Short definition
An O-ring is a round (toroidal) rubber ring that seats in a groove and seals between two mating parts under compression. Faucets are full of them — at cartridge bodies, at spout swivel joints, at handle stems. A worn O-ring is the most common cause of a kitchen-faucet leak at the base of the spout.
What it is
An O-ring is sized by its inside diameter (ID) and cross-section thickness. Standard material is Buna-N (nitrile rubber). For hot-water service, EPDM or silicone hold up better. For chemical or extreme-temperature applications, Viton.
In a faucet, O-rings show up wherever a moving or stationary joint needs a seal: where the spout swivels on the body, where a cartridge slides into its bore, where a handle stem passes through the cartridge cap, where a compression-fitting tubing nut seats. The rubber compresses slightly into its groove when the joint is assembled, and the seal holds because rubber is elastic and slightly oversized.
Why it matters to a homeowner
When a 10-year-old kitchen faucet starts dripping at the base of the spout — not from the spout opening, the base — the answer is almost always a worn O-ring on the spout swivel. The part is 25 cents. The fix is ten minutes: pull the spout, slip off the old O-ring, lubricate a new one with silicone plumber’s grease, slide it on, reseat the spout.
If you replace a faucet cartridge and it still drips, you may have either pinched an O-ring on installation or skipped one entirely. Always lubricate before reinstall — silicone plumber’s grease, never petroleum jelly or Vaseline (petroleum degrades rubber).
A $5 to $15 O-ring assortment kit covers most common sizes for both kitchen and bath faucets and is the right small parts to keep in your toolbox.
Common failure modes
- Aging hardens the rubber. Loses elasticity, leaks under pressure or movement.
- Heat damage. Buna-N degrades on hot lines over years; silicone or EPDM lasts longer.
- Pinched on install. Twisted or partially crushed in the groove. Leaks immediately.
- Dry install. No grease; rubber tears against chrome on first use.
- Mineral scale on the seat. New O-ring doesn’t seal until the groove is cleaned.
Common variants
- O-ring vs. flat washer. O-ring is round in cross-section; flat washer is rectangular. Different seal geometry.
- O-ring vs. quad-ring (X-ring). Quad-rings have four contact lines and resist roll-out in dynamic applications.
- Buna-N (standard) vs. silicone vs. EPDM vs. Viton. Each suits different temperature and chemistry. For potable hot water, EPDM or silicone is the safe pick.