750/500 valve cover bolts are a pain to seal up correctly. one bolt is so bad HD changed the design right after 2015. The bolts and rubber grommets
have to be change to get a good seal. The one behind spark plug is one that cause the most trouble. it often was stripped for the start . Aslo Hd no longer wants to supply the parts for that year.

My cure was making a copy of their bolt with slight longer thread. Removing the grommet from a 2016 model bolt and using it on the shorter bolt.
The threads are a M6-100.