Are the new ends the same length as the old ones? That may be the cause of them hanging up, needing to screw further into the rod socket.
Take your old tie rod ends and file a flute or two in them similar to what a tap looks like, and use them to chase the threads in the rod. It's probably just rust and gunk that's making the new ends hard to screw in. Use lots of WD40 or other lubricant. Also be sure the clamps are released if the rod sockets are split.
.812 - 18 would be 13/16-18 and I didn't see that size on the McMaster-Carr site.
