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How to Buy an Internal Bottom Bracket

So you've followed Step 1 of our how to buy a bottom bracket article, and you've determined you needed an internal bottom bracket. That's good, but there's still some work to be done!

Step 2: Selecting the type of bottom bracket interface

So you know you need an internal bottom bracket, but that doesn't mean you can use any internal bottom bracket; you need to select an internal bottom bracket that has the spindle interface required to install your crankset. Here are the most common types of spindle interfaces.

Square Taper JIS

JIS-bb.jpg

A very common bottom bracket crankset interface.

Commonly seen on older bikes and still available
today on less expensive cranksets.

 Octalink V1

 octalinkV1-bb.jpg

Version 1 of Shimano's Octalink interface.

Features 8 splines that are 5mm deep.

 Octalink V2

 octalinkV2-bb.jpg

Version 2 of Shimano's Octalink interface.

Features 8 splines that are 9mm deep.

 Power Spline

 power-spline-bb.jpg

Truvativ Power Spline interface is only compatible with Truvativ cranksets designed for Power Spline bottom brackets.

 ISIS

 isis-bb.jpg

Compatible with any crankset designed for ISIS bottom brackets.

Features 10 splines and a 21mm spindle.

Step 3: Selecting the bottom bracket shell width

Internal bottom brackets are designed to fit bottom bracket shells of specific widths. Some bottom brackets can fit multiple shell widths (for example a 68mm and a 73mm shell), while others are designed to only work with a single shell width. In order to select the proper bottom bracket, simply measure the width of your bottom bracket shell and select a compatible bottom bracket.

Note that bottom brackets compatible with E-type front derailleurs are differenciated by adding the letter 'E' after the bottom bracket shell width; 68E or 73E, for example. If you are using an E-type derailleur, you will need to select a compatible bottom bracket.

A note on bottom bracket thread types

There are two main types of threads on bottom brackets; English (aka British) threads, and Italian threads. Bottom bracket cups with English threads are tightened by rotating  them clockwise when standing on the drive side of the bike. Bottom bracket cups with Italian threads are tightened by rotating them counterclockwise when standing on the drive side of the bike. As you can see, English and Italian threads are opposite to one another; it is therefore important to match the thread type of the bottom bracket to the thread type of the shell.

One of the main problems with Italian bottom bracket threads is that the torque applied when pedaling is in the loosing direction of the bottom bracket - not good. The majority (probably upwards of 95%) of bikes have English threads. If you think your bike might have Italian threads, the easiest way to get confirmation is actually by measuring the bottom bracket shell. Italian threads are only on 70mm shell. If the shell is 70mm, threads are Italian. If the shell is not 70mm, the threads aren't Italian.

 

Step 4: Selecting the bottom bracket length

The final step is selecting the appropriate bottom bracket length.

Internal bottom brackets govern how far away the crankset will be from the frame centerline. The longer the bottom bracket length is, the further away from the bike centerline the crankset will be. In order to maintain a proper chainline, it's important to make sure the right bottom bracket length is used.

The length of the bottom bracket needed depends on the crankset design. If you are replacing your crankset, look at the technical documentation and see what bottom bracket length is required. If you aren't replacing your crankset, simply select the same bottom bracket length as was previously installed.

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