Have a couple of these which I think are common. I have one that will not come on. Swapped switch and no diff. Batteries are fit correctly and all threads front to back are tight. Suggestions? I am not sure how to test the drop in outside of the body as I don’t know where to touch the negative on it with bare battery and wire. Can someone guide me a bit on troubleshooting the business end?
Test it in the tube but use a wire to connect the circuit from the negative battery end to the edge of the tube. Wire, tips of pliers or even a plated Allen wrench could do it.
For a minute there I thought this was a thread about castration…
If there is a bad connection somewhere I would suspect the connection between the driver outer ground ring and the aluminum pill. Try prying the driver out of the pill then pressing it back in, and maybe wedge a bit of bare copper wire at the edge before you press it back in.
I was thinking of the "fix" humans do for a horse with a broken leg. 
Or, if possible, measure resistance between both elements. Yes, I thought the same when looking at that picture.
I think you have a driver/pill lack of ground issue. Pop the driver out, fold copper foil into a “T”, wedge it between the driver and the pill and lightly solder it to the ground ring on the driver and when assembled the pressure from the battery tube will push it against the pill since you cannot solder to aluminum.
Got nothing with drop in threaded into the tube and wire from battery neg to tube.
Is the pill the part with the spring? It is in there very solidly.
Popped that part out and ran wire direct to the edge of it…nothing.
Ya sorry, stupid title…
Ah I’m just giving you a hard time 
Can I bypass the driver and put battery power to the LED leads direct to see if it works?
So when I put my multimeter on the small bare metal patches next to the LED, in diode mode, pos to pos, neg to neg, I get no reading. Does this mean the LED is dead?
First, what voltage does your meter put out for its diode test? Many only do something like 1.5v or 2.0v which isn’t enough to light many leds. Better ones (for us) will put out 3.0v for the test. The led (or led mounted to the mcpcb) needs to be disconnected from the led wires and driver circuit, too. Did you try reversing the contacts to be sure? And if you can touch the actual led contacts rather than the board, that may work.
You can also try using a bare cell (or two 1.5v cells in series) just handheld with a couple of wires…then touch those to the contact points briefly to see if it lights up (briefly because this is direct drive so you don’t want to smoke the emitter). Can gently use a vise or trigger clamp or something to hold the wires on the cell, if that makes it less of a three-hand juggling act.
Thanks for taking the time to suggest. With 3v I couldn’t get it to respond whatsoever either with the two input points of the wires that come from the driver or by touching to the spots around the emitter….so seems to be kaput.