Get a set of the 4-40 Rocket City ball links. Tom mentioned that they are not his favorite setup and they used not to be mine, but I am a sponsored heli pilot for Robbe-Schluter helicopters, YS engines and a few other companies. They are NOTHING but ball links on a heli. That is all there is as they are slop free and there is no such thing as a straight 90 degree hookup on a heli. I fly a 90 size heli and for those of you who have seen one fly you know the EXTREME stress and leverage they put on one. I have NEVER had a ball link pop EVER and I'm been flying helis for over 7 years now and planes for over 20. I use RC hardware like Tom recommends on all my planes and a RC ball link on the servo end and the ball link come with a ball that spaces the connection away from the arm enough to stop just the problem you are having. It you are still scared then put a small washer on the bot that runs throug the ball then then it 100% can't come off, but there is no need. The only way a ball could pop off is you have a SEVERLY bad mechanical setup like it was way off center and even then it would bind the servo up first I think. Years of their use have proved them to me. The only way to go I think. I use only RC as they make a good high quality one. Ones like the dubro are not near as good.

Norman Ross Jr.