Hi. New to bike building. Actually I’m just rebuilding a bike using the original parts for the first time. Just new handlebars and and shifters and saddle eventually. Some other things need replacing though like the bottom bracket for example. I want to make sure I get the right size bottom bracket. There are so many numbers to consider while window shopping for square taper bottom brackets online. I definitely do not understand. I might find a bottom bracket with matching size numbers online, but it will often be accompanied with other size options that aren’t engraved on my original bottom bracket. I don’t know what I’m supposed to measure. I feel like this is probably something that is really easy to get right, but I’m not getting it unfortunately. Hoping to get some help please and thank you. Here are some pictures of my original bottom bracket.
by Glum_Natural_1408
2 Comments
The “BC 1.37 x 24” on your BB indicates that it’s a standard BSA (sometimes called ‘English’) bottom bracket. “BC” and “BSA” are interchangeable in this context – BC is the standard, BSA is the company that introduced it. It has a diameter of 1.37″ and 24 threads per inch – but all you need to know is that it’s a ‘BSA bottom bracket’.
The “73” is the width of the bottom bracket shell. 68mm is standard on road bikes, 73mm on mountain bikes. This is clearly a 73mm.
The only piece of missing information is your spindle length – the length of the axle from end-to-end. This could be 110mm, 115mm etc – just measure yours.
So you’re looking for a “square taper 73mm BSA bottom bracket” with your measured spindle length. So something like a Shimano UN300 will work (which are available with a spindle length from 110mm up).
When you measure the spindle, go from the flat end of one square taper to the flat end of the other, not the whole thing including the threaded section. That’s the number that matters. Shimano UN300 is dirt cheap and works fine for a rebuild, just match your measured length and you’re sorted.