Welp... thought I was gonna sell this light. Had a change of heart. Scroll down and read my dialog with Flashpilot further down.
One happy camper with a rejooved light!
Hope the info is useful to others here.
Is this too much work?
Asking in general.
Seems to be too much effort for many members.
Easy on him Jerommel, he just revived after several years.
From now on he will add images instead of links
I also have that same light, and for correct model designation, it’s a Fenix TK70 (4 x D nimh).
Btw, welcome back Dorp!
Thanks. ;
Welcome back dorpmuller!
If you’re able to mod, dont sell it! They don’t build em like this any more. Fenix designed the driver to also run 8.4V lithium. I removed the cell extension tube and I run my TK70 on a pair of 32650 cells. To take up the slack, I tossed in a D cell Maglite tail spring with wire bypass. If you can crack open the bezel, you can unscrew and de-solder the mcpcb (which is a large rectangular heavy copper direct thermal design) and reflow your favorite XML2 emitters. Easy mod IF you can crack open the bezel. Some lights were glued at the factory and impossible to open without destroying the light. Mine was not glued and still required more than 100 ft/lbs torque with a pair of large heavy duty strap wrenches and 2 determined guys. The effort was worth is as the light is now brighter, great tint, shorter and much lighter. I own more than 200 lights and my TK70 mod still remains in heavy use for its preferred form factor.
In summary, the only mod to my actual light was swapping emitters. The Maglite tail spring is removable and compressed by the stock TK70 tail spring. 
Now that’s a neat idea…both my Tenergy and Imedion D cells are almost dead, hardly a few mahs in them.
Now where to find those 32650 cells when even shipping them (lilo) is becoming a problem to my country.
I had this TK70 of mine modded by saabluster where he de-domed and installed 3 XML2 at that time. Currently the light is soundly sleeping back in its box.
Thanks! I'm gonna go try that. I think I have another 32650 around. I bought the light during the glue years, so I'm afraid of that. One fenix light did get destroyed trying to open it
Are the emitters on one board? I'm not set up for reflowing, but I have done singles by resting the star on a heavy old school soldering iron.
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Just went and looked...two 26650's with pvc pipe should work. Just gonna solder a button on one battery to make contact.
Thanks for this!
Now this is even better as I already have two Vapcell 26650s! (rated at 20A continuous)
Btw, the TK70 eats 9amps in Turbo with 4 x D nimh., and so far the ONLY light I own that can sustain Turbo mode until the batteries cannot sustain the load before it drops to High. No Step Downs!
Yeah, just go to the hardware store with everything in hand so you get a good fit and you're home!
Haha! Im glad I brought up the mod. The TK70 does so many things well that it will always remain a favorite. IMO, the amazing boost driver alone is worth the price of admission.
I thought about de-doming my TK70 but I already have a modded BTU Shocker (Same reflector as TK70 but 3 x 18650). I bought Trustfire 32650 cells from ebay several years ago. Also available through aliexpress and other places. These cells are good for around 6500 mah and will last many cycles through the years… so still the highest capacity cell up to this size for our hobby. You can also run your TK70 on a pair of good quality 26650 cells and make your own adapter from a pvc tube. Ah, I see tatasal already did! I still have my original box too, and my TK70 looks as new.
Someone ran their TK70 on 3 x 32650 cells, which threw the driver into direct drive to produce some larger lumens. As I recall, the over-volt messed up the modes so only max level was available. No permanent damage was done, but Id never risk my driver to try it.
Looking at cell voltage/storage/price ratio, this light should also run well on a pair of cheap 32700 LiFePO4 cells. HKJ recently tested well and they are dirt cheep. Test/review of VBatty 32700 3.2V 7000mAh (Grey) - #26 by Jerommel - Rechargeable Batteries - BudgetLightForum.com
It looks like these are probably the best quality: https://www.aliexpress.com/item/32983716184.html?spm=2114.12010612.8148356.6.4684771b3Kw64f
Also since 32700 LiFePO4 cells max at 3.65V, I wonder how the driver would handle 3 cells? With 3, that would be 10.95V LiFePO4 vs. 12.6V for lithium. Cheap enough to find out… if I didn’t already own too many 32650 cells I would! lol
Another thought. The driver reduces modes as alkaline/nimh cells become depleted, so there is no chance of damaging nimh. Im not so sure Id try that with lithium cells because I don’t like letting them go below 3.5V. Thats just my personal preference.
32700 LiFePO4 cells can safely go down to 2V and below… so another reason to run these if you’re so inclined. Then the low voltage protection should work well with the stock TK70 driver as cells become depleted.
If you can crack open the bezel, the mcpcb is a large copper triangle held in by screws and two large wires that you will need to de-solder. Unscrew the mcpcb first before desoldering to prevent the host from soaking up all the heat from your iron. Good luck and let us know how it goes.
If you get the LiFePO4 cells, make sure you use a charger that can handle that chemistry.
The TK70’s bezel glue is one hard nut to crack open…mine was accidentally blemished when saabluster opened it, but that was part of the risk in doing so.
Now the next move is to find a suitable ‘spacer’ to mimic the tube’s diameter for my 26650s.
I got lucky I didn’t kink mine with the strap wrenches. It wasn’t glued and still took several all-out attempts. Some people heat cycle the retaining ring with a blow torch then quench with a damp towel. But the reflector is plastic, which risks damage. Let us know if you figure out what size PVC tube works for your adapter.
Done! I'm withdrawing it for sale. I only had flat top 26650's so I carefully soldered a copper washer to + so it would contact the driver. Big difference over nimh also. I'm extremely grateful! You saved me from a grave mistake and I'll always appreciate. I'm not gonna mess with trying to open the head in view of what you mentioned about the reflector also. The light is still extremely capable as is. And a hell of a lot easier on over-60 wrists!
Gonna contact SB and see if he can move this to the mod forum.
Let us know what spacer adaptor you utilized for the batteries.
Thanks FlashPilot and dorpmuller.
I too am grateful for this thread…for years my TK70 has been forgotten!
Flash used a maglite spring. I used a spacer from plumbing parts that I cobbled together.
Good thing you came back! 
Im glad it worked out for you… and now you know what a gem you’ve got. Some TK70’s aren’t difficult to open up. Its the luck of the draw. Get a strong competent friend to help hold/clamp it down and give it another a try… you might get lucky without too much effort. Wrap the entire head in heavy masking or duct tape before you dig in. If it doesn’t pop open with a good amount of torque, throw in the towel and call it good. If you crack it open, you already know how to reflow emitters to make it that much better!
Any good high tension spring should work, as long as it applies a good amount of pressure to the cells. You don’t want to change the resistance across the spring, so solder in at least an 18 gauge wire bypass. I soldered a 16mm copper slug onto the small end of the maglite spring to maximize surface area and keep resistance low between the two springs.
I hope that makes sense. If not, let me know…