Here is the IndeeSZ 100mm truck set up with an AN-24a aircraft bolt king pin. This bolt pushes the threads clear through the base plate, so I added a short section of 9/16" tubing for an unthreaded spacer and then a full height 3/8x24 nylon lock nut below to secure the bolt. This way there are no threads at all in the base plate, where I have observed that the sharp steel threads can chew into the soft aluminum base plate material.   (9/70)