First measure the diameter of the rod to make sure it is a 1/4". Next go buy a brand new die that matches your Heim end threads and run it down the rod. The Heim now fits. A very sharp die is necessary so it can cut across existing threads every several rounds without getting diverted. Another approach is to use a thread gauge to find out what threads are on the rod and get a tap from a machine tool supply and run it in the Heim end. This leaves an invisible fix but a special order tap can be pricey.

Evan