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I'm swapping out a worn BB and crankset from a late 90s hardtail. I already have a new triple crankset that specs 122.5mm spindle length for proper chainline. However, the frame has a 73mm BB shell width, and these days a 73 x 122.5mm square taper BB is unavailable (sold out).

To maintain chainline, I'm going with a 68 x 120mm square taper BB instead. This will result in the non-drive-side being 2.5mm closer inboard, but I think I can live with that. Is there any drawback to this plan that I might be missing?

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The solution you are proposing isn't ideal but should work.

However, depending on the bottom bracket unit, the non drive side "lockring" might have a shoulder on it, preventing it from contacting the bottom bracket main part fully and therefore not supporting it as well as intended. Many types don't have a shoulder and this is a non-issue in that case, it will just disappear into the frame a bit (the threading generally allows this).

Otherwise you should be golden.

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  • Yes, good point about the nds "support"! – Armand Jun 9 at 13:04
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This might work assuming the lockring has enough support for the left side of the bottom bracket cartridge unit when used in wider than intended shell, but if you're swapping the crankset, it seems odd to use obsolete technology.

Square taper bottom brackets have a flaw, namely that the taper is too small. This causes the spindle to break.

Shimano attempted a bigger spindle with Octalink, but found there was not enough space for big bearings that thus failed quickly being small, and that the Octalink crank attachment has a loosening left crank bolt due to lacking press fit.

The solution finally was found with Hollowtech II: make a hollow big spindle integral to the cranks and move bearings outboard, thus making space for both big bearings and big spindle at the same time. Furthermore, Hollowtech II doesn't have loosening crank bolt problem like Octalink.

If buying both a new bottom bracket and a new crankset, I don't see any reason not to choose Hollowtech II.

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  • You're right about the better choice for a scratch build, but as noted I've already got the crankset (and I don't yet want to deal with facing the BB shell). – Armand Jun 9 at 18:12

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