i'm trying to roll dubro ball link on a 4-40 rod and it's going very hard.
after many efforts i got less then third link on the rod. is there any easier way to fully roll the ball link on the rod ?

Try a little soap on the threads for lubricant.

I put the rod in a cordless drill and carefully run it in about 2/3 of the depth. Dubro and/or Hobbico make a neat and cheap plastic tool to hold the rod end and/or clevis, or just carefully use your fingers, a vise, or pliers.

K&S also makes a link wrench.

Eric

If you have a 4/40 tap, try tapping the ball link first. It will thread a lot easier. It won't be smooth sailing, but I prefer a little resistance.