3 26650's definitely fit - I'm using the light that way. I got some wall beam shots - need to post them FF STL-V2 vs. SRK vs. TR-J18.
Slight update,
Parts are ordered and on it’s way. I ended up picking up the KD modified driver along with a bunch of AA/AAA springs. We will see how it goes in a couple of weeks time when I try and swap this driver in.
One question I do have tho is, is there a “need” to attach the driver to the flashlight?
What I mean is, when I took the original driver out to look at it, it was just sitting in the flashlight, held down by a spring (which is in turn held down by a retaining ring).
Looking at the guide for the driver upgrade on the smaller Tr-J12, I see that they where destroying the original driver to get it out, followed by soldering the new one into place… Do I need to do the same on the Tr-J18 or does the new driver just ’drop in”?
This is a pic of my J18 driver: https://budgetlightforum.com/t/-/10808#comment-289413
Can't recall exactly, but didn't have to do anything nasty to get it out or get it back in. Think I describe later in that thread how to remove it.
To install the new driver, you will need to de-solder the + and - LED wires, then solder them to the new driver at the places indicated for the + and -.
I ordered the I-O driver, looks the same, specs are the same, little cheaper, great support from Hank at I-O, rated for 3 batts, includes thermal pad.
Thanks Tom,
Yes I can get to the driver the same as in your pic, unscrew the battery tube, take out the retaining plate and the driver just “falls” out… I guess there is no need to solder in the new driver (onto the head) as shown in the thread where they modified the J12’s… yay 
Now, my next noob question. What is the thermal pad for? From what I see the driver just “sits” in the flashlight. There is not any “flat surfaces” for you to “glue down” the driver.
Actually I emailed Hank about the thermal pad - it goes in between the 2 boards. Haven't gotten the driver yet and haven't worked with a double board driver before, so not sure exactly how to fit it until I get it and see. I know the 2nd board just dangling by itself hanging by wires is probably not good - you want a path for the heat to go along, escape to. I think guys have been potting the driver in these cases, not sure (potting is taking thermal grease or epoxy and smothering the driver, and maybe filling the inside of the pill).
Got the I-O driver, installed it, wow! just wow! 3 simple modes, easy to install, fits perfect, sandwhiched in the thermal pads it came with between the 2 boards, cutting into pieces to fit. Measurements of the J18:
Before with 3 TF 26650 5000 batteries, as is, not fully charged but in the 4.05v range:
tailcap reading: 3.8A down to 3.1A, fluctuates, usually dropping steady
lumens: 3330 @start, 2940 @30 secs, throw: 34 kcd
After with same batteries as is, not charged in between:
tailcap reading: 5.78A
lumens: 4930 @start, 4726 @30 secs, throw: 48 kcd
Waiting on the TF batteries to charge, takes long on an i4, but should get 5000 lumens.
Wow!
I see one of these drivers in the fit-40s in the near future, thanks for the update tom. Any ideas on a more heavy duty tail switch or are we crossing that bridge when we come to it?
I'm taking my caliper home with me to measure the driver for my UniqueFire S7; if it's the same size, I'll be checking the possibility of completing either the resistor mod or hitting up I-O for the high-power driver. I'm just not as satisfied with my S7 since I have my King clone which is not quite as bright, but too close for the difference between a 3-LED and 7-LED light.
Anyone with experience with the resistor mod; what wattage resistor should be used? and what's a good source for a single low-resistance resistor of the appropriate sort?
Dunno what to do about the switch - where do you find better ones? Only thing I've seen is where O-L makes them out of copper - no idea how he does it, amazin stuff though. There is a lot of power going through it. The best I see is the conventional type rated for 2.5A, 240v. Can't recall now what's in the J18, though I had it apart a couple of times - it was flaky when I first got the light, did something to fix it, maybe just tightened it up, re-assembled.
