Ultrafire 501B tailcap question

I’ve got a pair of Ultrafire 501Bs I use for bicycling. One is a single mode that works great, and the other is a five mode that not only mode skips but is also difficult to get to work after changing batteries. I noticed the ring just under the spring in the tailcap was loose in the rive mode light, and changing tailcaps eliminated the problem I had getting it to work after changing the battery. I was able to tighten the ring (which solved the problem for a while), but the problem has returned with time. What can I do to keep the ring in place? I’m reluctant to try and take the tailcap apart and explore things any further, and hope some of you can enlighten me (pun intended) here.

would need a glue to stop the ring, but there is another solution: tighten the ring, remount the tailcap and to change the battery unscrew the head of the torch, do not unscrew the talcap.

That retaining ring completes the ground circuit to the battery tube when the tailcap's screwed all the way down, and that contact between tube & ring means removing the tailcap also tries to loosen the ring. Polishing the mating surface between ring & tube can help, along with a light coat of silicone lube. You can tighten the snot out of that ring, it's a solid stack of parts in the tailcap and I don't think you can break anything by really cranking it in there.

Thanks, I was wondering if I could damage it or not by using more force to tighten the ring.

Tail caps are cheap replacements. Don’t worry about over tightening. As long as its hand tightened. It should be fine. I realized that too much lubricant causes such issues also. Try wiping some off.