2

Do I need to be concerned with the difference between a:

and

apart from the difference in length of 2.5 mm (120 vs 122.5)?

3

No, they are basically identical. neither of them will have fabulous life expectancy.

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  • that's ok, it won't need to have! :) – ChrisW Nov 21 at 21:23
2

There are some secondary considerations:

  1. spindle length should match what your crankset is designed for, as this helps determine the "chainline" offset ( https://www.sheldonbrown.com/chainline.html ), the drivetrain's offset from the centerline of the wheels. The difference between the two you list will probably not be noticeable.
  2. The threaded "cups" are what attach the BB to the frame. The steel are a bit heavier and rust more easily. The alloy are lighter and usually found on the somewhat better models, so their presence may indicate better quality or materials in other parts of the BB as well. Whichever you choose, when you install it's very important to properly lubricate the threads with a good grease (or even anti-seize compound if the cup metal is different from the frame metal - i.e. alloy vs steel).
  3. A BB takes a lot of punishment, yet square taper ones are quite cheap. Often you can get substantial quality, material and longevity improvements by moving up from a $10 BB to a $20 BB, but you need to research the manufacturer and construction details and talk to someone with experience.
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  • Thanks :) this all tallies with what I'd picked up, so it's good to have it confirmed. The 122.5mm one was recommended to me by a very experienced friend (having measured my current BB), I was just intrigued if there was any difference between the A suffix, apart from the length – ChrisW Nov 21 at 21:56
  • so, the one I've linked to is alloy, but still pretty cheap, judging by what you're saying. What difference would there be between a £10 alloy BB, and a £20 alloy one? – ChrisW Nov 23 at 21:32
  • @ChrisW Personally I usually go for Shimano, because in my experience even their low-end BBs have good build quality; their prices are usually somewhat higher than others. The types of differences I'm thinking about are machining tolerances, ball bearing quality, assembly quality control, things that are difficult to spot from specs or even if you disassemble the BB! Due to demand in virus times, prices overall are higher. Here's a discussion of differences between two of Shimano's square taper BBs: forum.cyclinguk.org/viewtopic.php?t=133628 – Armand Nov 24 at 20:29

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