Thank you both. Your advice aligns with what I'm getting elsewhere. I will clean up the threads a bit, apply anti-seize, and carefully tighten the nut back up.

Hopefully I can get 20 years out of it!

Hi,
On my 82, it was the right hand side which had problem like yours (but some worse). I used the bike for more than 20 years like that.
Clean the threads as much as you can, buy new nuts, anti-seize and thread the nut properly and do not apply excessive force (run the bike and tighten until no exhaust leaks). And remove the nut every two years if you ride it often.
Hope this helps.
P.S. : there are people offering to put new material (solder) on the threads and thread them again. There is also the solution to put a new thread (tube threaded on the outside, after thinning the original exhaust part). Not cheap and quite involved for the maker....

As part of my maintenance checklist on this '82 R65 I just purchased, I removed the exhaust nuts to clean up the threads and apply anti-seize.

I found the left cylinder threads to be in rough shape (right side looks great). The nut spun off easily enough and spins back on smoothly, but I don't like this condition and am worried about making things worse when I go to tighten up the exhaust.

I noticed that the nuts do not match, with the left nut having a more pronounced bevel on the inner edge than the right. The threads look good in both nuts, so my theory is that someone struggled to remove the nut on the left side in the past (perhaps cut it off?), then purchased a new one and threaded it onto the damaged cylinder threads.

How would you guys proceed? I see that I can buy a 48mm thread chasing tool from BMW suppliers, but it is not cheap. Would that salvage these threads? 

WOW realy nice and clean bike. I'm also in your area.

I had day off and p?ine the sensor to base plate. Put back in place again  again. No spark so it  seems I have a faulty sensor