I recently used the Ace rethreader on the kingpin of some Indy Hollows. The nut didn't go on that easy before, after I used the rethreader the nut went on.. not as easy. So I am not sure the rethreader really helps with totaly destroyed threads on bolts. It seems to work for slightly damaged threads. I am curious to hear other opinions about it.
One of the hardest things is also getting all the extra metal from the nut or whatever out of the threads of the kingpins, so either half a turn on and quarter turn back with a shake, or get a really rough brush / metal option if possible, to brush out those other bits still stuck in there.
This helped a whole lot with people, especially with hollow kingpins, which seem to attract more of the other metal or have more issues with hollow kingpins - harder metal?!? and so often will mess up re-threaders if they are cheap, like the Ace versions. I found that the solid tool version was a whole lot better, as per post below, but the simple nut rethreaders can also work well enough.
Without having to go right into the posts, they are available from tool shops and online, UNF24 or NF24 3/8" rethreader.
Kingpin rethreader 3/8 UNF 24 / NF 24
Axle rethreader 5/16 UNF 24 / NF 24