BMX bottom brackets come in a variety of sizes. There is the internal diameter to match the type and size of bmx cranks that you have. There is also the external diameter which needs to match the type of bottom bracket your bmx frame has.
Bottom bracket sizing - internal diameter
There are 3 main sizes for BMX bottom bracket, the main two are 19mm and 22mm, but there is also a less common 20mm. These sizes are for 2 and 3 piece bmx cranks.
BMX bottom bracket sizing - external diameter
The outer diameter needs to match the BB shell size on the frame you are using. There are 4 types, Mid which is the most common, American, Spanish, and lastly Euro which is a threaded bottom bracket so it screws into your frame. The other 2 types, american, mid, and spanish are all press fit meaning they are pressed/pushed into the BB shell space on the frame.
| Bottom Bracket Type |
External Size |
Fit type |
| American |
51mm |
Press fit |
| Euro |
Various |
Threaded |
| Mid |
41mm |
Press fit |
| Spanish |
37mm |
Press fit |
Before purchasing a bottom bracket be sure to check what size the BB is on your frame, and the axle size on your cranks to ensure that you buy the correct one. If you are unsure we're always here to help with all your pre purchase BMX questions so just get in touch.