Singlespeed & Fixed Gear - impossible chainline

I'm converting my old steel Specialized MTB to singlespeed. I have a Surly singlespeed MTB rear wheel. I have Primo BMX cranks. I figured I would measure the chainline error and then buy a BB that would align them.

I figure I need a 148mm square taper BB to make the chainline perfect. I don't think such a thing exists. The longest Shimano BB I found was 127mm That would leave me over 1 cm offset between the chainring and freewheel.

Looks like I bought a crank and wheel which are not compatible in terms of chainline. Ooops. :(

Options:

1. Mount the chainwheel on the outside of the crank arm spider. Anyone do this? It fits better on the inside. There is a machined ledge there for it. The outside of the spider has countersunk holes that help allign the bolts. This option alone doesn't solve the problem, but it gets me closer.

2. Use a rear wheel with a narrower hub. This probably means a custom built wheel. Save the Surly wheel for another project.

3. Buy a set of MTB cranks. Save the Primo's for another project.

4. Get the longest BB I can find and don't worry about the allignment error.

What whould you do?

my conversion is 1cm more or less off. i ran it, i threw the chain once, and then made the chain really ****ing tight and it hasn't given me problems since. of course "just ****ing doing it" probably isnt the best solution, but if you must. you can probably also find chainring bolts that are a bit spaced out to make up a few mms difference.

...um, I think your math is off...

I measured from the center of the bottom tube to the end of the BB on the right side. That is 61mm. So, if the BB were symetrical it would be 122 wide. It isn't symetrical because the original crankset was a POS with chainrings rivetted to the right crank. I'm shooting for a 52mm chainline to match my Surly singlespeed MTB rear hub.

I put the right crank and chainring on the existing BB. It measured 39mm from center of bottom tube to center of chainring. I want to move the chainline from 39mm to 52mm. That is a 13mm increase. The right half of the BB would grow from 61 to 74. To make the left and right the same I need 74 x 2 = 148mm BB width.

I re-dished the rear wheel to get a better chainline on my MTB.

redishing doesn't change your chainline. you can try to respace the hub by adding or removing washers on the drive side to get correct chainline and then you would have to redish to get the rim centered.

Get cheap MTB cranks. The Surly has 52 cm chainline designed to line up with the big ring on MTB cranks, so it will, with a 118ish BB Just get shorter bolts and losre two chainrings.