Bottom bracket disassembly and installation - square hole and spline bottom bracket

We have learned some basic knowledge from previous articles about the bottom bracket and bottom bracket shell.  In this article, we focus on square holes and splined bottom brackets.

Square hole bottom bracket and splined bottom bracket

1. The difference between the square hole bottom bracket and the splined bottom bracket
The main difference between the two bottom brackets is that the way the bottom bracket is connected to the crankset and crank is different. The square hole bottom bracket only has 4 square surfaces in contact with the crank. The spline bottom bracket has 8 keys and 10 keys, so there are 8 or 10 contact points, but the installation and removal methods are almost the same.

2. Disassembly

The crankset must be removed before disassembling the bottom bracket to avoid hitting the chainring sprockets

(1) Shift the front derailleur to the large chainring to avoid hitting the chainring sprockets during disassembly.

(2) Use an Allen wrench to remove the crankset fixing screw in a counterclockwise direction. 

(3) Use the crank removal tool to screw into the crank screw hole and squeeze the center of the removal tool clockwise against the bottom bracket to separate the cranks.

Disassembling the crank

(4) Remove the crank arm downwards, taking care to avoid the chain pulling on the front derailleur. Disassemble the crank on the other side as described above

(5) Disassemble the bottom bracket. 

You need to pay attention to whether the bottom bracket shell is British thread or Italian thread. 

The threads on both sides of the British thread bottom bracket shell are opposite. The left side is a right-hand threading, tightening clockwise. The right side is a left-hand thread, tightening counterclockwise. 

The left and right sides of the Italian threaded bottom bracket shell are the same, so unscrew the bottom bracket counterclockwise. The square hole and splined bottom bracket are generally a cassette design, and the right side is the main shaft part including the bearing and the right bowl. The left bowl is used to support and lock during installation, so loosen the left bowl first when disassembling. 

① Use the bottom bracket shell tool to loosen the left bowl, being careful not to completely remove it first.

② Fix the bottom bracket tool on the bowl’s right side, loosen the right bowl clockwise, and then remove the left and right bowls.

Picture of the process of disassembling the bottom bracket

3. Installation

When installing, you need to distinguish the left and right sides of the bottom bracket. Generally, the larger side of the bottom bracket is on the right side, and the smaller side is on the left.

(1) Apply lubricating oil. Apply lubricating oil to the threads of the bottom bracket joint to avoid damaging the threads during installation.

Lubricating the bottom bracket

(2) Install the right side of the bottom bracket first, rotate it counterclockwise, tighten it slightly and then install the left bowl.

Installing the bottom bracket

(3) Use the special tool to screw the right bottom bracket until it is consistent with the plane of the bottom bracket shell, and then tighten the left bowl until the bottom bracket is locked.

(4) Hang the chain at the bottom bracket to prevent missing installation, and then install the crankset back to the bottom bracket. 

Make sure that the crank on the left side of the crankset is on the opposite extension line, and tighten the crankset screws with a hexagonal tool.

Thanks for watching. 

Don’t hesitate to leave a message if you have anything to share. We are looking forward to your idea and story. 




[1] ‘Cycling Bible’ Editorial Board. Cycling Bible [M]. 1st Edition. Beijing: Machinery Industry Press, 2013.

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