[Live_Wire]

I believe that I need a 50mm chainline on my bike from measuring. How do I know if a certain Bottom Bracket and a certain crank will give me that desired chainline?

 

[Homebrew]

I believe that I need a 50mm chainline on my bike from measuring. How do I know if a certain Bottom Bracket and a certain crank will give me that desired chainline?

Depends on the crank. ISIS and XT splined, you need a 118mm for 50mm. Some bb like RF have adjustable chainline.

 

[Speedub.Nate]

I believe that I need a 50mm chainline on my bike from measuring. How do I know if a certain Bottom Bracket and a certain crank will give me that desired chainline?

Just to add to Homebrew's post, ISIS, Octalink and the new outboard bearing cranks are pretty well "fixed" based on what spindles and/or spacers you use. Most of the manufacturers will have install instructions online that make mention of this. 118 will get you 50mm on Octalink & ISIS, with the exception of Shimano 572 LX, which require 126mm.

Square taper stuff can be a bit of a crap shoot, especially if you're dealing with a used crank. For instance a Sugino crank sits deep on a spindle than a Shimano, so a 113 with one might get you the same as a 110 with another. Race Face square goes the opposite direction, so a 108 might be required to accomplish what a Shimano 110 does. For square taper, a little trial and error may be in order, and an adjustable cup bottom bracket or an understanding, well stocked bike shop are a big help.