Sealed bearing UN55 BB. Left (fixed, non-drive side) screwed in just fine.

Right (drive side) just doesn't screw in, and I know it's a left hand (counterclockwise) screw.

The old hardware threads just fine. Threads not stripped.

Diameter and thread size measure OK. Item not mislabeled.

What the heck am I doing wrong??

Often there's slight axial misalignment in the threads. This isn't an issue (within reason) for traditional cop/cone BBs, but can be with a cartridge spanning both sides.

Simple diagnostic

Fire make sure the thread is OK, by screwing both sides in one at a time.

Then, with one side threaded in (if the cartridge is attached, the side with the cartridge) look from the other side and see if the end of the cartridge is centered in the hole. If not it's in a mis-aligned thread.

Often the fix is to thread both sides in together, so the deflection can be shared on both sides. Then as you near the end, get both sides as close to tight as possible, then full tighten the right side, and finish by tightening the left (if possible).

If one side is plastic and the other metal, reverse the sequence and full tighten the plastic side first. Also be sure the threads and all sliding surfaces are well greased, because you can end up with serious friction.

It also helps to screw the right side in first fully tight as the left side is tapered to tighten again the BB. It may be causing interference. Roger

The best solution is to have your LBS chase the bottom bracket shell's threads. Remember, on a decent frame, the bottom bracket should thread into the bottom bracket shell by hand. Having said that I can say that over the years I've had an occasional bit of trouble with Shimano's sealed bottom brackets. And if the non-drive cup is plastic, don't use it...get a metal one, and, as has been said, use lots of lube. Good luck!

Is the DS screwing in at all, or not even "catching a thread"?
If you can't get it to start, maybe the leading part of the thread got "dinged"?

I feel like a blithering idiot

Here's the scoop why I couldn't screw the sealed BB in:

There was interference from the screw that holds the cable guide on, that's underneath the BB shell.

Back when I had an open ball bearing set-up with a skinny little BB spindle having that screw stick down didn't matter. With a big fat UN55 sealed bearing unit, it did matter.

Backing the screw out: voila. BB screwed right on. That's about an hour of my life I'll never get back.

Chalk it up to a learning experience.

It's been said: You learn from your mistakes. But it's better if you learn from somebody else's mistakes.

I'll remove screw, grind it down, and reinsert.

Thanks for the help!

don't feel too bad. i think i may have done that a few years ago.

Been there....done that.....on multiple occasions.

might not even have to shorten the cable guide screw , the whole Cartridge is not a fat as the ends.