After cutting the head off the bolt,insert the smooth shank of the bolt into a drill or drill press. This is why you only want to cut off the head of the bolt and leave the shank to be cut later. I'm sure there are better ways of doing this but this is just how I did it. Using a flat file I removed the threads on the lower portion of the bolt.
Once the threads are filed off, you have to start checking the diameter. In my case, the target diameter was 0.325. How much thread you leave on the bolt above this area is entirely your choice. I left about 5/8 or 8 threads.
This picture (below) is really disproportionate. The filed end would actually be longer but this is just an example using a finished pin. That is also why it is crooked in the drill vise (there's no shank remaining to insert)
