Definition
A finishing nut with a closed, rounded dome on one end that covers the exposed thread of the bolt or stud it is screwed onto. The dome is solid metal, so the nut both secures the fastener and protects the threads from damage and corrosion while giving a clean, finished appearance.
Plain English
A nut shaped like an acorn, with a closed rounded top instead of an open hole. It screws onto a bolt and hides the end of the thread underneath the smooth dome.
Context Anchor
Seen in aircraft maintenance, parts catalogs, and inspections of hardware installed on the airframe or engine.
Derivation
Named after the acorn — the seed of the oak tree — because the closed, rounded shape of the nut looks like an acorn sitting on top of the bolt. Sometimes also called a cap nut or dome nut for the same reason.
Why Pilots Care
Covers sharp threads that could catch on clothing, controls, or wiring and protects against corrosion and FOD.
Intuition Check
Do not assume “acorn” describes a special aircraft function. It mainly describes the shape; the important feature is the closed, capped end.
Example Sentence 1
The mechanic torqued the bolt to specification and then threaded an acorn nut over the end to protect the exposed threads.
Example Sentence 2
Acorn nuts were installed along the wing fairing to give the surface a smooth profile and prevent snags.