When it comes to bottom brackets, there are a few important things to consider. Firstly, there are three axle sizes to choose from: 19mm, 22mm and sometimes 24mm. In addition, there are four different types of bottom brackets: American, Euro (threaded), Mid and Spanish.

Each type of bottom bracket has a different outside diameter, with the US size being 51mm, Spanish 37mm, Mid 41.275mm (or 1 5/8 inches) and Euro threaded. For example, if your frame's bottom bracket measures 37mm and your axle measures 22mm, you will need a 22mm Spanish bottom bracket.

Bottom bracket Outer diameter
American 51mm
Spanish 37mm
Mid 41.275mm (1 5/8 inches)
Euro Threaded

When installing a bottom bracket, there are some important tips to keep in mind. Always use grease first to ensure good lubrication. Additionally, make sure the bearings go straight in and don't forget the spacer between them. For press-fit bottom brackets, a rubber mallet can be used, but be sure to place a piece of wood between the hammer and the bearing to prevent damage.

Finally, if you are working with a threaded Euro bottom bracket, keep in mind that the left bearing must be screwed on counterclockwise. Look for markings on the bearings themselves that indicate which side of the frame they should be placed on, labeled "L" and "R." And most importantly, make sure you screw the bearing straight in to avoid damage or misalignment.