Source for Tie Rod Castle Nuts

I didn't expect this to be the snag in my project.

I have a perfectly good tie rod end, but need to replace the nut. Went to Home Depot, bought a shiny new 1/2" fine thread castle nut. It screws on just fine, but it is too tall. When completely tight, the cotter pin hole is still covered by the threads. Compared it to the other side, and this nut is at least 1/8" taller. Is there some secret source for proper tie rod hardware, or should I just attack this one with the grinder.
Author: D Cluley