dpayne

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.

absntr

First -

What crank are you running? A 109 is required by a Sugino 75. A Shimano 107 or a 110 (the driveside spindles are the same on both, the non is wacky - 107 is symmetrical).

What frame do you have? A Shimano 68 BB should fit into any English threaded frame. The BB should say which side is Right and which side is Left.

absntr

If you do want a perfect 109 BB, a Sugino 75 or a Dura Ace track hub will give you that.

dpayne

The crank is Sakae. I'm assuming it came stock on this 80's Bridgestone frame I'm converting. I had threaded an older Shimano BB in this frame without any problems, for the purpose of measuring chainline---but it had the fixed cup on the non-driveside. So-I'm not sure whether the driveside/non-driveside fixed cup has anything to do with the threading problem. I really don't want to change rear hub or crank if I can help it---I just want to find a bottom bracket that will work with my current set up.

dobber

The lower quality Shimano bottom brackets have the fixed threading on the left / non-drive side. The higher quality ones (5X, 7X, 9X) have them on the righthand side. The UN71's can be removed to make the poor mans Phil. And if you industrious with a grinder, you can make any Shimano BB adjustable.

The Shimano 107 will work nicely with the Sugino or Dura-Ace. As Absntr points out, the 110 has the same drive side length as the 107. This allows you to shim out the drive side 1 or 2 mm if necessary to get the desired chainline.

dpayne

Thanks for the info. So, how do I "shim out the driveside" on the un71? Also, what is the difference between un71, un72, and un73?

dobber

Shimano bottom brackets can be spaced out using........bottom bracket spacers

https://www.loosescrews.com/index.cgi...d=958586722377

I believe the typical rule is that you can move the cup out 1mm if the other cup is plastic/phenolic (current new ones like the 73 and 53), 2mm if its metal.