Buggered Threads on CounterShaft and nut

I have a 2008 WR250R (about 4K miles on it).

I was changing the stock sprocket from a 13 to a 12 tooth (gearing it lower to prepare for a trip to CO this August). I used an impact air wrench to remove the counter shaft sprocket nut. I pinged out the indentions and then let the wrench run. I noticed that there was some rust on both washers. After I got the nut off I noticed that the threads on the counter shaft and the nut appears to be buggered in a few areas. So much so that I can't screw on the nut anymore. I even added WD40 to reduce friction but with no success.

I was going to get with a mechanic to see if there's a way to clean up the existing threads on the countershaft and nut.

Has anyone ran into this before?

thx

Buy a good thread file & a new nut. Work slowly/carefully trying the nut often.

When I remove the nut to change sprockets, I don't try to dig the staked dents out of the sprocket, just turn it off & let the output shaft straighten the staked part of the nut.