Here’s one way to get the LED set in the middle of the pill
Take a spare o-ring which has a thickness the matches the gap between the led and the edge of the pill.
Cut the o-ring to fit
Now you can use the o-ring to keep the LED nicely in the middle while the thermal adhesive sets
I’m interested in how others do this
Usually I will put it on there by eye and then put the reflector down over it to see if I’ve got it right.
I recently got an Ultrafire drop-in that came with a black plastic isolation disk. The disk has a 3.5mm x 3.5mm hole for the LED in it (for an XP-G) and the whole disk fits perfectly into the top of the drop-in, almost snapping place. I think that disk could be used to center the LED. Once the glue sets up you could use the disk somewhere else.
Last dropin I built used a 16mm star. No real need for centering..^^
I’ve always centered and focused by hand. Some lights I don’t dare take apart because I know I’ve spent hours getting the focus juuust right. I am curious about these though:
http://www.kaidomain.com/product/details.S020262
Anyone ever try ’em? The look similar to what’s in my HD2010 and my V8 drop-in.
I’ve used that trick already when centering 14mm stars. Works great! 
Usually I just eyeball it.
They work perfectly with typical C8 reflector. I was a little bit skeptic how would it fit in 7.xx mm hole and still around emitter but they do and these are very thin so no change in focus or assembly problems.
But even with these protectors corners of led emitter (with those often exposed tiny square positive and negative pads) are still just a fraction of mm away from reflector hole. It would be much better to have C8 reflectors with at least 8mm hole and corresponding led protector like this (like recent non c8 torches)
I’m just curious from what flashlight is that pill ? Look similar to this one: http://www.dealextreme.com/p/cree-q5-270-lumen-3-mode-led-flashlight-blue-1x18650-102524
The pill is out of a Romisen RC-N3
I’m putting in an XM-L LED and a KD V2 driver. The plan is to use it as a helmet light for mountain bike night rides; with a 1S2P 18650 battery pack
Neat trick, but assumes that your emitter is centred to the base, which isn’t always the case if the base cutting machine isnt perfectly lined up with the pads for the emitters, or if your emitter isn’t reflowed exactly in the middle 0:)
And combine that with reflectors that are threaded on the outside, so when tightened tend to shift to a particular side, while your pill shifts to a different side (thinks of D10s), then you need to intentionally shift your emitter to one side to compensate, a game of small adjustments and mass frustration. (earlier models that started to use the XP-E, but before they had the centring rings worked out in the XP-G models)
Exactly, that’s why I do mine by eye with the reflector installed making adjustments until it’s perfectly centered on the reflector not the pill.