That’s my problem, the only high current switch I’m aware of is that mega bucks mosfet jobbie, not seeing that myself. I suppose the other answer is braid the spring and buy in a rook of switches, just swap em when they burn out. 
Just tested the J18 out on 2 ICR KK's, reads 5.2A, does 3630-3570 lumens throw of 35 kcd, very steady during the first 30 secs. Nice light at this size, easier on the components and switch too!!
When/if the switch does decide to melt/cook/die… where would we be able to find replacement switches? Are they fairly standard?
And I to am not ready to spend mega $$ on a new switch, if they last few months to a year at a time i’m good
As for the driver, I ordered the KD custom programed version…… still waiting it to arrive, will report back when I get it installed.
I thought they were standard switches, you can get at FastTech, I-O, FancyFlashlights, etc., but will confirm. The ones I've been buying are stamped at 2.5A for 240v 1.5A 250v - I've replaced a lot of them, mainly because of quality issues, squeeks, etc. I never melted one, so I'm not sure where/how it breaks down. I saw in one of O-L's mods, he took a switch apart in all it's pieces, then fabricated replacements parts all in copper, I think from copper sheet material - that switch must be awesome -- beware of oxidation though!! (O-L's new thread on that topic).
Update: Just confirmed it's a 1.5A 250v rated switch, Omten brand name - I got 2 spares already of the exact brand name. Good thing is it's an actual brand name, and should be better quality then what you get in a typical C8 for example (not the XinTD - it's a quality switch already). Buy them here for 50 - 60 cents each:
http://www.intl-outdoor.com/omten-reverse-clicky-switch-pbs101-5-pcs-p-566.html or
http://www.fasttech.com/products/1610/10001865/1135000-15a-250v-pushbutton-onoff-switches-2-pack
Maybe they can be taken apart and beefed up?
2nd Update: Crap! Maybe I replaced it already? can't be sure now...
I’ve got a pack of the omtens, I did my zy-t08 switch making sure it was not the source of low run current. I’ve just found out a host that’s inbound has a 3a switch, I’ll see if that has any id on, may be a bit more robust….
Final #'s on the J18 on fresh off-the-charger TF 5000's, 5.87A measured at tailcap:
5,235 @start - 4,998 @30 secs --> Darn, just short of 5,000!! Impressive @start though... 
There are some driver issues though - slow mode changing, buzzing in low, maybe slightly in mid - gonna check the ground/mount now... Gotta get a wall beam shot -- freak'n thing is blinding on the snow!!
Try replacing with a 10A Judco switch.
O-L has done this in his Mag mod's so he would be the guy to ask about connecting it to the tailcap.
I have read forum post after forum post to the point of almost being nauseated. I have the Trustfire TR-J18 that ive had for about a week now. I am currently running the flash light with four 1.2 volt AA’s @ 2700 mAh’s.
From my limited understanding that would be 1.2 volts x 4 = 4.8 total volts @ 2.7 milliamps per hour? Correct?
I do have 26650 x 3 batteries that are rated at 3.7 volts and 4000 mAH coming in the mail by Tenergy with a battery charger that can and does charge batteries independantly from each other.
My biggest question is that since I am going to be increasing the voltage to 11.1 (which is over twice what I am currently running), am I going to see the luman and lux increase?
I laugh almost evertime I see the “Caution: Hot Surface” sticker on the bell of the light because it never puts off any heat needless to say but there is no way someone could possibly hurt themselves.
Thanks in advance.
That J18 must be sayin: Feed Me! Feed Me!
It's amazing it even works!
@Tom E.
Thank you for your reply.
I have to correct my previous post. The batteries are AA and NOT AAA.
So do you think I will see an increase in lumens and lux?
You are greatly under driving it, even with AA's, so the light is probably ok. Others may know more exactly what's going on with driving it at that voltage, but my best guess it will be ok, and yes, you should see a lot more lumens and lux. Think mine did over 3,000 stock with 3 TrustFire 26650 5000's - definitely brighter than a good, stock SRK.