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 in side diameter and the cross-section diameter (thickness). The Danco O-ring will repair your leaky faucet. Faucets usually have one or more O-ring to prevent water from leaking out around the spout. Replacing worn-out O-ring is an inexpensive alternative to replacing the entire faucet fixture.