Definition
A coiled stainless steel wire thread insert installed into a tapped hole to create a stronger, more wear-resistant set of internal threads, or to repair threads that have been stripped or damaged. The insert is screwed into a specially tapped hole and provides a permanent female thread that accepts standard bolts or studs.
Plain English
A small spring-like metal coil screwed into a hole to make new, stronger threads — often used to fix a stripped bolt hole so it can hold a bolt securely again.
Context Anchor
Seen in aircraft maintenance when repairing damaged screw threads in engine or airframe parts, especially in softer metals such as aluminum.
Derivation
Heli-Coil is a brand name (now genericized in maintenance use) combining 'helical' (spiral or coil-shaped) with 'coil.' The term describes exactly what it is: a helical coil of wire used as a thread insert.
Why Pilots Care
Restores reliable fastening strength in critical aluminum or magnesium parts without replacing the entire component.
Analogy
It is like putting a new metal liner inside a damaged screw hole so the screw has clean, strong grooves to hold onto.
Intuition Check
Do not read “Heli” as meaning helicopter here. A Heli-Coil insert is a spiral thread-repair part, not a helicopter component.
Example Sentence 1
The mechanic installed a Heli-Coil insert in the cylinder head where the spark plug threads had been stripped.
Example Sentence 2
Heli-Coil inserts are commonly used in the crankcase when original threads can no longer hold case bolts securely.