When replacing O-rings for carboy fermenters ensure you purchase the correct O-ring for your home brew equipment. O-rings come in different sizes to fit various carboys:
AMPI - 10mm thick with a 245mm diameter
Atlas - 7mm thick with a 270mm diameter
BMW/Rheem - 10mm thick with a 190mm diameter
Davis - 8mm thick with a 245mm diameter
VB - 8 mm thick with a 220mm diameter
* Make sure you measure the O-ring you want to replace so your new O-ring is a correct fit!