I would have liked to use both original kingpin bolts but I can’t since I had to drill and tap out the bottom mounting hole on the fishmouth to 9/16-18 UNF. Both of the original bolts were made from 5/8″ hex head bolts. They both have a center drill hole on one end which means they were modified in a lathe using a tailstock. The top bolt is drilled for safety wire and was centered on the hex end and the bottom bolt was centered on the threaded end.
I like the bottom bolt better since it has longer threads. I will use this bolt in the top but it has a few problems.
First it was not fully threaded which means it had locked up on the threads instead of the 5/8″ diameter shoulder. I suspect this was a contributing factor into how badly the bottom hole was messed up. I used a 1/2-20 UNF die to cut threads up to the shoulder.
The next issue to address is that the shoulder is not long enough to allow the bolt to seat on it. The bolt hex will lock up on the bushing instead. This will unloosen the bolt when you turn the handlebars to the left. It will tighten when you turn to the right. No problem as long as you plan all your rides using right turns only.
I used my toy lathe to make the shoulder longer by turning about 1/32″ off the bottom of the hex.
The next issue is that this kingpin was made from a 5/8″ hex head bolt that measures .611″ under the hex. This is too loose in a .625″ ID bushing so I decided to bore out a FB912-6 562″ ID diameter bushing to .612″ in my toy lathe.