Ron,
If I remember correctly the die is threaded 1/2"-20, so you should be able to use a common 1/2-20 bolt to make your expander. An old machinist trick to make a threaded item "run true" is to turn a threading die onto the thread and then "chuck" the threading die in the lathe. Since threading dies(not rethreading dies) are split, they will close on the thread( when the chuck is tightened) and keep the bolt from unscrewing itself.
Mike