I have a 71 Challenger which I just bought a rebuilt gear box from Napa. The old pitman arm fits on the new shaft but the old nut does not. It seems to small. What size is the 70-71 nut suppose to be?

Ok, how about this, do both size gearboxes use the same nut for the pitman arm?

1) Could the threads be slightly damaged / be cleaned up with a Die (shaft) / nut (tap?)

Note: The O.D. of the shaft "should" be larger than the I.D. of the nut, as the threads inter-mesh when screwed together.

That said, they could indeed be different size, (new shaft versus old-nut,) but, if they are ...

2) You could always stop by a hardware store and buy a couple grade 5 or grade 8 nuts to exactly fit the New shaft, and New fasteners are usually a good idea anyway.

I used to own a '70 Challenger R/T 383 4bbl, and may still have the gearbox in my garage. Oh well.

Didn't they supply a nut and washer for the new box?

I'm fairly certain they should have...

Who knows what shaft went back in, I guess that depends on who actually rebuilt it and what part of china the parts came from.

Maybe take it back to napa and have them set you up with what it requires.

If that's a no-go and depending on your area try Harry's Steer & Gear on Groesbeck, near 14 mile.
They're just south of the old Mancini location.

Good guys there, on a friday at closing time they rebuilt my 1.06 federal steering pump in an hour for $60, including a granaded valve that needed replacing.

If your not too far from them you still have a little time before they close, and look... it's friday.