Bottom Brackets are what your crank arms attach to spin.
Cartridge or Sealed
All the moving parts are housed within the tubing of a sealed bottom bracket, making them relatively easy to install, but basically impossible to repair. If the axle does not move freely or experiences up and down play, the whole bottom bracket needs replacing.
On a sealed bottom bracket, there is one free cup (which can be removed from the rest of the bracket) and one fixed cup, which is truly fixed in this case. The fixed cup is reverse threaded.
Identified off the bike by the roughly 1" tubing around the axle.
- Install the fixed cup first, into the chain ring side of the bike. Remember it is reverse threaded (Lefty Tighty, in this case). It should go in tightly (bottomed out), then be backed off (loosened) a quarter turn.
- Install the free cup second on the opposite side of the bike. In the case of the sealed bottom bracket, the free cup should also be on as tightly as possible. The threading on the free cup should be Righty Tighty, Lefty Loosey.
- Re-tighten the fixed cup (Lefty Tighty, again) to ensure a tight, even fit (as per Shimano instructions)
Cup & Cone/ Unsealed
These have a lock ring and have either a threaded or cotter pin axle.
Commonly found on inexpensive BMX bikes.