O-rings are one of the simplest, yet most engineered, precise and useful seal designs ever developed. They are available in various metric and in standard sizes. Sizes are specified by the inside diameter and the cross section diameter (thickness). The Danco O-rings will repair your leaky faucet. Faucets usually have one or more O-rings to prevent water from leaking out around the spout. Replacing worn out O-rings is an inexpensive alternative to replacing the entire faucet fixture.