Heli-Coil Drilling Data Reference

Thread repair insert reference data. Tap drill sizes, STI tap sizes, and standard insert lengths (1D, 1.5D, 2D) for inch and metric sizes.

Inch Sizes (Unified)

Thread Size	Tap Drill	STI Tap	Insert 1D	Insert 1.5D	Insert 2D

Metric Sizes (ISO)

Thread Size	Tap Drill	STI Tap	Insert 1D	Insert 1.5D	Insert 2D

About Heli-Coil Inserts:
Heli-Coil (wire thread inserts) are used to repair damaged threads or provide stronger threads in softer materials. The oversized hole is drilled to the tap drill size, then tapped with the special STI (Screw Thread Insert) tap. Insert lengths are specified as multiples of the nominal thread diameter: 1D (standard), 1.5D (typical), and 2D (heavy duty).

Why Heli-Coil Data Matters

Heli-Coil inserts rescue stripped threads in aluminum, cast iron, and other soft materials by providing a hardened steel thread in an oversized tapped hole. Getting the right drill and tap size for each insert is non-negotiable — the wrong hole size means the insert won't seat or won't hold.

Each thread size requires a specific oversize drill and a special Heli-Coil tap — standard taps won't work
Insert length is specified in multiples of diameter: 1D, 1.5D, and 2D — use 1.5D for most applications, 2D for high-stress joints
Heli-Coils actually improve thread strength in aluminum — the insert's hardened coil outperforms the original tapped thread

Heli-Coil Drilling Data Reference

Why Heli-Coil Data Matters

Related Tools

Stay Sharp