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ASME B18.29.1-2010 (2017) — Helical Coil Screw Thread Inserts: Free Running and Screw Locking (Inch Series)
📏 ASME B18.29.1-2010 is the first ASME standard devoted to helical coil screw thread inserts in the B18 fastener system, superseding the 1993 edition and reaffirmed in 2017. It delineates dimensional data for inch-series helical coil screw thread inserts and the threaded holes (STI holes) into which they are installed, covering Unified Fine (UNF) and Unified Coarse (UNC) thread series in sizes from #1 through 1½ in., in both free-running and screw-locking types. Appendices cover insert selection, STI taps, gages and gaging, installation, and removal tooling. The helical coil insert was invented in the 1930s, found early acceptance in aircraft built and serviced by the Allied air forces during World War II, and has since become the standard means of thread enhancement and repair in aerospace, automotive, and industrial original-equipment design, production salvage, and in-service repair.
💡 A helical coil insert is, in essence, a diamond (rhombic) cross-section stainless wire formed into a coiled thread which is screwed into an oversized tapped hole; once seated, it provides a standard, wear-resistant, corrosion-resistant internal thread — “damaged once, repaired permanently.”
1. Scope and Basic Concepts 📌
Clause 1 of the Standard states the scope: it delineates dimensional data for the inch-series helical coil screw thread inserts and the threaded holes into which they are installed, covering free-running and screw-locking types in Unified Fine and Unified Coarse thread series from size #1 through 1½ in. The Appendices describe insert selection (Appendix A), STI taps (Appendix B), STI thread plug gages (Appendix C), installation (Appendix D), and removal tooling (Army A-A-59158).
As with most B18 standards, the inclusion of dimensional data does not imply that all described products are stock sizes; consumers should consult manufacturers on availability. The Standard also incorporates by reference ASME B1.1 (Unified Inch Screw Threads), ASME B1.2 (Gages and Gaging), ASME B18.18.2 (Inspection and Quality Assurance for High-Volume Machine Assembly Fasteners), ASME B46.1 (Surface Texture), and ASME Y14.5M (Dimensioning and Tolerancing).
⚠️ After installation, the insert provides a standard-size internal thread, but the prepared hole must first be tapped oversize to the STI configuration. Tapping the hole as an ordinary thread leaves no room for the insert wire — a classic field error.
2. STI Tapped Holes and Insert Fit 🕳️
An STI (screw thread insert) tapped hole is larger than the corresponding standard thread by one insert wire diameter, so that when the insert is wound in, its inner surface forms a standard thread form. Table 1 of the Standard gives STI threaded hole data for each size; installation requires taps marked “STI” (Appendix B) and acceptance gaging with STI thread plug gages (Appendix C). Common inch sizes and their typical applications are summarized below.
| Nominal Thread Size | Thread Series | Insert Type | Typical Length (dia. multiples) | Typical Applications |
|---|---|---|---|---|
| #4-40 | UNC / UNF | Free running / Screw locking | 1D–3D | Instruments, electronic equipment |
| #6-32 | UNC | Free running / Screw locking | 1D–3D | Appliances, automotive accessories |
| #8-32 | UNC | Free running / Screw locking | 1D–3D | General machinery, tooling and fixtures |
| #10-24 | UNC | Free running / Screw locking | 1D–3D | General machinery, hydraulic components |
| 1/4-20 | UNC | Free running / Screw locking | 1D–3D | Machine tools, dies, engines |
| 3/8-16 | UNC | Free running | 1D–2D | Power equipment, large fixtures |
| 1/2-13 | UNC | Free running | 1D–2D | Heavy machinery in-service repair |
| 3/4-10, 1-8 | UNC | Free running | 1D | Large castings and housing repair |
💡 One-diameter length (1D) is the basic length and restores the load capacity of the original thread. Longer inserts (2D, 3D) enlarge the shear area and are used where the parent material is soft or the loading is high — but they require deeper tapping.
