If I’m fitting a square taper BB, 68mm shell, what width BB do I need? It’ll be for a 1×8 set up on a regular 135mm QR hub. I didn’t know they came in five Seven different widths!

Are you replacing an existing BB?

If so, you can measure the existing square taper BB as a starting point. You can then choose the one that adjusts your chainline to be in the middle of the cassette. If going smaller be sure the crank arms aren’t going to touch the chain stays.

What chainset? it dictates the BB required.

No, not replacing, it’s a new (old) build. Ill be using a Truvativ triple chain set, but just setting it up for single (middle) ring up front.