I’ve used Artic Alumina thermal adhesive epoxy to glue on LEDs to the pill sometimes, but I just recently read a post from Richard/RMM on his sales thread about parts needed to a triple S3 where he wrote this: Thermal Paste (do not glue MCPCB to pill!). Is this specific for the triple S3 build? Or is gluing MCPCBs to the pill a bad thing in general, even if I don’t ever want to remove the LED?
… but why would you never want to remove the LED? No hope for an upgrade? Or have you found the perfect emitter and you’re not letting the rest of us in on the secret? Inquiring minds wanna know!
AFAIK, if you glue up a single emitter, it’s tricky to get it aligned (emitter — reflector/optics — head — body) properly before the glue sets up. A triple (especially a tiny one like in an S3) might become impossible.
As for me, glue requires too much of a commitment to an LED that (I hope and pray) will be outmoded soon (not soon enough, most likely, or I’d wait).
The triple isn't glued because as the pill is screwed into the head, at some point the optics will bind against the bezel before the pill is fully seated. If it has no freedom to move you may snap off the support legs of the optic. According to my reading, I haven't built one yet.
Otherwise, the main advantage of adshesive is where there's no reflector or screws to hold the star to the reflector, e.g. an aspheric FL.
Well, the “never want to remove the LED” part of my question is theoretical. I understand that gluing is pretty silly if you want to replace the LED in the future, it’s the most obvious reason not to. My question is if there are reasons beyond that, say thermal conductive reasons or something. I have a few lights with cheap hosts where I have glued on the LED because there was no simple way of keeping it in firm contact with the pill, they kept popping out slightly. Besides aligning and permanent fixture, are there any reasons not to glue with something like Artic Alumina thermal adhesive epoxy?
But then it twists the wires. That can be addressed, but my bigger concern is that the mcpcb can lift off, and it will get air bubbles under it when you try to press it back down.
You can glue the LED on before fixing the driver into the pill, avoiding any push from wires while the glue sets.
I was responding to Gj.
Twisting isn't usually a problem for a couple of revolutions, depending on the specific case of course.
I don't see why the mcpcb should lift off in the short time between setting it and installation. There should be very little force trying to lift it. 
Mine lifted off when I changed the optic. When I build another one, I'll glue or solder the mcpcb. I'll lube the o-ring with water so the bezel can spin more easily, and clamp down the optic to further stop it from spinning while screwing on the bezel.
Thermal adhesive (glue) isn’t very permanent. You can pry the PCB off with a screw driver quite easily.
That depends on the adhesive and the quality of the bond. For example, poorly bonded silicone is very easy to remove. You can pry it off with a finger nail if you have to.
But tightly bonded Arctic Alumina is darn near impossible to remove. The only success I’ve had is to soak it in alcohol for a couple days and then try to pry it off. Even then, if I did a very good job mating the surfaces, an alcohol soak won’t do any good. Then I have to heat it until the epoxy melts, at which point, you can kiss the LED bye bye. Heat is a method of last resort for salvaging just the pill.
I also removed AA some times.
I have a contact spray which is made of alcohol with some other additives(it’s named kontakt chemie 60) this stuff loosens the AA, at least I think that it does it I let it soak then I try to twist the star with pliers. After removing the star I use a dremel to polish the rest of the glue away. It works but I don’t matter if the led breaks because I want to replace it anyway…
The biggest disadvantage with glue is that it is not so good in terms of heatsinking…so I try to use arctic silver under the star.
That sounds like a great solution except for the minor issue that it won't work. I've tried. Leaving the MCPCB free to rotate during assembly is the best option with the fewest opportunities for something to go wrong.
If you're worried about the wires twisting, give the MCPCB about a half turn in the opposite direction before installing the pill, so that the movement from tightening puts the wires back close to their original orientation. But even that's just not necessary. Unless the wires are too short, it will spin 2 or 3 full turns either direction before the wires bind, and the MCPCB only turns somewhere between 1/2 and 1 turn during assembly/disassembly. It's a non-issue.
Also a non-issue is disturbing the paste layer between MCPCB & pill. It doesn't leave any air bubbles or anything else.
To keep the twisting to a minimum, stick your finger onto the front of the TIR while unscrewing the pill. It will spin a little bit at first, then when the TIR clears the o-ring the pill & TIR can be removed without anything moving around.