The past comes back to haunt me. I used to make parts for 12'th scale electric cars back in the 80's. It looks like you are using a dowel pin for a kingpin in your picture, toooo hard to machine. Try an 1/8" mild steel pin with a groove cut to accept an e-clip to fit just under the upper mount. Or drill thru the bottom end and use a longer pin with e-clips on the outer ends to retain the pin. If you still want to use the screw in method you will need to enlarge the upper portion of the spindle to recieve threads. Then take a #12 set screw and turn down a major portion of the threads to 1/8" leaving enough threads to thread into the upper portion of your spindle. One threaded spindle. The reason for the thread being on the top, if you would break it, it would be easier to remove.
If it's not nailed down, it's mine.
If I can pry it loose, it's not nailed down.