Question on Luxeon led's

My question is, how hot do those LED's actually get? I am planing to heat sink it, but am not sure how much heat I will need to dissipate.
I was thinking for my next project I will try modifying a d-cell Mag-Light with a Luxeon Rebel Tri-Star LED. The led gives about 700 lumens from 700ma current.
Also has anyone tried using an RC engine heat sink head to cool their LED's. Thought they might work well as an exposed head of a flashlight because of the verity of shapes, colors, and styles. I know that those heads should cool any LED for sure.

You can buy Maglite heatsinks in the Custom Buy/Sale/Trade forum. That Luxeon tri star is bright, but you'll need 3 reflector or optics to use it, and the LEDs are close together meaning you can't get big reflectors to fit. Smaller reflectors = wider beam. If that's what you are going for, then that'll be perfect. Also, if it is 700 lumen at 700 mA, the LEDs are probably wired in series, meaning you'll need a boost circuit to hit the ~9-11v the LED will need.

The Cree XM-L is a larger single die LED that could be a better choice. Depending on the output bin, it can do 700 lumen at 2-2.5 A at ~3.2v. I did a mod with that LED, 3 D maglite, maglite heatsink, Cree XM-L on a 14mm round board (I'd get a smaller one if you can), and a 3 mode 2.8 A driver.

You have to dissipate 60-100% of the power you put into the LED. For 'robust' design I suggest planning on comfortably handling 100% of the power put in (Suppose the flashlight is head-standing on an opaque surface). It's also nice to handle temperature rises gracefully, like when lights turn on in pockets. By thermodynamics, if you don't dissipate as much heat as is produced, temperature rises until balance is reached. We like our lights to be "in balance" below 60C under all reasonable conditions. This is quite hot!

LEDs give about 75-100 lumen per watt, so at 700L you're going to have to handle about 7-10 watts. Since the Luxeons are in series, 0.7A * (3.4*3)V = 7W of heat produced. Do account for driver losses; so add about a watt of heat. You'll need to dissipate 8W of heat at a reasonable temperature to run continuously. Broadly speaking, heat exchange is proportional to temperature difference. Also broadly speaking, a light 20 degrees hotter than "ambient" will dissipate twice as much heat as one 10 degrees hotter than ambient.

In short, make good thermal contact with the outside of the Mag Lite (Hence buying ready-made heatsinks) and you'll be in pretty good shape at only 12W.

Thanks guys, that helped a lot. I will definitely look at all my options and go from their. I might even decide to build a flashlight body from scratch.
This RC head is similar to what I have laying around.

Thanks