Very close but Yes they can stack.
KB - I'm also interested in more details on the FT03 mod. Looks like you saved a lot of space on the driver side.
I'm thinking, but haven't check measurements, you did this:
- reamed out the center of the driver retaining ring to make it thinner so the battery can fit through it (probably can dremel it down)
- tossed the original plastic piece around the driver spring
- removed the driver spring and replaced with a small flat copper slug (or something like this?)
- replaced the tail spring with one that will compress completely flat (dunno where to find one like that, but I could rig of something equivalent I think
Not sure if the first 3 would get the batt as low as you show there in the tube, but it would help. If you used a lathe or something like that to mod the tube or head, I'm screwed.
Update: Ohh, ok. You added bored out copper rings, so you used those for extra spacers, and I see you filled in the gap on the outside with a black o-ring. Think that's the trick...
Why is it that so many 18650 lights have stoopit-long springs but all the 26650 lights I have are contortionist-tight?
I got an old River Rock 2×C light that I tried stuffing in a regular 26650 but it was too tight a fit. Might try “boring” it out a bit with a sander or something, as it’d be worth it to stuff one of these in there and take up the slack later.
If you need to increase the inside diameter of the battery tube use one of these with a drill.
Kewl, tnx!
Thought I’d be grinding away forever with one of those cylindrical sanders.
For the FT03, making those copper rings (or ring) as a spacer, probably fitted under the brass driver retaining ring, is probably not an option with the tools/supplies/skills I have. I could though perhaps use copper wire formed in a loop. As long as the spacer is conductive and relatively even thickness, and doesn't interfere with inserting the battery, should work.
Took some measurements on my SP70.
Even with the springs really squished, and the threads not fully engaged, noway are a pair of 800s going to fit.
Looks like the tube needs another 23-25mm or so for a comfy fit.
All the Best,
Jeff
If that new 26650 6400 mah Vapcell pans this mod doesn’t gain much.
Yep, I ordered a pair of the K64's from Ali. They went up in price from $20 for 2, to $25 for 2, plus tax, plus shipping.
For me in NY, that would be about $29 for a pair. The 26800's cost me $17.35 for a pair from Aloft - big difference.
Plus I got a Reese's Peanut Butter Cup in the box 
. Ok, a little weird but it tasted good...
Texas shooter asked about a 26800 extension tube for Astrolux in Fins sales thread. Fin said he submitted the request to Astrolux, but I doubt it will materialize without a lot of requests from others. Artiet59 mentioned, “All astrolux models / tubes you mention do work together, but ft03 unfortunately does not. Ive tried…” That’s too bad since I need several for my fleet of FT03 lights. Maybe they will make 2 versions. It shouldn’t be too much effort for them to produce them.
For those who need a copper contact disk, they can be found at esty.com in all sizes.
I’ll be surprised if those will compete with the QB26800 above 20A sustained, but I’ve been surprised before. Im looking forward to your impressions and hope for the best.
true - the QB26800 is rated at 30A continuous, the K64 is rated at 15A continuous.
I’m seeing those two points also. I paid $59.27 for 8 Mooch tested QB26800’s, half the amount for untested K64’s. But they need a custom tube. Most of the listed lights operate under 15 amps. Except 7 LED MF01 mini, and FT02S that’ll need all the amps a battery can push. Future lights like Astrolux EA01S will definitely want the highest. I’m looking for both runtime and output. It’s QB26800 for me, I’ll gladly pay extra for the extra tube.
Hummm,
I clearly need a lathe. The tooling. Someone to show me how to use it. And a place to put it.
Other than that, I’m ready……
Done it on the drill press… made an arbor out of thread all nuts and a socket with a diameter close to what I needed… use a rotary file, tootsie roll, or sanding drum to open up the I.D. of the copper ring with about ,003 stock press fit to the O.D. of the socket arbor, press the copper rings on using the next size up socket which so happened to also fit the arbor nut…you can use any material to space the front of the tube out… the battery tube threads are bare…………………………………………………………………………………………… 
Here are more pics…of the mod…
EDIT: Instead of using 2 rings to extend the bat tube at the front…(head)…I’m using one in the front and one in the cap… yes you do have to open up the I.D. of the driver retainer ring also…
215Wh/kg and 600Wh/L is pretty low, especially since that’s only at 1.36A discharge.
And the more current you draw, those numbers drop to significantly less.
A high capacity 18650 or 21700 can do much better specific energy and energy density at higher currents.
Those of us going with single cell lights this cell tops in capacity with practical output. I also have lights in 18650, 21700 and several other sizes. I think you may have missed the flavor of this thread and that is to take something different and run with it. In this case the 26650 lights and how to incorporate a 15mm longer and better battery.
That long and thin back spring looks like it could add a lot of resistance and drop a lot of voltage under load at 20amps. You might get more output with a bypass or shorter spring
This is far from the highest capacity single cell.
There are plenty of other sizes above 26650, such as 38120, 38140, 40120, 42120, 40152, 66160, etc etc etc
Those can get to 10Ah-40Ah+
Might as well use something better than 6800mAh if you have to custom make your flashlight anyways to fit a new cell size.
I’m sorry, I did not mean to offend you and set you on this path.