Today I am writing two separate articles. The first one being how to identify which size bearings you need, and the next being the assembly of them into the frame (minus anything to do with chains, to keep it simple). I may also write a quick article to put it out there about chain alignment and do a detailed one later.
So the first issue. Which size bearings do I need!? Well, let’s say you have a frame, and cranks. Those two variables are what make bearings. You will see two sizes associated with bearings. The outer diameter, and the inner diameter. The outer diameter is completely to do with your frame, and the inner is completely to do with your cranks. They are basically unrelated.
First thing we’re going to figure out is your frame size. As said, there are 4 popular sizes for bearings. American, Mid, Spanish, and Euro.
American Bearings: rest in a cup pressed in to the frame
Mid/Spanish: pressed directly in to the frame
Euro: Rest in cups threaded in to the frame
Now how can you tell which frame you have? Simple. Measure it. If you have to think to yourself “How do you measure with threads on the frame?” then you have a Euro BB. Euro is the only one that threads in to the frame (as of right now), and are rarely seen in park/street BMX, albeit the norm in racing.
The other three are somewhat more difficult. If you want to simply measure the inner diameter of your frame’s BB shell, and match that up with the size of the bearings and know “okay, X diameter goes with Y bearings.” Then check out the chart below.
Here’s a simple chart I drew up:
That is the classification of the outer diameter, the Mid/American problem. I did not include Euro on the list simply because it is threaded, and if there are threads, that should be identification enough of which BB you need. If there are any doubts though, talk to your local shop or the manufacturing company of the frame.
The next issue is the inner diameter. 19, 20, and 22mm are the popular sizes in BMX. 20mm is rarely seen, usually by Haro as a stock bottom bracket. Aside from that, it isn’t seen very much.
19mm is seen popularly on 48-spline interface cranks.
22mm is seen usually on pinch-bolt style cranks.
If you don’t know which size you need, measure or ask. 19mm is 3/4”. 22mm is about .85 inches. That measurement is the outer diameter of your spindle where the bearings would rest, not the spline interface.
Here is a table of millimeter-inch conversions: