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Helical Coil Insert Tap Drill Chart

Commonly published drill sizes for STI (screw thread insert) taps used to install helical coil wire thread inserts in inch and metric threads.

Ref CT-0062ASME B18.29.1

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A helical coil insert is a coil of diamond-shaped wire that screws into a tapped hole and gives a standard internal thread. It repairs stripped threads and strengthens threads in soft metals. The hole is tapped with an STI tap (screw thread insert tap), which is oversized so the installed insert's inside matches the nominal thread. STI taps are marked with the nominal thread they produce after the insert is installed, not their actual size.

How to use it

Find the final thread size the bolt needs. Drill with the STI drill below, tap with the matching STI tap, install the insert with the correct tool, then break off the tang if the insert has one. Inch STI drills are larger than standard tap drills by roughly one-third of the thread pitch.

Chart

Final thread	STI drill	Drill diameter
#4-40 UNC	#31	.1200 in
#6-32 UNC	#26	.1470 in
#8-32 UNC	#17	.1730 in
#10-24 UNC	13/64	.2031 in
#10-32 UNF	#7	.2010 in
1/4-20 UNC	H	.2660 in
1/4-28 UNF	G	.2610 in
5/16-18 UNC	Q	.3320 in
3/8-16 UNC	X	.3970 in
3/8-24 UNF	W	.3860 in
1/2-13 UNC	17/32	.5312 in
M3 × 0.5	3.2 mm	3.2 mm
M4 × 0.7	4.2 mm	4.2 mm
M5 × 0.8	5.2 mm	5.2 mm
M6 × 1.0	6.3 mm	6.3 mm
M8 × 1.25	8.4 mm	8.4 mm
M10 × 1.5	10.5 mm	10.5 mm
M12 × 1.75	12.5 mm	12.5 mm

Key points

Drill sizes can vary slightly between insert makers and by material. The insert kit's instructions govern.
Drill and tap deep enough for the insert length plus thread runout. Insert length is given as a multiple of nominal diameter (for example 1D, 1.5D, 2D).
Use the installation tool made for that insert size. Prewinders help in larger sizes.
Tanged inserts need the tang removed in through holes and usually in blind holes, unless the hole is deep enough to leave it.
Gauge the finished thread with a standard go/no-go plug gauge after installation.

Common mistakes

Using a standard tap instead of an STI tap.
Drilling with the standard tap drill for the final thread, which leaves no room for the insert.
Installing the insert too shallow; the top coil should sit about a quarter to a half turn below the surface.
Leaving the tang in a blind hole that is too shallow, so the bolt bottoms on it.

Governing standards: ASME B18.29.1 (helical coil screw thread inserts, free running and screw locking, inch); ASME B18.29.2M for metric. Insert manufacturer's installation data governs.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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