How do I measure a bottom bracket? I need to replace the bottom bracket bearing on my fixie. I plan to get a Shimano UN26, but I note that there are a number of different sizes. The sizes seem to be expressed as 68-110, where 68 is the shell width and the second number varies according to the table below, from Shimano's tech specs.



I assume the second number relates to the width somehow.

How do I measure my existing bearing? Is it length from end to end of the axle? Given that the only tool I have for this is a ruler (no calipers) what's the best way of measuring my existing bottom bracket to ensure I get the right size?

Update



I removed the bottom bracket and it had "103" stamped on the axle. I also measured and it was exactly 103mm from end to end. So it looks like the measure is the length. I went to my LBS and they assured me a 107mm (coincidently the shortest one they carried) would be fine. I installed it and my chainline is marginally out (I guess by 2mm). So I'm going to try to find a 103mm from somewhere else.

Thanks!
The numbers 113, 115, etc. mean the total axle length in milimeters.

On a singlespeed (fixie, internal-geared-hub), the important measure is how much the right side of the axle "shows out" (not always perfectly symmetric), and how much the crank can "sink" into the tapered square, which may vary from model to model even with the same nominal spindle length, and with how much torque you apply to the bolt when installing.

I have two single-speeds (fixie and IGH), which required some "fine tuning" to get a straight chainline:


One had a 73mm bottombracket tube on the frame, so I had to use a slightly shorter spindle, because it is installed off-center (2.5mm to the right because of extra "frame width" on the right side, the nut on the other side didn't have a lip and could be inserted fully inside the bb thread);
Other was regular 68mm, but two models with same spindle length had different offsets on the crank (because of taper, lip width of the shell, and some other obscure factors) and I had to install a cheaper model (generic instead of Shimano), it was the only one that fitted, and I'd rather have an aligned chain than a slightly better bottombracket.