Square taper bottom brackets

[Garth]

Are the square tapers all the same size?

 

[fossyant]

Sort of - just different shell sizes (68mm road and 73mm MTB) and different axel lengths depending upon chainset. Remove it to find out length of axel.

 

[dan_bo]

But don't expect a shimano crank to sit well on a campag BB-or is it the other way round?

 

[MartinC]

The short answer is no. The longer one is a long explanation of nearly. There are 2 standards - JIS and ISO. Most are JIS but Campag (and their lookalikes - e.g.Ambrosio, Miche, Neco) and some Stronglight ones are ISO.

 

[fossyant]

Ah flippin Italians......

 

[Garth]

Thanks guys, I know the lengths are different, it's the size of the square I meant. But that's answered my question.

 

[andrew_s]

ISO and JIS are both a 2deg taper, but an ISO is marginally smaller at the end than JIS.
The end result is that if you put an ISO crank on a JIS BB it doesn't go on as far as it should, and a JIS crank on an ISO BB goes on too far. The difference is about 3 or 4mm or so on what the chainline should be.
Problems mostly occur with JIS crank on ISO BB, where going on too far means the crank bolt gets tight on the end of the axle before the crank itself is tight on the taper. Tolerances are such that it's sometimes OK, and sometimes it isn't.

( I hope I've got the difference the right way round )

 

[NickM]

( I hope I've got the difference the right way round )

You have

JIS cranks sit ~4.5mm further onto an ISO axle than onto a JIS axle.