I know the tapered shaft goes into a tapered hole and the taper creates an interference fit. But suppose the taper on the shaft was machined 1/4" too low on the shaft. Now the tie rod housing is making contact with the center-link or ball joint arm before the tapered shaft contacts the tapered hole. As you tighten the nut it starts pulling on the ball inside the joint, possibly cracking the housing allowing the shaft to seat in place. Now the castle nut is too far down on the shaft causing the OP's problem. You guys are most likely right, it's just the wrong castle nut, but what if the above is the problem, he just puts a washer under the nut to raise it to spec, drives down the road and it comes apart? You don't take chances on critical components and a tie rod end is definitely a critical steering component.