A Bottom Bracket is the bearing system within your bike that connects the cranks to the frame. Bottom brackets come in many different shapes and sizes but luckily in BMX there's only really a few options.
Most BMX bikes currently have a MID Bottom Bracket, this represents the size of the bearing required to fit into your frame. Mid is a push fit style Bottom Bracket and the bearings literally sit in the frame. Along with mid you can also get SPANISH Bottom Brackets, spanish is a slightly smaller fitting than mid but is essential very similar to mid. Spanish BB's are named so because there were invented by spanish BMX Brand Fly Bikes. However from 2017 onwards Fly cease to use the Spanish system in their frames and now use the Mid style BB system. Next up, predominantly in race BMX bikes is EURO. Euro is a mountain/road bike standard, these Bottom Brackets are threaded and the bearings sit in cups that thread into the frame. Finally we have the oldest BB type and thats called USA. USA is the oldest BB style in BMX and is predominantly found in older style frames and bikes. Visually the biggest BB for a BMX, the USA style BB's normally had press fit cups with bearings within the cups.
Finally, because you Bottom Bracket connects you frame and cranks together it is important to get the right size Bottom Bracket for your crank axle. Most BMX cranks are 19mm, 22mm or 24mm so if your frame has a MID Bottom Bracket and you Cranks have a 19mm axle, you need a MID 19mm Bottom Bracket. Another example would be if your frame has a Spanish Bottom Bracket and your cranks have a 22mm axle, you need a Spanish 22mm Bottom Bracket.
As Always if you require any assistance or just want some good advice, give us a shout and we'll try our best sort you out. Sales@foundationbmx.com.