I think he’s right. The thickness of the white one is a lot smaller than 1mm.
Lumintop support told me the white O-ring is 16mm x 0.8mm.
And what function does this rubber ring perform? Moisture protection? I spun off the shank and pulled out the tube with the rubber ring, then when I put everything in place, I couldn ’t tighten the shank, the clip was almost free to rotate. I was only able to tighten the shank well after I removed this rubber ring. What did I do wrong?
Which o-ring are you refering to?
If the one on the clip, it is for anti rotation as mentioned before. It can prevent full tightening of the tail cap on certain lights.
Moisture protection comes from the black o-ring near the threads.
I meant a white rubber on a black tube
Helps to keep the tube from shorting against the body.
With this sealing rubber, I cannot tighten the shank, the clip is not fixed rigidly, moves.
Wait are there two white O-rings? One on the clip, and one on the switch tube? I may have re-assembled one of my lights wrong. But the switch tube is anodized, so it doesn’t short.
Maybe I didn ’t install the rings correctly. How many rubber rings should be on the tube? White rings where should they stand?
From your picture, the left-most white ring is in the wrong place. Lift it out of the groove below the threads and slide it down over the black o-ring. It should be resting on the pocket clip, between the pocket clip and the black o-ring.
When you screw on the cap, the white o-ring presses against the clip and prevents it from rotating.
The second white o-ring in the right of the picture is meant to help keep the black inner tube centered and reduce the chance of a short between the inner tube and the switch. However, sometimes it can cause problems. If your light isn’t working right, you might try removing that o-ring completely and see if it works without it.
I don’t think anyone really knows. It’s not for moisture and it’s not needed to prevent a short circuit. I don’t think it’s needed for centering the tube. It may provide a bit a tolerance and pressure to keep the center tube pressed against the tail switch.
It may be to prevent the inner tube from rotating. If it’s not there, the metal lip will press against the outer battery tube. Then if it twists it can wear the anodized finish and create a switch short. Just a theory.
The other white o-ring is just to keep the clip steady. For some lights it can prevent the tail cap from fully tightening.
Make sure to always tighten the tail before the head.
That’s how mine arrived, and I started fiddling with it because of loose clip and erratic operation.
I changed it as described above:
I tried that and it did keep the clip from rotating but screwed up operation of the light. All I got from engaging the tailswitch was a fast weak flicker.
So I moved the white ring back to “the wrong place” and the light’s behaving.
I swear. Too many variables with this design.
It probably prevented you from tightening the tail all the way.
Firelight2 Thank you very much for the clarification. I did everything as you said and everything turned out, the flashlight works, the tail is tightened, the clip for the pocket does not rotate. Everything is fine. Apparently I have a flashlight from the latest releases and the board in the shank is tightened with a threaded nut, this is very convenient, since nothing falls out when disassembling.
I CANNOT lossen the tailcap on two brand new lights.
I received 2 x FW3a and 1 x FW1A today.
I cannot for the life of me remove the tailcap off two of the lights. Ive ripped holes in a bath towel, used pliers, rubber and hurt my hands.
I want to replace the battery tube with smaller tubes and remove the stock clip, this is my right. They shouldnt be tightening this thing so far its basically welded just because they cant design a proper light.
I know what im doing, let me do it. One of the switches also doesnt feel right and is causing it to not register clicks, i want to fix it.
Any ideas on how to loosen it?
Between this, the FW21 battery tube size issue im not particularly impressed.
You might need to assume it’s glued. Have you tried adding heat from a heat gun or a hair drier?
That would suck if they started gluing the tailcaps.
None of my FW3 series lights have glued tailcaps. Some of them were screwed pretty hard together, but nothing I couldn’t unscrew by hand after applying some 3M indoor stairway grip tape to for extra grip.
I just got a brand new FW4A, and the tail cap was not glued.
In addition to my stuck caps, my 1 day old FW3A XPL 5k is broken from the factory.
It was flickering from the first use a little and when it got hot the tint went extremely blue.
I removed the lens and i can see the LED PCB is floating free from the driver and the negative wire isnt soldered properly, in fact its come off.
I dont even have a working Soldering Iron at the moment due to Covid-19 shipping issues, been waiting 6 weeks 
How is the LED board meant to sit in the head? Does it sit in a thread? Or is it just held onto the driver with thermal paste and pressure from above?
No doubt the board wasnt removing any heat into the body and it was causing the light to go blue under extreme heat, possibly desoldering the wire with it.
If i had an iron id be happy to reattach it, but if its part of a bigger issue of a free floating PCB i will send it back or something, along with the stuck tail cap!
It’s a simple copper star with thermal compound underneath it.
Later model FW3 series lights also have a small screw that sits in a divot on the underside of the star to keep the star from rotating when the bezel is screwed on. The latest models are the same, but have an extra-thick copper star.
If the star isn’t aligned properly during assembly, the screw head on the shelf meant to sit in the divot on the bottom of the star might not be in the divot. If that happens, the screw head will keep the entire bottom of the star from sitting flat on the shelf. This could result in the star overheating and the driver wire desoldering itself like you observed.
The fix is fairly straightforward, but unfortunately does require a soldering iron. You will need to align the star and resolder the driver wire. Preferably after removing it completely and applying fresh thermal grease.
I’m not sure why you can’t remove the tailcaps on your lights though. Until your post I’ve never heard of any FW3 lights having glued tailcaps. They tighten them a lot in the latest batches because that’s what’s necessary for reliable operation. But they certainly aren’t welded and with some grip assistance you should easily be able to unscrew them.