BB-UN26 (-K)- I am replacing the bottom bracket on my Specialized Sirrus and it accepts that type of bottom bracket. The problem is that one of the only models I can find for that have the plastic cups that have reviews saying that it cracks. Are there any compatible bottom brackets that would also work with that? My crankset is a Shimano FC-M391.
Here are some more stats I found about the crankset and bottom bracket combo online: Spindle length: LL 123, chain line- 50mm, shell width=68 / 73 mm (1.37 X 24 T.P.I.)



EDIT: I found a UN 55 bottom bracket with a 68x122.5 length and people in the reviews of the stock bottom bracket said that would work. Possibly?

Doesn't the "K" have a specific mounting for the FDER?

You need to match the current bottom bracket's spindle length so, if you can find a UN 55 that's the same spindle length as what you have, you're good. BTW, those plastic non-drive side cups are no problem unless you are too ham-handed with them. If you torque them too much they can break but they are not that fragile. That cup is just a spacer and stabilizer and the stresses on it are minimal.

To install these bottom brackets properly, install the cartridge end (usually the drive side) first and torque it to spec. Then thread in the plastic cup and tighten it firmly but no more. When you remove the bottom bracket, remove the cartridge side first. Done that way, the plastic cup will give no problems.

If you have difficulty finding a correct Shimano BB with a metal NDS cup, you can get a Problem Solvers "UP-CUP": Problem Solvers to replace the plastic one. Or if you currently have a metal cup you can likely re-use that. It is just an adapter which has no wearing parts.

Sorry for my lack of knowledge..I probably know what it is once you say it but what is a FDER?

Front Derailleur

Ah....How would a bottom bracket affect FD mounting, though? It is a clamp on type Acera FD

Some BB's have a mount for the FDER.
I DON'T KNOW if the "K" type is one of these, thus my question. I can't remember the designator for that type.
IF your BB is also the mounting for the FDER, you need the same type of BB.
If NOT, it's moot.

Ah...I saw some pictures online and it is definitely not. UN-55 with 123mm length it is.

FWIW it's the E designation that means a BB with space for a FD.

Never install a BB or crank or carbon parts without it.