Rear Differential filler plug

Typically the plug protrudes the cover past the threads some. The taper of the threads on the plug is what causes them to tighten up and to seal. If you limit how far they can go in then the plug may not tighten enough. The other thing is if you have a Powerlock limited slip or a full case locker the plug can go too far in and hit the case. Very bad.
Cheap and easy to replace the cover.