. Found out the stator was gone and taken out by the shrapnel from this stopper assembly. Luckily it did not grenade the whole engine while hitting a big jump.
This is a REAL problem with 2014 YZ250F and not just an "internet" issue. Bikes with only a few hours are needing full rebuilds. Here are some pics of the stopper from my bike.
My dealer worked with Yamaha to replaced the damaged stator and fish out the remaining pieces from that bearing out of my engine. My son LOVES this bike (just to ride KX250F) and recently bought another 2014 YZ250F but had the dealer replace the stopper and impeller shaft with updated part before taking delivery.
Check your bike...