Old cups came out just fine.

New Shimano BB doesnt want to thread in on the right side. It gets REALLY tight after the first few turns. L holder screws in just fine.

Bad part?

-SP

Get something like an old tooth brush (metal bristles would be better)
Apply a bit of penetrating oil or similar to the threads and "scrub" that side.
Use the metal cup and run it in & back a few times.
Repeat with the brush and wipe with a rag.

+1
Make sure to move the brush circumferentially, that is along the threads not across them. Also inspect the cartridge threads carefully for any small flaw.

Both new and old parts are clean and free of damage.

....but I would have done the same things you guys suggested too!

I'm going to try another old BB and see if it screws in correctly.

-SP

What are you using to install the bracket? A socket tool with a long handle? Or something shorter than that?

Unless you ordered the wrong part or it's defective, I'm inclined to think that the binding is nothing more than new parts into old threading.

Apply grease to the threads inside the BB shell and leave the BB mech threads clean.

Was the old BB a loose-ball unit with steel cups? Because of the way they were adjusted, they could be machined to turn pretty easily when threading them in. The newer cartridge units with aluminum cups will fit tighter because the aluminum is expected to deform a little on installation, and all of the threads need to support the torque. Better that they start out a little too snug than too loose.

That said, if it doesn't feel right going in, some caution is a good thing.

Another possible issue is interference from the bolt that secures many BB cable guide onto the shell's underside. The bolt can protrude inside the shell and contact some BB units but not others. NE Ohio has many shops which can chase (and I suggest facing too if chasing is done) the shell to insure good thread conditions. And these same shops, hopefully, have the skill to examine the threads before blindly running a chaser through to see if there's any prior issues. A empty shell chasing often costs around $25. Priceless to keep from causing more thread damage from over enthusiastic leverage. Andy.

One should probably ask the question as to-
WHAT BIKE?
Possibly you have a bike with one of the "other" thread "standards" such as Italian, Swiss or French, where we are assuming you have British threading?

https://sheldonbrown.com/harris/botto...ets.html#sizes