Lets take these 2 cranks and matching bb.

Sugino RD - 103mm BB spindle

Sugino 75 - 109mm BB spindle

According to what I have googled, these both produce a 42mm chainline. How? Wouldn't the Sugino 75 stick out 3mm more on each side?

Or is it just based on how the cranks themselves are formed and how they fit into the bottom bracket, and spindle length means very little?

Want to swap the cranks on my steamroller. The current bottom bracket is 103mm spindle, so a bit confused on what cranks I can use.

Thanks.

They are of two different kinds of spindle, RD one is JIS and the 75 is ISO. Sheldon's got an article about their interchangeability.

You need to change your spindle length according to what the new crankset need.

Ah makes sense. Had a feeling it was the way they fit together that made the final size.

Thanks.