The spring can be a pain to reinsert once you pull it out. You need some fine tipped straight needle nose pliers that will enter the loop in the top of the spring. Then, you insert the spring and turn clockwise while applying enough pressure to make the spring compress into the center hole and lock down.
Or, you can find a spring with a wide base and just push it in with your finger. The spring needs to be shiny stainless steel for good conductivity. The spring from my extra Sigelei Zmax battery cape fits perfectly and provides greater force. You don't want something that applies too much. All that pressure is exerted on the circuit board and the top cap (which is held in place by a pink o-ring).
My Provari spring was shot after a year and a half of use. After reforming it and re-installing it, I could still shake the Provari an get flashes from the LED. That's when I tried the Sigelei spring.
The bottom cap where the spring is will build up black contamination, too. To really clean up the battery cap, you need to remove the spring, but have those pliers handy!
I use Ox-Guard on my threads just because Lowes was out of Noalox. Both are rated equal in the reviews. Clean threads and good springs make for a good and reliable vape.