3. Materials and Chemical Composition 🔬
Table 2 of the Standard specifies the chemical composition of the insert wire. The mainstream product is 18-8 chromium-nickel austenitic stainless steel wire, which combines good corrosion resistance with adequate strength and formability. Formability of the wire is verified by bend testing to ASTM E 290, mechanical testing is performed to ASTM A 370, and aluminum sheet/plate inserts must conform to ASTM B 209 where specified.
| Element | Requirement (wt %) |
|---|---|
| Carbon C | 0.15 max |
| Manganese Mn | 2.00 max |
| Silicon Si | 1.00 max |
| Phosphorus P | 0.045 max (per ASTM A 370) |
| Sulfur S | 0.030 max (per ASTM A 370) |
| Chromium Cr | 17.00 – 19.00 |
| Nickel Ni | 8.00 – 10.00 |
4. Free-Running vs Screw-Locking Inserts 🔩
Free-running inserts present a smooth continuous internal surface, so a screw turns in without extra resistance — ideal where precise torque control, frequent assembly/disassembly, or low friction is required. Screw-locking inserts incorporate a locking feature over part of the thread that produces a sustained prevailing torque on the screw, resisting loosening under vibration; the values are verified against Table 5 of the Standard. Locking is typically achieved by a locking coil (or coils of polygonal cross-section), and the insert shares the same STI hole and dimensional system as the free-running type.
⚠️ The prevailing torque of locking inserts has different acceptance limits for first installation and for reuse. Verification must follow the Standard’s specified procedure and torque-test spacer — do not rely on “feel” alone.
5. Engineering Practice Essentials 🛠️
- 🔧 Repair sequence: drill out the damaged thread → tap the STI oversize thread with an STI tap → clear chips → wind in the insert with the installation tool → break off the driving tang per Appendix D → gage the finished thread. (The tang is the drive tab at one end of the insert; after seating, it is snapped off at the root with a punch.)
- 📐 Hole depth and bottom clearance: for blind holes, ensure sufficient tap depth and clearance below the hole per the figures and Table 1 so the insert seats without protruding.
- 🔍 Gaging: check the tapped hole with STI thread plug gages (Appendix C), then verify the assembled internal thread with standard thread gages.
- 🧰 Removal: when an insert must be replaced, extract the remaining coils with the removal tool per A-A-59158 and reinstall a new insert.
6. Frequently Asked Questions ❓
❓ Q: Why must an STI tap be used for the hole?
A: The STI tapped hole is one wire diameter larger than the standard thread. A standard tap produces a hole too small to accommodate the insert wire; only a tap marked “STI” (Appendix B) yields an internal thread that returns to standard size once the insert is seated.
A: The STI tapped hole is one wire diameter larger than the standard thread. A standard tap produces a hole too small to accommodate the insert wire; only a tap marked “STI” (Appendix B) yields an internal thread that returns to standard size once the insert is seated.
❓ Q: How do I choose between free-running and screw-locking inserts?
A: Choose free-running when you need precise clamp-load control or frequent disassembly; choose screw-locking in vibration-prone service where loosening must be resisted. Locking inserts add prevailing torque — accept per Table 5.
A: Choose free-running when you need precise clamp-load control or frequent disassembly; choose screw-locking in vibration-prone service where loosening must be resisted. Locking inserts add prevailing torque — accept per Table 5.
🔍 Q: Should I pick a 1D, 2D, or 3D length?
A: 1D restores the capacity of the original thread. Use 2D or 3D where the parent material is soft or loads are high to enlarge the shear area, and increase the tapped depth accordingly.
A: 1D restores the capacity of the original thread. Use 2D or 3D where the parent material is soft or loads are high to enlarge the shear area, and increase the tapped depth accordingly.
❓ Q: Why is the driving tang removed after installation?
A: The tang transmits torque during installation. If left in place it blocks the screw. Per Appendix D, snap it off at the root once the insert is fully seated and clear the fragment; use a notched removal tool to back out the insert if replacement is ever needed.
A: The tang transmits torque during installation. If left in place it blocks the screw. Per Appendix D, snap it off at the root once the insert is fully seated and clear the fragment; use a notched removal tool to back out the insert if replacement is ever needed.
