If you're not so sure about building one yourself, but the idea of a custom Mag floats your boat, you'll want to check out Jayrob's XM-L Maglite sales thread on CPF! Jayrob wasn't the first to modify Maglites with 10+ Watt LEDs, but he'll build you one exactly the way you want it. Jayrob has impeccable feedback because he backs up what he sells 110% and he goes over every detail with you so you'll make an informed choice. He has options for custom battery voltage monitors, superior reflectors, even a cut-down 1D stubby version--check it out!
Now, you budding flashoholic, you. I bet you're thinking, "the good old Maglite is decent: they're made in USA, tough as nails, and they are fairly inexpensive here in the States." Well, I would have to agree with you. I practically guarantee this Mag build will put a grin on your face if you've ever owned or used what is the quintessential flashlight to so many.
With a modicum of skill, resourcefulness, and patience you can modify your incandescent or LED Mag with a Cree XM-L LED, turning it into a powerhouse flashlight capable of 1000+ emitter lumens, regulated output, and multiple brightness levels. The great thing is it retains the familiar Maglite styling. A majority of high-output flashlights these days try to sell tacticool features and hyper-militant styling and simply look tacky (pocket-shredding "assault crown bezels?"). The Mag design is timeless, practical, and the body makes it a great mod host (it's a big mass of aluminum).
Below: six-thousand word essay on real-world performance
Step 1: It ain't all about flux.
Don't get hung up on losing a flux bin or two if you want a particular tint. The tint we chose for my buddy necessitated a low-ish T3 flux bin--it produces about a third less light than an XM-L U3. That might seem like a big difference, but if you want a warmer/high CRI tint, you have to accept a hit in output.
It's a game of give and take. For me, Quality > Quantity.
Update: Cree has since released the XM-L2. It's a little brighter. Generally, I recommend it if you find an XM-L2 with a desirable tint or if tint does not matter to you.
Step 2: What type of cells for your D or C Mag?
Hosts, Cell chemistry and Number of Cells:
You can build a light that can run on High (about 3 Amps) with alkalines. However, the current drawn on High mode will run alkaline cells into the ground very quickly. As in, <1V per cell in 10 minutes. Medium will play nice with them and will still be bright. You can use High mode in bursts (like a turbo mode) with alkalines, but that isn't really ideal.
If you need use alkaline cells, you can use a 3D (4xC cells), 4D (4xD or 5xC cells), 5D, or a 4C Maglite. I recommend hosts in the 4 and 5 cell range if you don't have NiMH cells. This way, when the cells run down to around a volt apiece or less they will maintain regulation in High mode. You can get away with alkaline cells, but it's like filling up your daily commuter F-22 Raptor with Regular Unleaded.
I recommend NiMH cells. If you can use them, you can use smaller hosts. For example, a Mag 2D fits 3xAA or 3xC cells, a 3C accommodates 3xAA or 3xC, and a 3D Mag fits 3xD, 4xAA, or 4xC. A 4-Pack of AA rechargeables isn't expensive, and AA NiMH also provide longer regulated runtime than disposable D cells.
If you have good quality (like AW, Redilast, BatteryStation's IMR, Callie's Kustoms) lithium-ion 18650/25500/26650 cells and practice proper safety precautions, you can use one cell (4.2V fully charged, around 3.5V resting when depleted). This option is nice since you can use shorter hosts like the 2C (though I'd love a nice cut-down 1C...)
Step 3: Components you need:
1.) The LED
2.) An 8xAMC7135 linear regulator driver
3.) The heatsink
4.) Arctic Alumina thermal epoxy
Optional but recommended: Mag makes a deeper reflector for its newer ('DL' prefix to serial #) LED lights. I snagged three from ZBattery.com for about $13 shipped. They make a nicer hotspot with a better tint with neutral and warm XM-L LEDs than the stock incandescent reflector. If you go this route, you need a heatsink that lets the head screw down enough to cover the barrel O-ring. To achieve this with the deep reflector, I machine my heatsinks so they're roughly 30mm tall for D-size Mags.
1.) I buy from IlluminationSupply. ShiningBeam is another good source for the LED. As a bonus to those living in the States, both vendors offer very fast and inexpensive shipping. Customer service is great.
2.) Again, I've ordered 8xAMC7135 drivers from IlluminationSupply. ShiningBeam sells an essentially identical one.
3.) You can join CPF and orders heatsinks or complete drop-in modules from Jayrob, and another member, H22A, sells heatsinks in his sales thread as well. I've bought 3 from him and they're great quality. I machine my own original design now. I bought 12 feet of aluminum round stock and learned how to use a lathe and mill. If you want to be get creative without the heavy machinery, you can use copper plumbing end-caps found at the hardware store. You can grind/sand down the copper piece and/or the Mag tube to fit, or wrap it in aluminum foil if it's a little small. YMMV.
4.) For the thermal epoxy, check out ebay.
Step 4: Stripping down to the bare essentials...
If you buy new stock or have a light made in the past 2 years or so, you might need a T8 Torx wrench. I bought a a fold-out Torx set by Kobalt from Lowe's for about $7. I had to use a Dremel to grind down the shaft thin enough to fit inside the switch hole.
Unscrew the grub screw enough so you can slide the switch out of the 'back' end of the light.
Now that the switch is out, remove the bulb holder. Cut off the bulb tower so the switch is flat on top (I trimmed the tower on this build the rest of the way later on, no pics unfortunately).
Step 5: "It's soldering time..."
The positive lead, which is the red one in the picture, needs to be soldered to the positive V-in on the AMC driver board. The negative lead soldered to the side part of the switch goes to the driver's negative V-in