Evidently there's a lot of P3 pilots cracking from the stress being caused by the hysteria around this subject.
No one knows for sure the cause, probably something in the plastic (bad mix/curing/molding). That's also my opinion, based on my background with this matter. It's pretty safe to assume it's not caused by unbalanced props, because some RC shells also crack and there's no spinning anything on the TXs, no torque, no braking, no twisting and no serious stress of any kind.
Also, the occurrence of cracks is random for the most part: some arrive cracked, some crack after a couple of flights, some (apparently the majority) never crack. Some crack with old motors, new motors, old shell and new shell. Some crack with arm and motor reinforcements, some don't. There's no pattern other than the plastic used in some batches, for both the AC and the TX body.
All that means (to me) that it's hard - if not impossible - to say for sure if some preventative measure actually works, or by how much. My opinion is that if you got a bad shell (from, say, a bad batch), it'll crack regardless of overtightened or loose screws, zip ties, electrical tape, stiffening add-ons, prop guards, hard breaking, whatever. And if you're lucky, you may have 1000 or more crack-free flights. Which, even though not comforting for owners of cracked P3s, seems to be the rule.
