Elaborating on this because of a mention in the Best Zoomable Flashlight thread Best Zoomable Flashlight Review $5-$40 range, Sipik SK68, SK98, UF T20 Odepro KL42 (G700 flashlight alternatives)
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So far, the “3W Police 2AA” threads fit most of the #3 zoomie, and all of the SK68 zoom and COFLY KX - FH10 zoom flashlights I’ve tried
Xref, Keltex also has a good thread on these: Sipik SK68 battery tube extension mod
I’ve used this one: cheap silver extension tube for sk68 ? - #5 by hank
http://chinaprices.net/buy-3w-white-led-aa-handy-camping-flashlight-torch-buyincoins—buyincoins-046351910
You use two of the sections with the ten ribs. Those sections have one male and one female thread.
With the #3 and SK68, the male thread fits into the zoom flashlight body in place of the tailcap.
So you can stack two of the “11-rib” sections, and you use the zoomie tailcap.
(in that case, you have the other battery tube section, tailcap, and emitter as a poor 1AA flashlight)
Or you can stack the one 11-rib section with the other section and keep the “3W” tailcap.
I guess this needs a picture, I’ll get one showing the variations.
With the Cofly zoomie, the female thread fits the zoom flashlight body in place of the tailcap, and you keep the “3W Police” tailcap switch, which has a female thread.
(in that case, I forget if there’s any way to use the leftover parts.
For a few years I had no problems; the last few #3 zoomies I tried had incompatible female threads though, so the extra section wouldn’t fit.
I can still find #3 zoomies for $2 intermittently, on sale at AliExpress (last ordered some a week ago though the price bounced back to $4 as of right now)
Not sure if they’ll have compatible threads til I have them in hand to try out though.