I have a stress crack in one of the arms. Because I can't send it in for free warranty repair, I installed the UAV BITS strong arms for removable prop guards, am I'm happy to report that after 19 flights (11 of those without prop guards), the crack has stayed stable. I think it'll stay stable, so I'll stop periodically taking off the strong arm to verify the crack. This is the crack before installing the strong arms: Crack, picture 1 by nonflyingbrick posted Feb 14, 2016 at 10:35 PM Crack, picture 2 by nonflyingbrick posted Feb 14, 2016 at 10:35 PM Next time I can I'll buy the strong arms for flying without prop guards, because now when I fly without guards the strong arms are secured with only 4 out of the 6 screws. As for the cause of the crack, I think it's possible at least one time I flew without the 2 motor screws that go where the prop guards are screwed, because when I take out the prop guards I was often too lazy to put the short screws back on for transport, leaving just 2 screwed on every motor. If that was the case, it's most likely it would have caused the crack. Lesson learned, always put all 4 screws on even if just transporting.