Helicoil Tap Sizes (STI)

Q: What is the Helicoil (STI) tap size for 1/4-20?

Use the instant lookup on this page (select 1/4-20) or filter the chart. The table provides key STI threaded-hole diameter ranges from ASME B18.29.1.

Q: What does STI mean?

STI stands for Screw Thread Insert. STI taps create an oversized internal thread so a helical coil insert can be installed.

Q: Is this chart only for inch threads?

Yes. This page covers the inch-series table from ASME B18.29.1. For ISO metric tapping, use the Metric Tap Drill Chart page.

ASME B18.29.1 (Inch Series) • Chart + Calculator

Helicoil (STI) taps create an internal thread that is larger than a standard internal thread so a helical coil insert can be installed. This page provides an instant lookup and a filterable chart of key threaded-hole diameters from ASME B18.29.1 (inch series).

Unified tap drill chart & calculator Metric tap drill chart & calculator Roll form tap drill calculator

Helicoil Tap Sizes Chart according to ASME B18.29.1 standard (Helical Coil Screw Thread Inserts – Free Running and Screw Locking (Inch Series)). The tapped hole which the helicoil insert is to be installed shall be in accordance with ASME B1.1, except that the diameter is larger to accommodate the wire cross section of insert. Acceptance of the threaded hole is determined by gauging with STI GO, NOT GO (HI), and plain cylindrical gages according to System 21 of ANSI/ASME B1.3M.

Helicoil inserts are screw thread bushings coiled from wire of diamond shape cross-section. They are fit into thread tapped holes to form internal threads with nominal size. Helicoil inserts are diametrically larger than the tapped hole. During the assembly operation, the diameter of the leading coil is reduced due to applied torque and this permits it to enter the tapped thread.

Helicoil tapping hole according to ASME B18.29.1-1993

Instant Helicoil (STI) Tap Size Lookup

Select a thread size to view key STI threaded-hole diameters from the ASME B18.29.1 table: major diameter minimum (Dmin), pitch diameter range (D2min–D2max for Class 2B), and minor diameter range (D1min–D1max).

Thread size

Thread

Selected

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Nominal

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TPI

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Dmin (Major, min)

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D2min (Pitch, min)

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D2max (Pitch, max, 2B)

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D1min (Minor, min)

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D1max (Minor, max)

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Suggested drill (midpoint, in)

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Nearest standard drill

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Helicoil (STI) Tap Size Chart

Filter the chart to quickly find a size. You can copy or download the visible rows as CSV.

Filter chart (type: 1/4-20, 8-32, 10-32…)

