Replacing worn-out o-rings is an inexpensive alternative to replacing the entire faucet fixture and is an easy DIY installation. Sizes are specified by the inside diameter and the cross section diameter (thickness). Faucets usually have one or more o-rings to prevent water from leaking out around the spout. These tough and pliable Buna-Nitrile Rubber (NBR) O-Rings will help provide long-term use for your plumbing needs.