Short definition
A bonnet nut is the large external nut threaded onto a faucet body that retains the stem assembly or cartridge. Loosen it to pull the stem; tighten it to seal the stem against the body. On compression faucets it’s the second nut you remove (after the handle); on cartridge faucets it captures the cartridge from above.
What it is
After you pull a faucet handle, the next thing you see is the bonnet — usually a hex nut (7/8 to 1-1/4 inch on residential compression stems, 1-1/16 to 1-1/2 inch on tub and shower valves). Underneath it sits the stem, the cartridge, or the cam assembly, depending on the faucet type. The bonnet’s job is to clamp that internal mechanism against a seal at the faucet body so water doesn’t escape up through the handle area.
On older two-handle compression faucets, the bonnet works in tandem with a smaller packing nut that sits inside it and squeezes packing material around the stem. On modern cartridge faucets, the bonnet simply retains the cartridge, sometimes alongside a horseshoe U-clip.
Why it matters to a homeowner
When you’re rebuilding a dripping bath faucet and the procedure says “remove the bonnet,” this is the part. The classic mistake — confusing the small inner packing nut for the bonnet — leaves the stem trapped in the body and turns a 20-minute repair into an hour of frustration.
The other common headache: a 50-year-old chrome-on-brass tub valve where the bonnet is galvanically seized. Field cure is penetrating oil, a deep-socket faucet wrench, and gentle heat. Levering with channel-locks rounds the flats and makes the job worse.
Common variants
- Bonnet nut vs. packing nut. Bonnet retains the whole stem in the body. Packing nut compresses packing around the stem to stop a leak at the handle. Older two-handle faucets have both.
- Bonnet nut vs. cartridge retaining clip. Newer cartridge faucets sometimes use a horseshoe U-clip in addition to (or in place of) a threaded bonnet.
- Hex bonnet vs. deep-socket bonnet. Tub and shower valves often use a recessed bonnet that requires a deep-socket faucet wrench rather than a basin wrench.