the Klean Kanteen website says the mouth is 54mm (2.125in) wide. I can't find the specs for Nalgene anywhere and don't have one available to measure. Can anybody tell me how they compare? It looks a little smaller in pictures.
Also, can anybody attest to whether or not they fit in equipment pouches built for Nalgene bottles? They look a little taller, and they have a slightly higher capacity.
I think the mouth is the same. I need to double-check my current day-pack rig, but I'm 99.99% certain I'm running a matched set of these
SmarTube drink-tube adaptors with the same widemouth lids on both a Klean Kanteen and a Nalgene. There may be a difference in the way the companies "spec" the lid, because Nalgene calls their widemouth bottles a 63mm cap. However there are multiple
websites were you can see people reporting that KK, Nalgene, and 3rd-party (capCAP, etc.) tops all interchange.
I like to carry at least one stainless drink bottle because it doubles as a way to boil water in emergencies. Yes, the taller stainless bottles generally fit into the same spaces Nalgenes fit into, but because they're taller, they aren't necessarily as secure unless the pocket is deeper than normal.
My last two hiking rigs have been Mountainsmith Tours. The pre-2012 version had pockets that were barely tall enough to hold the taller bottle, but had an easily adjustable tensioner so you could cinch it after inserting the bottle, even while on your back. The 2012 revised pack had slightly taller pockets, which seems good, but the adjuster was changed and tall bottles interfere slightly with the load straps now. Overall, the bottle is held more securely, but its such that you can't reach back and easily grab the bottle anymore, hence I added the hydration tube adapters so now I can drink from the bite valves without pulling the bottle.
(Not my pack, but you can see Nalgenes in 2012+ pockets here)