One of my beloved Phantom developed cracks the the usual spots. One way to deal with it is to install snap on/off prop guards using the guard base as reinforcement to the motor mount area. The proper way though aside from a purchase of a new shell and the hassle of a complete tear down is to coat epoxy in the motor mount bed. This not only strengthens the bed area it will also help absorb the high frequency vibrations that I feel weaken the plastic used in manufacturing. When laying the epoxy take care not to get any on the motor mount posts or the motor guides that keep the motor straight. Also don't get epoxy on the edges of the shell where the lip edges join in a groove.