I have a 6503 crankset that doesn't have a small chainring. It uses an octalink bottom bracket. I want to use it how it is now, with just the two larger chainrings, but I was wondering what size bottom bracket I should use.

The triple says it needs a 118.5 v1 spline, and the double is 109.5,

My question is whether the crank arms themselves are different, or if they're the same and the triple needs a longer BB because of the third chainring.

This triple crank has mounting holes for the third chainring that protrude a little, and could be cut off. I am hoping that with this done it will be compatible with the double sized BB and have the correct chainline.

Thoughts?

Chainline for the triple crank is 45mm, which is where the middle ring lies. Chainline for the double crank should be 43.5mm, which is where the point in between the two rings lies. Assuming that the chainring spacing (distance between middle of the two rings) is 6mm between the 2 rings on your triple, then you'd now have a chainlin of 48mm (45 + 3/2). You therefore need to move the crank in by 4.5mm (48 - 43.5). To achieve this, you would need a BB that is 9mm shorter (4.5 x 2), and so the 109.5mm (118.5 - 9) would be ideal. However, perform some of these measurements yourself and redo the calculations if necessary. I would not expect that you'd have any frame clearance issues with the rings or the inner mounting studs.

Wow, excellent answer. Thank you!