Definition
The enlarged, formed head on the shop end of a solid rivet, created when the rivet's shank is compressed (upset) during installation. The upset head, together with the manufactured head on the opposite side, clamps the riveted parts together.
Plain English
When you drive a rivet, one end already has a head on it from the factory. The other end gets squashed flat against the metal during installation, forming a new head. That new, squashed head is called the upset head.
Context Anchor
Seen in aircraft maintenance and sheet-metal repair when inspecting driven rivets.
Derivation
‘Upset’ here is an old metalworking term meaning to shorten and thicken a piece of metal by hammering or pressing it. So an upset head is literally the head formed by upsetting (compressing) the rivet shank.
Why Pilots Care
The shape and size of the upset head tell an inspector whether the rivet was installed correctly. A properly formed upset head means the joint is holding the structure together as designed; a malformed one is a sign the rivet must be drilled out and replaced.
Intuition Check
"Upset" does not mean angry or disturbed here. It means the metal has been compressed and spread into shape.
Example Sentence 1
After driving the rivet, the mechanic checked the upset head against the inspection gauge to confirm it was the correct diameter and height.
Example Sentence 2
Inspect each upset head to confirm it is evenly formed and free of cracks.