Because my posts are always long winded, I'll start with the conclusion:
Good news - the Argo HP can be modded to accept a 18650 Li-Ion cell and still run it at full brighness.
Bad news - the mod is pretty much irreversible. I don't see CR123 cells going back in after all the material that has been removed. (Although one might be able make a spacer to enlarge the 16mm CR123 cells diameter to 18mm)
Long winded part:
Start by completely disassembling the Argo HP. The positve battery cap is a snap fit, not threaded. It's on pretty tight, but using a pair of padded jaw pliers with a "twist and pull" motion will remove it without damage. Remove the bezel, lens and two screws that secure the LED module. The module is also a little bit of a tight fit. Just use a small bladed screwdriver and carefully work it out along the circumference, "coaxing" it to the front.
Now the ugly part. I was hoping the battery contacts would easily slide out of their retaining rails, but no such luck. The contacts have a pair of "teeth" on them that pretty much ensures once they are installed, they stay that way. Another problem is that the contacts aren't very thick, so they became a little bent up during removal. I had to reshape them and grind down the rough spots. If anyone discovers a neater way to remove them, please share.
Once you are down to the shell of the body, you can start boring it out. It took me about 90 minutes of filing with a rat ******* file to remove enough material for the 18650 cell to fit. (It may not have been a rat ******* file, but I like to say "rat *******") (place rim shot here) (OK - I stole that line from another post, but you still laughed, right?)
Because there are no longer any guides to hold the battery contacts, you have to rely on the 18650 cell to keep them in place. So you want the final fit to be snug. (But not so snug the positve contact cuts through the cell's insulation. Talk about a disaster! Wish I had a protected cell for this project, but I don't). If I end up with a short, I'll be kissing my Argo goodbye.
When assembling the light, start by placing the positive contact on the cell and installing from the positve side. Install the positive end cap. Now carefully slide in the negative contact strip. Be sure leave a little bit of the negative contact strip sticking out beyond the threads so it makes contact with the battery cap. Before installing the PCB/LED module, measure the
voltage on the contacts, verifying 3.7V is present.
As far as charging goes, I'm leaving the cell installed and using alligator clips. I don't think it's worth the risk of monkeying around with removing and reseating the contacts to do a charge.
Here's some pics:
ELM