For the many Maglite fans:
Someone here recommended Burndy Penetrox A for aluminum-to-aluminum and aluminum-to-copper threaded electrical connections.
I picked up some yesterday, tried it on the tailcap of my 2D Maglite, which is old enough that the anodization has worn off of the threads.
WOW! The tailcap now screws on very smoothly.
I found the Penetrox A (catalog number P8A) at a large electrical supply house -- for half the price of buying it online.
BTW, Gunner, I'm making progress on the Luxeonated MX991\U. I think I have a scheme for driving a 3W Lux from 2 D cells, full output down to 0.9 volts per cell. A 1-watt would be more "practical" because of longer battery life, but I think I'm gonna go 3 watts on this one just for fun.