It is a Shimano BB UN 73 square taper with a plastic non drive side ring/cup. At first install on new frame, the plastic seemed warped and would not go all the way in leaving a couple of mms on both drive and non-drive sides. I installed it on my road bike and it looked fine so it is *most likely* not warped and a possible problem with the new frame.

What would be better/cheaper between getting the threads tapped and cut deeper on the new frame or just getting a spacer, like those SS/fixie folk use to achieve a perfect chainline, to just cover the gap? I am leaning towards the spacer since that probably is cheaper and my girlfriend might not want to spend any more money for her bike so I would pay for it. Or could I just leave it. The gap isn't huge, but there are some threads exposed on either side and to me, that isn't a good way to set up a bike - operable, but just sloppy.

Thanks for any input.

Check the shell width first. The cups are supposed to be tight against both sides of the cartridge. The drive side is supposed to be screwed in all the way but exposed threads on the non-drive side don't matter much as long as the BB is properly supported.There are different types/sizes of non-drive side cup so you might be able to find one at your LBS that would fit better (eg from a different model BB or one meant for a 68mm vs 73mm shell, etc).

Shell width is correct, checked be measuring and numbers listed (68mm) - should have said so before.

I suppose I could let the threads expose only on the non-drive side. Did not think that was optimal.

Is there a screw or plastic thing protruding into the bb shell? (To hold bb guide on for example). And the drive side cup should be completely flush against the frame, the left cup may or may not be

Are the BB shell threads bare metal or are they painted?

Paint can clog the threads but you can clean them out with a brass brush.

No screw or plastic things and the threads seem clean, but it is a cheaper, new frame so it may need prepping. rmfnla, the paint thing seems intriguing, I'll look into it more.

Make sure you torque the plastic cup to spec and not anymore - they crack easily.

I think it's actually designed-in that the loose cup does not thread all of the way in. Otherwise, the bottom bracket bearing shell might not be properly supported inside of the frame. Ideally, the loose cup not only supports the outer diameter of the bearing shell, but also clamps tight against the end of the bearing shell. If the loose cup clamped tight against the frame, it would take very tight tolerances to insure that it was also clamped tight against the bottom bracket. All of the bikes I own have exposed threads on the loose cup side (drive side tightened up against frame).

One tip for installing the plastic cup: I would not attempt to torque that cup to the spec that Shimano gives. I don't even use a torque wrench on the plastic side. What I've done is to torque the fixed side to spec then unscrew it about half a turn. I then install the plastic side and tighten it until snug then retorque the fixed side. This clamps the bottom bracket tight against the plastic cup but without having to stress the plastic side very much. Seems to work for me.