Shown is a "standard" 1-inch pin on the left compared to an odd pin with tapering threads on the right.

This pin had a Brookfield CD 121 [030] tightly wedged onto it when I purchased it.

If you look at the third thread up on the pin on the right, you will see a smooth shiny area where the insulator made good contact against the pin - none of the narrower threads made contact with the insulator.

Many insulators will not catch the threads on this pin at all - the CD 121 only made good contact because it was squashed out of round (which was why I purchased it in the first place).