The time-tested way to smooth ball links:
All you need to ensure perfect fitting ball links is a small pliersor needle nose pliers, and a heat gun.
If a link is too tight on the ball, you can gently squeeze the link with a needle nose pliers while it's on the ball. Squeeze a little bit, then check the fit and squeeze a little harder if it's still tight on the ball.
After a while, ball links become loose from wear (or oversqueezing). A trick to return the link to new condition is to heat it up with a heat gun. Heat it carefully, just until it starts to have a shiny look, then dip it into a glass of water to cool it. Voila! You have a like-new ball link!
The reason the fit of a link to the ball may be too tight or too loose is that the plastic is not fitting properly to the ball. Polishing the ball will work in the short run, but it may result in a link popping off. The point that Dwight makes is that the problem lies with the link, and not with the ball. Dwight's technique is easy and results in a custom mated fit between the plastic link and the ball.
Chuck the ball in a Dremel tool. For Miniature Aircraft links, you can chuck the 2.5mm stud. For loose balls, fasten the ball with a spacer to a 2mm or 2.5mm screw long enough to chuck into the Dremel tool.
Thread the plastic link to a spare connector rod long enough to use as a handle.
Snap the link to the ball and while holding onto the connector rod turn on the dremel. At first there will be drag as the ball spins at high speed inside the link, but as the plastic heats up it will turn very freely. Note: Miniature Aircraft links will heat up very fast, only spin it for a second or two.. Try your hardest at this point not to deform the plastic as you turn off the dremel. If you introduce any sideways pressure on the link as it cools on the ball, then the link will be ruined.
When the link has cooled, it is ready for use. It is perfectly mated to the ball and will provide a slop-free frictionless connection.
Either of these techniques work well. If you haven't optimized your ball links yet, you will be pleasantly surprised at the difference it can make. Pay special care to the tail rotor pushrod and bellcranks to get them as free as possible, and you will notice a dramatic improvement in tail response and accuracy.