Pictures against the 35W HID on the right
Very good step by step instructions. I think that anybody who feels confident enough to fellow your procedure will and up with a awesome light. These TN31’s can be really beastly when modded. Heck, they are very impressive bone stock.
This is very concerning… I have tried to get some clarification from David at Thrunite but it’s like trying to get blood out of a stone.
I’m worried about using my TN31 now !, especially on longer runs.
Thanks for the info Alex
I'm suspecting they want electrical isolation, which is what you lose when you have a direct copper thermal path, like in a SinkPAD. Interesting, I've asked this before about the SinkPAD's but guess no one seems to know. Original MCPCB's has the substrate layers to electrical isolate the middle pad of the LED, but with a SinkPAD type of MCPCB, you lose that isolation.
as I’m sure you realize, the middle pad is neutral, and doesn’t need electrical isolation…
so that use of tape is really strange
Then why the substrate layers in standard stars? Thought it was mentioned they were there to electrically isolate the middle pad for the battery negative running through the body. Maybe something left over from older design LED's?
Update: this CREE App Note explains the issues: CREE - XLamp_PCB_Thermal.pdf. They say:
"The pad is electrically isolated from the anode and cathode of the LED and can be soldered or attached directly to grounded elements on the board or heat sink system."
Not sure though if the housing could be considered grounded.
but I wouldn’t be surprised if it’s simply cheaper to produce MCPCBs with a dielectric layer under the thermal pad than without. I imagine that in common designs, the dielectric that isolates positive and negative traces is the same as that which ends up beneath the thermal pad. Thus, eliminating/removing the dielectric in the thermal path would necessitate an extra step. Just a guess, and I could very well be wrong.
Slim - could very well be, but makes you wonder why ThruNite engineers designed it with that kapton tape there... Electrical isolation is the typical use of it, but maybe they were just playing it safe and being overly cautious, thinking it wouldn't impact the heat transfer much. You would think they would do thermal testing, but who knows what really goes on in the design/prototype phases with these manufacturers.
I finally got the info from David at Thrunite. He tells me that there shouldn’t have been Kapton tape on the back of the sinkpad.
He says that it should be directly coupled with thermal glue.
Kinda weird though, where did the tape come from then ? unless the sinkpads come with a Kapton tape backing and they didn’t remove it this time.
Ok, I was offbase here... Wow, this sounds like a ThruFire brand 
. Maybe this is some of the quality problems people have been experiencing with ThruNites lately.
What I suspect is that it was some sort of surface protection for the copper PCB and it was supposed to be peeled off prior to instalation, it is like the very thin kind of tape the factory puts on the back of new watches and the user has to peel it off before use.
When I first removed the led PCB the copper back looked the same color as the copper wire used in transformers, that is what gave it away, then when cleaning it to remove the white thermal grease, it started to peel off.
I was able to source an extra stock TN-31 copper PCB from another member who moded his light, when it arrives will check the back for the presence of the tape and post pics if its there.
That’s my guess too 
My TN31’s MCPCB did not have tape under it when I modded mine. Just thermal paste.
I tend to agree with AlexGT’s observation about what it is and I’m putting it down to poor QC on Thrunite’s part.
I’m glad I haven’t bought one yet - I’d have to pull it apart just to check lol
too much of a Laser + spill for me anyway, so if I did it would be to mod it anyway…
which is feasible with these great instructions 
wow thanks for the how-to! just wondering which SMD resistor that one is. unfortunately i don’t have a pic of the tn31 so i just stole turbobb’s k40 pic, i hope he doesn’t mind. is this the one? i hope tn31 has a similar pcb
Yes that's them.
Yes that is exactly the one, you just solder another smd resistor over it and the current will increase, BTW while you are soldering an extra resistor you can also upgrade the base of that spring with more solder.
awesome! can’t wait to do the mod! so it boosts all levels, right?
oh btw don’t use Flitz on any thermal interface. it gets into the pores of the metal and inhibits thermal transfer. instead use thermal grease to polish the surfaces. doing this impregnates the metal with conductive particles
i learned that the hard way when i was working on a watercooled 172-Watt thermoelectric cooler
The resistor mod does not alter the other levels as far as I can tell, only the highest level is what gets boosted.
The digikey part number for the 1206 0.082 ohm SMD resistor I used is P.082AVCT-ND,
Thanks for the tip OC, I did clean both parts with alcohol before using AS5, and I know the results are better than what came from the manufacturer, I can tell right away the led module heating up fast when in use, so the thermal transfer must be working as planned.