A few weeks ago, after some flashlight trading and swapping, I came short one 18650 battery. When fiddling around with two flattop 18350 batteries to replace the 18650 I took out of my SureFire 6PD, I accidentally shorted the tailswitch. It did not die totally but the switching action was rather unpredictable. After some careful destruction I found out that a coiled spring, half the diameter of a ballpoint pen spring, had melted like a blown fuse. The most common SF tailcap is the Z41, the standard LockOut TailCap (momentary push button pressure switch, rotate the tailcap for constant-on). Not available any more, but a Z41 tailcap upgrade can be found all over the internet for about $15. My SF 6PD has a Z59, the compact clickie tailcap (momentary push button pressure switch, push button hard to click constant on/off). It is a bit taller and is what we call a forward clickie. If you are looking for a real Z59 you can buy new old stock or refurbished, but only after you’ve sold some of your organs. On the other hand: I certainly was not planning to buy 3 pieces of $hxx for $5 from AE.
The tailcap combo of my SF consists of two metal parts. They hold a white piece of plastic with the actual spring mechanism in it. This piece is nowhere to be found on the internet. 
I noticed that the interior of the metal parts had some likeness with that of lights like the well known S2. My worries would be over if I could find a ring of the right size to keep a common switch in place (find as in: not having a lathe). So I *oogled for “DIY Adapter Brass Driver Pillar”. And found a brass retaining ring of 22mm*3mm on Kaidomain. And as bonus I picked up a short brass pill for an S2.LINK
That pill is out of stock on FastTech and elsewhere.
After a month my order arrived and I saw that the diameter of the KD ring was correct, but the pitch was not. But I noticed that the threads on the also arrived S2 pill were much more like those of the SF. When I tried it went in all the way, into my empty tailcap. Now all I had to do was figuring out what part of the pill could be used to fixate and center a switch board in that tailcap. Being a “think twice before doing once” type of person I first made some kind of construction drawing. 
Having done that, it was just a matter of removing enough material from the pill to create the retaining ring I wanted. To be precise: 3.8mm from the side with the shelf and 1.7mm from the side with the retaining ring.
The left side of the drawing shows the BEFORE situation, the right side shows the AFTER situation.
Then it was just a matter of putting in the usual tail cap items from my spare parts box. I already had a SF rubber tailcap cover. But I asked myself: if an ordinary 1288 switch on a PCB fits, how about a lighted switch? It did! So the last picture shows (in order of assembly) the parts that saved my SureFire 6PD. 
Comments:
- The translucent tailcap cover is 14mm and has to be forced in gently, by repeatedly pushing back the parts that pop out again till they get tired of it and stay in.
- The GITD ring replaces a thin rubber ring and is added to gain a bit of extra travel for the tailcap threads.
- The momentary action of the old tailcap is no longer with us. Now it is just a click-on click-off switch.
- My drop-in is the EagTac (now Sportac) single mode XM-L2 U2. IDK if the lighted tail switch works with other drop-in’s.
Afterword: not only is my 6PD usable again, it also has a lighted tail cap now.
Therefore it faces promotion to become my nightstand light.
