The metal on the car's rotating post is harder than what's in the arm.
(I'm pretty sure the post is steel.)
If you rotate the arm around on the post you'll chew out the soft metal that's inside the arm's hole.

You have to sort of wiggle the arm back and forth to let the old grooves find the splines, then it will go down onto the post a bit further.

The post is slightly tapered so it tightens as you crank down on the nut. This also lets it deepen the grooves in the arm's hole, up to a point. I finally decided that when changing to a new (or used) arm, you do need to force the nut down hard to cut the grooves deeper in the arm's collar hole. That way the arm doesn't come free of the post.

I think I found a couple different spline patterns, so if that's true, earlier and older arms might not really be interchangeable.

As someone else pointed out, the left and right arms are different. Not by much, but different.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.