I've practically memorized Sheldon Brown's chainline article and have searched and searched through the forum archives, but am still confused about finding the right BB to get the chainline I need for my FG conversion.
After doing all the math, I figured out that I needed a BB with spindle length of 109 mm (68 English/ISO) to get that elusive, perfect 42 mm chainline in the front. I'm running an IRO fixed/free rear hub with an EAI 15t 3/32" track cog. Rear chainline measures out at almost exactly 42 mm, as far as I can tell.
So, I go to the LBS, and they don't have a 109, but they do have a 110 in the Shimano un73. So, I get it back home take it out of the box, and realize that 1) the left/right orientation is the opposite of the sealed BBs I'm used to seeing---that is, the fixed cup was on the right and the other cup on the left---and that 2) the driveside spindle end is much shorter than the non-driveside spindle end. The measurements I had made were based on the driveside and non-driveside spindle ends being of equal length. Actually, I've determined that what I REALLY need is a measurement of about 25.5 mm on the driveside spindle end (the portion of the spindle that protrudes past the fully tightened cup on the driveside)
I decide to go ahead and try to thread it in anyway, just to see how it worked. I could not get it to thread in properly, either on the right or left side.
SO, my questions are:
1) Are certain BB models made to thread into the driveside and some in the non-driveside?
2) If so, which of the Shimano models will work on my bike (possibly the un72?---what's the difference in these models?)?
3) Given that the driveside and non-driveside spindle end lengths apparently vary, how do I find a BB that will give me a driveside spindle end length of 25.5?
I would greatly appreciate any help. I'm so close to completing my fixie build, and this last damn detail is just taunting me and making me insane.