i have the strong arms installed on my P3P - but I forgot to add nail polish as a weak to medium thread locker.

As recommended by UAVbits I have been checking the screws on each arm after each flight.  I am finding that due to vibration some of the motor screws do end up less than tight. I have been tightening about half of them roughly.  They are not loose enough to come off but I like them really snug (not over tightened).

So remember to check and tighten and also use a little nail polish before you set the motor screws ( I will do it next time).

I have installed prop guards, and when I pulled the original screws they were definitely snug and had blue thread locker on them. I reinstalled the new screws with a little thread locker on them and snugged them. I see some people are saying "hand tight" is good, but I think there is a lot of torque in the area (hence the cracking). So, how tight is tight enough?

Nail polish is your friend.  I used a little dab of nail polish fifty flights ago, all this loosening went away permanently.