What do these bottom bracket measurements mean?

[Runes]

I need a bottom bracket for Octalink cranks. Like this one:

http://www.chainreactioncycles.com/shimano-es300-o...

I know that the first measurement (68 and 73) refers to the width of the bottom bracket shell, but what does the second measurement mean, and how do I know which one I need?

[CoolC]

Second number is the length of the axle.

[Runes]

Thanks

How do I know what length I need? (I'm not replacing like-for-like.)

[CoolC]

Good question.

What are you changing from and to? There may be a comparison chart somewhere on the net.

[Runes]

CoolC said:

Good question.

What are you changing from and to? There may be a comparison chart somewhere on the net.

The frame I'm putting it in most recently had a Hollowtech II crank and BB. I now want to put this FC-M521 Hollowtech I (Octalink) crank on it. It doesn't appear to be on this Sheldon Brown page.

[CoolC]

There's a note on page 10 of this to say use 121 on the M521 but with a 1.8mm spacer.

http://productinfo.shimano.com/download?path=pdfs/...

TBH changing from one crank and BB to another can turn into a right royal pita trying to work out what works with what.

[Runes]

Aha, that's great, thanks. I'll give it a go and see if I can get it working.