There are a lot of bottom bracket standards right now on the market, so we won't blame you if it's becoming a bit confusing. On top of the many standards available, the fact that the bottom bracket selected must be compatible with the frame (ie. the bottom bracket shell) and the crankset is not making things any easier. But fear not - we've hopefully put together a series of short articles that will have you selecting your new bottom bracket in no time.
Note that we will be only focusing on sealed cartridge type bottom brackets. The reason for this is that all modern bikes are spec'd with this type of bottom bracket, and older bikes can (and should) be upgraded to cartridge bottom brackets at their next replacement.
Internal, external and press-fit bottom brackets
There are three categories of bottom brackets; internal, external and press fit bottom brackets.
Internal bottom brackets thread in the bottom bracket shell, and their bearings reside inside the shell, hence the term internal. There are internal bottom brackets designed to fit 68, 70, 73, 83 and 100mm threaded bottom bracket shell widths.
External bottom brackets also thread in the bottom bracket shell, however their bearings reside outside the shell, hence the term external. There are external bottom brackets designed to fit 68, 70, 73, 83 and 100mm threaded bottom bracket shell widths.
Press-fit bottom brackets are press fitted (not threaded) in the bottom bracket shell. There are press-fit bottom brackets designed to fit 68, 73, 79, 86.5, 90.5, 91.5 and 95.5mm threaded bottom bracket shell widths.
Notice that the bottom bracket shells for internal or external bottom brackets are exactly the same. Namely, they are threaded shells of standard widths. Therefore, internal and external bottom brackets are interchangeable and either can be used (with the appropriate crankset).
Step 1: Determining what type of bottom bracket is needed
You can determine what type of bottom bracket you need simply by comparing the pictures above to your bike. This way, you will be able to tell if your bike currently has an internal, external or press-fit bottom bracket.
A note on threaded bottom bracket shells
If you are only planning on replacing the bottom bracket and keeping your current crankset, then you will need to select a bottom bracket of the same type as currently installed. However, if you are replacing both the bottom bracket and crankset, you can choose between a bottom bracket of internal or external type and a compatible crankset. As a matter of fact, if your bike has a threaded bottom bracket shell, both internal or external bottom brackets can be installed as long as a compatible crankset is used.