the tail cap oring is a one way to let battery gasess vent out as far as I know, or to allow you to turn the head without air getting compressed. I think the seal is very good on the head part, that is why they had to do that on the tailcap. So yeah, make sure your orings are well lubed and tight, put a normal one on the tailcap as Reptilezs said, and you should be set!