Thread Insert / Heli-Coil Drill & Tap Size Chart

Thread-insert preparation lookup

Search by the parent thread, then filter metric, UNC, or UNF rows. This bounded lookup shows manufacturer-listed drill recommendations for two material groupings, the STI tap designation, and common insert-length multiples. It covers standard Heli-Coil wire-thread-insert examples, not every insert series or parent thread.

MECHANITEC / ENGINEERING REFERENCE

Thread Insert / Heli-Coil Drill and Tap Size Chart

A bounded Heli-Coil wire-thread-insert preparation lookup for metric, UNC, and UNF examples, with insert drilling kept separate from ordinary tap-drill sizing.

1–4 of 4 matching rows (4 total)

Thread Insert / Heli-Coil Drill and Tap Size Chart — page 1 of 1
System	Parent thread	Thread family	Pitch or TPI	Insert series	Drill, aluminum (in)	Drill, aluminum (mm)	Drill, steel/magnesium/plastic (in)	Drill, steel/magnesium/plastic (mm)	Aluminum drill designation	Steel drill designation	STI tap designation	Insert length options	Source	Value status
metric	M6 × 1.0	metric_coarse	1.0 mm	Heli-Coil standard metric	0.2461	6.25	0.2480	6.3	6.25 mm	6.30 mm	M6 × 1.0 STI	1D, 1.5D, 2D, 2.5D	STANLEY Heli-Coil HC2000 Rev 12	Manufacturer table example; verify selected insert series and material row.
metric	M8 × 1.25	metric_coarse	1.25 mm	Heli-Coil standard metric	0.3281	8.33	0.3320	8.43	21/64 in	Q drill	M8 × 1.25 STI	1D, 1.5D, 2D, 2.5D	STANLEY Heli-Coil HC2000 Rev 12	Manufacturer table example; verify selected insert series and material row.
inch	1/4-20 UNC	UNC	20 TPI	Heli-Coil standard inch	0.3281	8.33	0.3320	8.43	21/64 in	Q drill	1/4-20 STI	1D, 1.5D, 2D, 2.5D	STANLEY Heli-Coil HC2000 Rev 12	Manufacturer table example; verify selected insert series and material row.
inch	#10-32 UNF	UNF	32 TPI	Heli-Coil standard inch	0.2420	6.15	0.2460	6.25	C drill	D drill	#10-32 STI	1D, 1.5D, 2D, 2.5D	STANLEY Heli-Coil HC2000 Rev 12	Manufacturer table example; verify selected insert series and material row.

Search and sort apply to all 4 rows. Print and CSV include only the 4 rows shown on this page.

The drill size prepares the hole for the insert’s external thread. The STI tap forms that larger internal thread. The finished installed insert then provides the parent thread size. These are three different operations and dimensions.

Why an STI tap is different

An ordinary M8 × 1.25 or 1/4-20 tap cuts the finished parent thread directly into the material. An STI tap cuts the larger special thread that receives the outside of the wire insert. After the insert is installed, the insert’s inside diameter provides the specified M8 × 1.25 or 1/4-20 thread.

That is why the chart must not be combined with a standard tap-drill chart. A standard tap drill normally targets a percentage of thread for the final internal thread. An insert drill targets the insert’s external thread and installation geometry. Using the wrong drill or tap can produce a loose insert, damaged parent material, poor thread fit, or an unusable repair.

How to use the chart

Identify the parent thread and confirm whether it is metric, UNC, or UNF.
Identify the insert manufacturer, series, free-running or locking style, and required insert length.
Select the current manufacturer drill row for the workpiece material. The aluminum and steel/magnesium/plastic columns are not interchangeable by convenience.
Drill to the specified depth, remove chips, and check the entry condition required by the installation instructions.
Tap with the matching STI tap. Do not substitute a standard tap with the same parent-thread designation.
Install the insert to the specified depth, then remove the driving tang when the selected system requires it.
Inspect the installed thread with the required plug gauge, fastener, or production inspection method.

Metric, UNC, and UNF examples

The metric rows use M6 × 1.0 and M8 × 1.25. The inch rows separate 1/4-20 UNC from #10-32 UNF. UNC and UNF are not interchangeable labels: they have different pitches and require the matching insert and STI tap.

The chart preserves source drill designations such as 21/64 in, Q, C, and D, and also shows converted millimeter values for planning. The letter designation remains important because the rounded millimeter display is not a substitute for the specified drill.

Insert length and engagement

Insert length is commonly described as a multiple of the parent-thread diameter, such as 1D, 1.5D, 2D, or 2.5D. Choose the length from the joint design, material strength, available depth, and the insert manufacturer’s recommendation. A longer insert is not automatically better if the hole is blind or the installation depth is limited.

Keep the tapped depth, insert runout, countersink or lead-in, and fastener engagement within the installation specification. A correct drill diameter cannot compensate for insufficient depth, damaged STI threads, chips in the hole, or an insert installed below the required position.

Common mistakes

Using an ordinary tap-drill value for a Heli-Coil insert hole.
Using a standard parent-thread tap instead of the matching STI tap.
Choosing a metric insert because its nominal diameter looks close to an inch thread.
Rounding a letter drill to a convenient metric drill without checking the manufacturer’s limit.
Mixing free-running and locking insert installation requirements.
Ignoring the workpiece material or using a row from a different insert series.

For ordinary internal-thread preparation, use the tap drill size chart. For metric pitch identification, use the metric thread pitch chart. For inch thread-family comparison, use the UNC and UNF thread chart.

Sources and limits

The bounded values come from the STANLEY Heli-Coil HC2000 catalog and the related STI tap documentation. These documents define the manufacturer context for the examples. Confirm the current catalog row for the selected insert series, parent material, and installation tool before production or safety-critical repair work.

Frequently asked questions

Can I use a normal tap drill for a Heli-Coil insert?

No. A wire insert requires a larger preparation hole and a matching STI tap. A normal tap-drill chart targets a finished internal thread, not the external thread of the insert.

What tap do I use for a 1/4-20 Heli-Coil?

Use the matching 1/4-20 STI tap specified for the selected Heli-Coil system. The parent-thread designation alone does not authorize a standard 1/4-20 tap.

Are UNC and UNF insert taps interchangeable?

No. UNC and UNF have different pitches. Match the insert, drill row, and STI tap to the exact parent-thread designation.