94 410D track rod end

So, I'm doing the off side track rod end on my 94 410D. The guy in the parts shop swears the TRE he gave me is the correct one, but the diameter of the threaded section is visibly bigger than on the old one, and it doesn't want to fit into the rod. I don't have a caliper so I can't say exactly how much bigger, but I'd say there's a couple of mm in it anyway. Enough to stop it screwing in comfortably.

I'd swear it was the wrong part, except... the threaded end of the rod itself is designed with a slot, almost as though it could be widened to take a bigger diameter TRE.

Any thoughts? Should I return the part? Should I use a blunt implement to try to widen the hollow threaded section of the track rod? Should I just give up and take it to the professionals?

Any help appreciated.

Where's the original track rod end?

If the threaded end of the TRE is larger in diameter I doubt the threads would be the same pitch. You can check the thread pitch by laying the threads against each other (new with the original). Probably the reason why it stops after a certain number of turns; the pitch doesn't match the original.

I would consult another shop if possible or bring the track rod into the shop that sold you the end.

The original track rod end is lying on the bench next to the new one!
The thread pitch is similar, it's the diameter that's really way different. It's so much wider it won't even get a start, let alone stop after a few turns.

Going back to the shop tomorrow with both the original and new parts...