The o-ring went under the lens focus ring; between the lens focus ring and the heat sink. Or, on another laser, it would go under the lens focus ring, between the lens focus ring and the host. As you screw in the lens, at some point it makes contact with the o-ring, the o-ring provides tension.
At the infinity focus point, the clearance between the lens ring and the housing (heat sink) is about 2mm. This o-ring looks 3mm thick.
Everything just happens to work perfectly... as the lens is screwed in, then when focused to the infinity point, the lens ring is "jammed" down into the o-ring. But not so hard as to damage the o-ring.
This laser does not get hot, really... it's duty cycle is listed as "continuous", and the maker, garoq, says that one forum member / beta tester left his on for over > 10 minutes with no problems. I've run mine at least 5-7 minutes at a time, and the housing gets "a little" warm. So I don't think it will get so hot that it damages the o-ring.
Also, the o-ring "holds tension" still through burning distances (which kind of suprised me); but this acrylic lens is touchy that way... a LITTLE movement/turn goes a long way.
I suspect that his 405nm Survival Laser might get too hot for this solution (when using a G1 lens), and his 445 Survival Laser PROBABLY gets too hot for this ; but then again, those lasers have shorter duty cycles.
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Got the PM; that's okay, with this o-ring thing I don't need teflon tape anyway; and I have 1 roll of teflon tape on order anyway.