Joe,
I would re-think your control setup if I were you. Don't use a plastic servo arm with a ball joint of any kind. Here's why: since the ball joint is mounted on top of the arm, it creates a tangential load which causes the arm to twist and eventually break. To visualize this, imagine mounting the servo arm to a servo that can't rotate while you push and pull on the linkage. It willl cause tha arm to twist. That's what happens when you fly. If you MUST use the ball joint then use either a wheel or a metal arm. These are much more resistant to this type of load. I know some guys get away with plastic arms and ball joints but I REALLY wouldn't recommend it.

I only use plastic arms with a clevis. Since the clevis transfers the load in the same plane as the arm, it doesn't create this evil twisting force. That being said, I don't like to use a clevis on anything except a pull-pull cable linkage. The typical metal-pin clevis will eventually wear the plastic arm and create slop in the linkage. This has only a small effect on the cable tension but it will be very noticeable on a rod-type linkage.

You can also use the ball-joint on a plastic throttle arm because there is very little load there since the throttle should rotate very easily.

John Pavlick
Team Black Magic, Tech-Aero Designs