Before you get any of these make sure you have 4-40 taps, both left and right. They come untapped and the rubber is very hard, much harder that a DuBro. They will not self thread using a bolt. The hole is deep so you can tap it a full 1/2". Once tapped, links are very easy to turn with just enough hold to keep them from rotating on their own. I tested one with only 4 turns in the threads and it held for as hard as I could pull on the pushrod. In my bag of 10, all 10 of the balls were tight and very free. In my last bag of DuBro about 1/2 were too tight to use.
Ken
Just curious, does anybody make a 3.75-40 tap just for ball links?
I think you want one with a different class of fit, Class 3x for one or both sides of the interface. . But, there is a similar question about getting class 3 4-40 taps. You might do better on the male side, dies are adjustable, or single-point threading. In plastic you might get an interference fit since it will spring out a bit while tapping it.
I only have a thread on one end, the other end has a pretty classic-looking 3/32 music wire with a keeper. If you have left/right threads like a turnbuckle, then you have to worry about them turning themselves and do something to prevent it.
Brett