Okay, I was just installing a Shimano UN53 on my Bridgestone. It all came out fine with the help of liberal amounts of grease, but there's a little bit of threading showing on non-drive side, maybe a mm or less. This is supposed to be there for placement of a lockring, right? Or am I mistaken and there should be no threading showing? It's getting to be a pain in the ass to keep taking out the bottom bracket, regreasing everything and then putting it back in all because the left cup doesn't quite want to go all the way in.

Sounds like you just need the lock ring.

Check out the Park Tools adjustable bb guide here:

https://www.parktool.com/repair/readhowto.asp?id=93

Okay, sweet. I was getting contradicting opinions from the people on the phone from my LBS so it got confusing.

no no no

UN53 = cartridge bearings, no lock ring needed. The left side doesn't really do anything but stabilize things a little bit and keep goo out, it's plastic for chrissakes. Just screw it in hand tight and give it a little nudge past that, maybe 1/8 turn. Doesn't really matter how many threads show, generally like two, but if I remember right and the BB I'm looking at right now is a 53 the thing has a bit of a ridge on it and you would never get a (useless) lock ring on there anyway.

UN53 BB (68mm road version):

Aight, cool. Glad to know it doesn't make a difference that there's a tiny amount of threading still visible. One of the people on the phone at the LBS was talking about using a cup and how it'd be "sub-par", but I wasn't really hearing what she was trying to say.

By the way turns out mines a UN54 and doesn't have that threadless ridge that the one you showed above does, the thread goes all the way to the end except there's possible a hair width length where they end. And yes there is 2 threads visible...

Yeah, don't torque the left side down too hard or it might crack. Worst thing that can happen is it backs out a little and nudges up against the crank arm, you probably wouldn't notice for like a week. I wouldn't take it apart to clean it or anything, but I don't grease the plastic ones, I'd rather it just gall in there. It's plastic, it's not going to get stuck and if it does it can just sit there and make friends with my next cartridge BB for all I care, it's not like it's a wear item. Probably the least worn item on the bike, actually.

Some threading showing is fine.