A bolted joint with any chance of bolt rotation should be assembled with a castle nut and cotter key. The bolt itself should be installed head-up whenever possible. The combination can be aggravating if the nut is out of sight (as it is with this aileron bellcrank), because it is impossible to see the cotter key hole without a mirror.
Save time and bad language with this simple trick. Carefully mark the hole alignment on the bolt head with a felt tip pen prior to insertion. With a clear indication of alignment, it becomes easy to position the nut, and insert the cotter key by feel.