The DuBro Heavy Duty 4-40 ball links are used by many for our controls.  I recently bought a package, but the ball  joint was very stiff.  No amount of exercising, lapping, or oiling would make it work smoothly.

I then bought another package, and they all worked very well.

We work hard to make our control systems with minimum friction.  Ball links, when they are good, add very little friction overall.  If you get one that doesn't work smoothly, I guess it must be discarded in favor of another one.

It looks like a manufacturing problem where all products are not quite the same.

Floyd

I use the ones from Tom Morris. They seem to work very well. The only time I used the Dubro units was on the Cobra I built. They got tighter over time.