How are bottom brackets measured?

[swee'pea99]

On one like this, where it's described as 68mm x 113mm, what's 68mm? And I presume 113 is the width of the BB, but is that precisely from one external 'face' to t'other? Or what? How many different kinds are there? (I'm looking for something to fit an old British frame - is there a 'standard' it's likely to be?)

 

[Steve Austin]

Most bottom bracket shell widths are either 68 or 73 (thats the bit of the frame that it screws into.
Axle lengths vary from about 103 up to 120+ (thats the long middle axle bit, that the cranks go onto)

Lots of standards, and you mostly need the BB that the crankset needs eg it will be 68x whatever axle length the crankset needs.

 

[swee'pea99]

Brilliant - thanks Steve.