I have had my Nano for years, and it works great.  Except for when it either gets loosened in my pocket and comes apart, or when it tightens in the pocket and runs the batteries down.  I've concluded that this happens because the O-ring that seals the head has aged and doesn't provide enough resistance to turning anymore.

Does anyone have an idea of what size O-ring I need to replace mine?  I can't find anything except batteries as "replacement parts" on Streamlight's site...

ACE Hardware is your friend.

For a free one call SureFire, but you'll have to wait @ 2 weeks or spend .25 at a HomeDepot, Lowes, etc

OK, so just "the right size" will work?  That's great to know.  I'll get cracking on measuring and then hit the hardware store.