Disclaimer: the following contains a lot of my usually useless babbling. So, if you want to read up on the essential information, just scroll down until you’d notice any text written in bold characters. Those are the only vital informations.
Imagine you flunked the stress echo at work, and were being sent home. What would you do?
Righto - go home and start playing with your torches! It seems to relax me a lot.
So, I have those two torches right in front of me; the 4Sevens Quark X AA² tactical (XM-L of unknown bin, and too lazy to look it up right now - google is your friend, as well!), and my trusty Fenix LD20 XP-G R5 (thankfully laser-engraved on the head, or I wouldn’t have known).
I had always wanted a Fenix by the time I saw one in reality, a P2D clipped to a police officer’s vest - they were rather expensive back then, but still not as expensive as a, let’s say, $urefire (I always type this dollar sign for a purpose; no offense). When I found an LD20 on amazon for 40 quid (€48, $62 US), I bought it. I never really appreciated the greenish tint on low, but I always loved the ramp-down from turbo to low when untightening the head - just like a conventional incandescent bulb would dim. Softly. Smoothly. Big fan of that! The torch itself saw little use, mostly as a backup light, and has been dropped a couple of times. Still performs beautifully (once I decide to actually use it).
The 4Sevens I ordered just in time, last year, when someone here at BLF pointed out they were selling the remainding stock with the old logo at a ridiculously low price; which still wasn’t quite “budget”, but almost one third off the original retail price. When it finally got delivered, I took it to work and played lots with it. I was re-programming it countless times, during a promotional event we hosted for Jaguar-LandRover Ltd. back then, frosty weather and nowhere to skive off for a regular Camel. What would you have done? - Anyway, the 4Sevens saw almost no real use at all (still, it has been scuffed and scratched already) - mostly because those pouches both Fenix and 4Sevens are supplying just don’t work for me. They are great, and they look great, but still - in daily use, they are impractical, at least for my purposes and habits. A loop-style holster works best for me. No velcro flap, please! (Velcro is great, by the way. Love it! -just not for flaps on EDC pouches)
I am wandering off, again, aren’t I? Right, where was we - ah yes, interchangeability.
Low and behold: The (square!) threads on the heads of both the Fenix LD20 (and presumably the LD10, as well) and the 4Sevens Quark X AA² are matching perfectly on both battery tubes!
Now, I have no idea how this would prove to be useful information; but hopefully, to someone, it may be. A replacement head for the Quark X AA sporting the still-fancy Cree XM-L retails for 39 yankee dollahs (don’t ask for the price I’d have to pay in the Old World 
), so this mod won’t even pass as a “cheap upgrade” for a Fenix LD20. Still, it is working!! I’ve never seen an LD20 as bright as with the head of the Quark X AA² screwed on. Of course, the Fenix’ head will work on the 4Sevens torch, as well. With the same pesky seaweed-type of greenish tint on low…
photos, as promised last night. Here we have both torches:
and here you can see the square threads of the battery tubes. I wouldn’t have thought they would match perfectly.
Quark X by Fenix, working as if it was intended to.
