Do you need to buy a replacement square taper bottom bracket for your (possibly somewhat older) bike, but you're not sure what spindle length you need? Here's a quick way to check. You just need a set of calipers and the correct allen key for removing your crank arm bolts. No need to actually remove the crank arms.
I'm assuming you've already determined the shell width you need, e.g. British threaded 68mm or Italian threaded 70mm, and you just need to know the axle length shown below.
If you've already gone through the trouble of using a crank puller to remove both cranks, then this is a no-brainer: just use your calipers to measure the axle length directly. No need to even remove the BB from the BB shell. However, if like me you have a more pedestrian toolkit of Allen keys, wrenches, and no crank puller, then try the following.
When the cranks are installed, the end of the axle is recessed slightly below the face of the crank arm, and the crank arm bolt (usually 5mm Allen key) is screwed into it. If you remove that bolt with your basic Allen key set, you're then looking at the end of the BB axle.
In the crude diagram below, where the crank arms are shown in purple, If you can measure the distance B between the two faces of the two crank arms, then the distances C and D from the face of each crank arm to the ends of the BB axle, you can subtract the latter two measurements to find the axle length A=B-C-D.
- Remove the crank arms bolts using an Allen key (probably 5mm or so).
- Place a hard flat surface like a small hard cover book against one crank arm face, then use the depth probe portion of the calipers (item 3 in the image below) through an opening in the chainrings to measure length B between the two caliper faces.
- Again use the depth probe to measure the two shorter distances C and D from each crank arm face to the end of the BB axle.
From the Wikipedia Caliper Page.
I recommend for each measurement, repeat three times and average the three measurements. When I tried this on my 56cm 2010 Specialized Tricross Sport, I got the following measurements in mm:
This gives a final BB axle length measurement of A = 139.93-11.13-11.93 = 116.87mm.
Looking at the options available for the Shimano BB-UN55 Square taper BB in shell widths 68mm, I see 107, 113, 115, 118, and 127mm. Of those, the closest to what I measured is 118mm. I later properly removed my cranks at a bike co-op and found that my BB is indeed a 68mm x 118mm.
I hope this helps you determine your BB axle length if you do not have a crank puller at hand. If you really want to be sure ahead of buying a new BB, see if your city has a bike co-op where you can swing by and use a crank puller to remove your cranks, allowing direct measurement of the axle length with calipers.
Thanks for reading!