The right hand side bottom bracket will not thread in by hand. It goes in a few turns but then I can’t turn any further. The old bb still goes in fine.

They are the same model etc.

any ideas?

Have you cleaned and greased the threads in the shell? Use a wire brush. New threads may be slightly tighter tolerance. What type BB?

Yes I did clean out with degreaser, toothbrush and isopropyl alcohol.

it is a Tiagra RS500 BB.

Put it in by hand as far as you can. Check it is square. If it is use a tool and slowly wind it In.
Keep checking it is square and that it doesn't tighten up more as you wind it in.
A tight thread is different from a wrong or broken thread. You can tell the difference if you are slow and carefull.

A wire brush is more effective than a toothbrush at clearing debris from threads. Even better is chasing the threads with a bottom bracket tap, but those are expensive. You can fabricate a functional thread chaser from a sacrificial bottom bracket cup by grinding debris channels across the threads with a Dremel tool:

Thanks - as you say I am ultra cautious about cross threading it.

How many times can you tap the bottom bracket threads?

That is something that has to be done very carefully. Retaps can do more damage than good.

I'm guessing that the cups are not removable or transferable.

If the threads are not actually damaged (as in only poorly cut from the factory or heavily clogged) then running in a proper piloted tap won't really remove much material. This is called chasing the threads and one can do this many times to a BB shell (if they know how to). However if there is actual damage then the taps will tend to remove the material where it shouldn't be and it's this removal that one has to be very careful about because it's a lot more involved to replace that material.

Thie situation is one that can go well or really bad dependent on the person doing the work. I don't think learning how to chase threads is best done long distance. If the cup won't thread in with mild wrench effort I suggest taking the frame to a shop that knows about this stuff and has the correct tools. Andy

Sound advice 👍

Is there a cable guide on the underside of the BB shell held by a screw? I've had situations where the screw is long enough to interfere with the BB "housing" and cause binding. Hopefully I have my rights and lefts correct.

Oops. I should have read more closely. Identical BB.

Is it the "pipe dope" that's on the threads of the new one? That will cause a snugger fit when new and help them to stay in place, but wears away after an installation/removal or two.

one side of the BB will go in clockwise and that the other side will go in counter clockwise.

a long shot but you never know.

if the bike shop chases the threads, they can reface the BB at the same time with the same tool which will lead to a smooth running system, all for about $24.95 .

Spot on - it was the ‘thread locker’ on the bottom bracket threads that were giving the extra resistance. I pushed through and used the tool to finish off.

Thanks for all your helpful comments 👍

Got to this thread a bit too late to comment; however it's a real joy to learn that the issue was resolved without any drama. Good riding, mate.

Chasing existing threads with a sharp tap shouldn't be a problem; all it will do is push any debris in the threads out of the way. If the threads have been damaged by e.g. cross-threading, dented shell, etc., shell material will be removed by the taps, and you can only remove so much material before it becomes a problem. English or metric thread shells with threads damaged to the point where cups are no longer secure can be reamed and tapped with fresh Italian threads, or a threadless cartridge can be used. Severely damaged Italian threads can also be addressed with a threadless cartridge.