Thread	No./Nominal	TPI	Dmin (Major, min)	D2min (Pitch, min)	D2max (Pitch, max, 2B)	D1min (Minor, min)	D1max (Minor, max)	Suggested drill (in)
1-64	1	64	0.0933	0.0832	0.085	0.0764	0.0823	0.0794	#47
2-56	2	56	0.1092	0.0976	0.0996	0.0899	0.0961	0.093	#42
3-48	3	48	0.1261	0.1126	0.1148	0.1036	0.1104	0.107	#36
4-40	4	40	0.1445	0.1283	0.1308	0.1175	0.1252	0.1213	#31
5-40	5	40	0.1575	0.1413	0.1438	0.1305	0.1373	0.1339	#29
6-32	6	32	0.1786	0.1583	0.1611	0.1448	0.1527	0.1487	#25
8-32	8	32	0.2046	0.1843	0.1872	0.1708	0.1781	0.1744	#17
10-24	10	24	0.2441	0.217	0.2203	0.199	0.208	0.2035	13/64
12-24	12	24	0.2701	0.243	0.2464	0.225	0.234	0.2295	#1
1/4-20	1/4	20	0.315	0.2825	0.2864	0.2608	0.2704	0.2656	17/64
5/16-18	5/16	18	0.3847	0.3486	0.3529	0.3245	0.3342	0.3294	21/64
3/8-16	3/8	16	0.4562	0.4156	0.4203	0.3885	0.3987	0.3936	25/64
7/16-14	7/16	14	0.5303	0.4839	0.489	0.453	0.4639	0.4585	29/64
1/2-13	1/2	13	0.5999	0.5499	0.5554	0.5166	0.5273	0.5219	33/64
9/16-12	9/16	12	0.6708	0.6167	0.6225	0.5806	0.5918	0.5862	19/32
5/8-11	5/8	11	0.7431	0.6841	0.6903	0.6447	0.6564	0.6505	21/32
3/4-10	3/4	10	0.8799	0.8149	0.8216	0.7716	0.7838	0.7777	25/32
7/8-9	7/8	9	1.0193	0.9471	0.9543	0.899	0.9119	0.9055	29/32
1-8	1	8	1.1624	1.0812	1.089	1.0271	1.0421	1.0346	1
1 1/8-7	1 1/8	7	1.3106	1.2178	1.2262	1.1559	1.173	1.1644	1
1 1/4-7	1 1/4	7	1.4356	1.3428	1.3514	1.2809	1.298	1.2894	1
1 3/8-6	1 3/8	6	1.5915	1.4832	1.4926	1.411	1.431	1.421	1
1 1/2-6	1 1/2	6	1.7165	1.6082	1.6177	1.536	1.556	1.546	1
2-64	2	64	0.1063	0.0962	0.0981	0.0894	0.0947	0.092	#42
3-56	3	56	0.1222	0.1106	0.1126	0.1029	0.1086	0.1058	#36
4-48	4	48	0.1391	0.1256	0.1279	0.1166	0.1229	0.1197	#31
6-40	6	40	0.1705	0.1543	0.1569	0.1435	0.1503	0.1469	#26
8-36	8	36	0.2001	0.1821	0.1849	0.1701	0.1771	0.1736	#17
10-32	10	32	0.2306	0.2103	0.2133	0.1968	0.2041	0.2005	#7
1/4-28	1/4	28	0.2964	0.2732	0.2765	0.2577	0.2646	0.2611	G
5/16-24	5/16	24	0.3666	0.3395	0.3433	0.3215	0.3288	0.3251	P
3/8-24	3/8	24	0.4291	0.402	0.4059	0.384	0.391	0.3875	W
7/16-20	7/16	20	0.5025	0.47	0.4744	0.4483	0.4561	0.4522	29/64
1/2-20	1/2	20	0.565	0.5325	0.5371	0.5108	0.5186	0.5147	33/64
9/16-18	9/16	18	0.6347	0.5986	0.6035	0.5745	0.5826	0.5786	37/64
5/8-18	5/8	18	0.6972	0.6611	0.6661	0.637	0.6451	0.6411	41/64
3/4-16	3/4	16	0.8312	0.7906	0.7961	0.7635	0.772	0.7677	49/64
7/8-14	7/8	14	0.9678	0.9214	0.9274	0.8905	0.8994	0.8949	57/64
1-12	1	12	1.1083	1.0542	1.0608	1.0181	1.0281	1.0231	1
1 1/8-12	1 1/8	12	1.2333	1.1792	1.186	1.1431	1.1531	1.1481	1
1 1/4-12	1 1/4	12	1.3583	1.3042	1.3112	1.2681	1.2781	1.2731	1
1 3/8-12	1 3/8	12	1.4833	1.4292	1.4364	1.3931	1.4031	1.3981	1
1 1/2-12	1 1/2	12	1.6083	1.5542	1.5615	1.5181	1.5281	1.5231	1
1-14	1	14	1.0928	1.0464	1.0527	1.0155	1.0243	1.0199	1

Note *: Countersink angle 120° +/- 5° .

Note: The minimum drilling depths allow for countersinking the drilled hole to have a good starting thread. The drilling depths also allow 3/4 to 1 1/2 pitches insert set-down with countersink below the top surface of the tapped hole.

Definitions:

STI Plug Style Tap: This type of tap has over 4 pitch thread chamfer. Plug style tap is widely used in through holes and where there is sufficient room at the bottom in blind holes.

STI Bottoming Style Tap: This type of tap has only 1 to 2 pitch thread chamfer. Bottoming style tap is ideal for blind holes that require threads to be close to the bottom of the hole.

Threads per inch: Number of full thread per an inch length.

Supplements:

FAQ

What is the Helicoil (STI) tap size for 1/4-20?

Use the instant lookup above (select 1/4-20) or filter the chart. The table provides key STI threaded-hole diameter ranges from ASME B18.29.1.

What does STI mean?

STI stands for Screw Thread Insert. STI taps create an oversized internal thread so a helical coil insert can be installed.

Is this chart only for inch threads?

Yes. This page covers the inch series table from ASME B18.29.1. For ISO metric tapping, use the metric tap drill chart.

Reference:

ASME B18.29.1-1993, Helical Coil Screw Thread Inserts - Free Running and Screw Locking (Inch Series)