Loosen the nut, drop in the new washer, torque, (ok,  count threads), and go fly!

Final (I hope) whacking of the dead horse.  Yes, math and geometry are required - Oh, how I wish the mounts themselves had slots in them...

 

Attached here is a view of a shouldered nut (this is not a self portrait).  Do not do this.

 

Grayerhawk