There's a thread around here from some while back that has a lot of details on swapping the optic out for a reflector. Here's a copy and paste I had saved to my computer from a message thread here at least a year ago - some relevant info: "Spent about 2 hours last night hand filing down an SO17XA (no dremel at the moment) and finally, Voila! Perfect beam and hotspot on my FFII! Mine actually fits inside the copper head instead of having a lip, so I "borrowed" the O-Ring from the body of my MiniMag AA to temporarily replace the thin O-Ring in the front of the bezel until I receive the O-Rings I ordered from the Sandwich Shoppe..
To sand down the sides of the reflector, I used the small-rounded sanding bit. To shorten the top of the reflector I used the round-sanding disk, so I could just "sit" the reflector on the disk in a straight up-and-down direction and let the wait of my finger tips force the reflector top to the sanding disk.
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When changing the optics, you have to be careful when screwing the bezel down. Several things are happening - the front O-ring (lens seal) is being compressed into place, and the optic is being secured. Due to the tight tolerances on the length of the optic and the space it has to fit in, sometimes the bezel can compress the O-ring too far, and push the sandwich slightly away from its seating. If you unscrew the bezel about 1/8th of a turn or less when that happens and press the sandwich into place firmly, making sure the snap-ring is tight against the back end of the sandwich, proper operation should be restored. The length issue is less critical with the Fraen, because it is slightly shorter than the NX05.
When doing this install, make sure that the bezel is seated down far enough for the front O-ring to seal properly, and that the sandwich is firmly set in place, and there should be no problems. The head uses 2 different sizes of O-ring in front of and behind the accent ring, to give the correct tensions to each side. Te head will still rotate to and off, and the bezel will still stay in place, even if the bezel doesn't seem to be as tight as you might think it should be. Waterproofing will be maintained.
The reason the low level doesn't work is because the sandwich is not touching at the front face until heavier pressure is placed on it from the battery, due to the issue as above. I am not sure why there is the delay in activation, but it may have something to do with the amount of spring pressure in the battery train.
Double_A is correct - don't over-tighten the head after activating high level! The FF can crush-in the bottom or nipple of the battery, which will prevent activation of low level if the crushing is severe. It doesn't need to be bottomed out to work reliably.
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Then I did the O-ring spacer mod to the NX-05 that cy & Fitz suggested (9/16" OD x 7/16" ID x 1/16" Danco O-Ring from Lowes (#96755, sold in 10 pack, $1.79)), & it turned the FF2 into a whole different & much better light!!
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the bezel-side should have the larger O-ring, and the body-side should have the smaller O-ring. That gives more friction between the head and bezel than between the head and body, so the bezel is less prone to unscrewing.
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technique for thinning the reflector to fit the FireFly:
1. Find a small faucet replacement washer, the kind that is conically-beveled on one side and flat on the other.
2. drill out the center hole to accept a #12 flat-head screw, 1¼" to 1½" long.
3. Slide the faucet washer's flat surface onto the screw.
4. Slide the reflector onto the screw so that the reflective surface mates with the faucet washer's bevel, and fasten with a nut. A small washer under the nut wouldn't hurt, but I didn't use one.
5. Chuck the assembly into a hand drill.
6. Hand hold the drill and spin the reflector (with very light pressure) against the edge of a (not too aggessive) belt sander where the belt is unsupported by back plate so it will have a little give under pressure. Make sure that at least one of your hands/wrists is supported for better control.
This technique
1. gives fine hand control for even sanding;
2. avoids creating hotspots in the reflector's plastic.
3. accomplishes perfectly even thinning in a few minutes;
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The opening in the ring should be over the lettering on the sandwich bottom. These should cure the no-low-level issue."
