In this article I’m going to explain how to install a square taper bottom bracket. The process is pretty straightforward.
Let’s get started.
What’s a square taper bottom bracket?
The bottom bracket spindle—it is a spindle and not an axle since it “spins” on the bearings—left and right side has a simple box-shaped or square end.
This end is slightly tapered into a wedge shape, narrow at the end increasing in width. The crank is pushed onto the wedge once the bolts that fit into either end of the spindle apply enough pressure as you crank them to the appropriate torque.firstcomponents.com
The BB-UN300 square taper bottom bracket I bought was new. The non-drive side has an aluminium cup. There are also bottom bracket version with nylon cups.
What size and length bottom bracket do you need?
I initially bought a bottom bracket with the wrong spindle length. It was a 132mm width but I needed a 123mm. I don’t know why I bought the wrong one since I’d already thrown away the old one.
The diameter of the cup is usually 68mm. as was mine.
The best thing would obviously be to buy the exact same version you’re replacing. But since that proved to be impossible with the Thun bottom bracket version I replaced, I decided to go for a Shimano.
After having bought the wrong version I found a database on Sheldon Brown’s website that included the correct size that corresponded with the crankset you we’re using.
Which way to replace your square taper bottom bracket?
I constantly had to look up videos about which way to turn the cups of the bottom bracket for getting them into the shell until I found the following video:
The guy in the videos states that for installing:
For all intents and purposes almost all bottom brackets everyone goes to loosen forward! Both sides forward! You have to go forward in order to make some progress!BikeManForU
So naturally the opposite goes for installing a square taper bottom bracket: both sides have to go backwards!
Step 1. Applying grease before installation
Bottom brackets are notorious bicycle parts know for seizing into your frame. It’s no wonder I ruined the plastic bottom bracket cup.
A newly installed bottom bracket in a properly maintained bicycle that isn’t ridden in wet conditions too often probably won’t seize. That being said it’s a good thing to apply grease on the threads to prevent it from happening again.
And installing an aluminium cup into a steel frame increases the risk of calvanic corrosion.
The most common examples of galvanic corrosion of aluminum alloys are when they are joined to steel or copper and exposed to a wet saline environment.pfonline.com
Step 2. Installing the bottom bracket
As said before, installing the bottom bracket is achieved by rotating both cups backwards. That’s counterclockwise for the drive side and clockwise for the non-drive side.
You start with the drive side first. I use a Parktool BBT-32 bottom bracket tool which fits into the splines of the bottom bracket. I use an adjustable wrench to tighten it.
Make sure the cup sits flush against the frame to prevent contaminants and/or water from getting onto the threads.
Next up is the non-drive side. The non-drive side cup does not have a flange so it can always go in as far as it should to tighten against the bottom bracket.
Since frame sizes are standardized (in this case 68mm. width), it should fit exactly inside the bottom bracket shell.
And that’s how you install a square taper bottom bracket! If you like this article give it a thumbs up. If you have any questions or suggestions for new articles, let me know in the comments below. Cheers.