Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B18.29.2M-2005 (2017) — Helical Coil Screw Thread Inserts: Free Running and Screw Locking (Metric Series)
📏 ASME B18.29.2M-2005 is the metric-series companion to ASME B18.29.1-1993, developed by ASME Subcommittee 29 (Threaded Inserts) and reaffirmed in 2017. It delineates dimensional, mechanical, and performance data for metric-series helical coil screw thread inserts and the threaded holes (STI holes) into which they are installed, in both free-running and screw-locking types. Appendices cover insert selection, STI taps, installation, and removal tooling. The helical coil insert technology dates to the 1930s; the regular metric series was delineated in Europe in the 1950s and entered production in inch-using countries in the 1960s. Today metric inserts are widely used for thread enhancement and repair across automotive, aerospace, and general industry.
💡 A notable feature: Clause 2 of the Standard states that no ISO standard exists for metric helical coil screw thread inserts, and no ISO work is expected for some time — making this ASME document the most important normative reference for the product worldwide.
1. Scope and Characteristics 📌
The Standard sets out dimensional, mechanical, and performance requirements for metric helical coil screw thread inserts and the threaded holes into which they are installed, covering free-running and screw-locking types. As usual for B18 documents, inclusion of dimensional data does not imply that all products are stock sizes; consumers should consult manufacturers on availability. The Standard references ASME B1.13M (Metric Screw Threads — M Profile), ASME B1.16M (Gages and Gaging for Metric M Profile), ASME B1.21M (MJ Profile), ASME B18.12 (Glossary of Terms), ASME B18.18.1M (Inspection and Quality Assurance for General Purpose Fasteners), and applicable ASTM material and test standards.
⚠️ Metric and inch inserts are entirely different thread systems and are not interchangeable. Even where outside diameters look similar, M-profile pitch, thread form, and STI hole dimensions differ completely from the UNC/UNF system.
2. STI Tapped Holes and Metric Insert Fit 🕳️
As with the inch series, the metric insert requires the hole to be tapped oversize to the STI configuration (Table 1) before the insert is wound in. Figure 1 defines the tapping depths, which are essential for blind-hole installations, and Table 3 gives insert length data while Table 4 gives insert dimensions. Common metric sizes and typical applications are summarized below.
| Nominal Thread (dia × pitch) | Insert Type | Typical Length (dia. multiples) | Typical Applications |
|---|---|---|---|
| M3 × 0.5 | Free running / Screw locking | 1D–3D | Electronics, precision instruments |
| M4 × 0.7 | Free running / Screw locking | 1D–3D | Appliances, automotive electrical |
| M5 × 0.8 | Free running / Screw locking | 1D–3D | General machinery, drive components |
| M6 × 1.0 | Free running / Screw locking | 1D–3D | Machine tools, engines, hydraulics |
| M8 × 1.25 | Free running / Screw locking | 1D–3D | Power equipment, chassis parts |
| M10 × 1.5 | Free running / Screw locking | 1D–2D | Heavy machinery, dies |
| M12 × 1.75 | Free running | 1D–2D | Large equipment, structural joints |
| M16 × 2.0 and above | Free running | 1D | Heavy-equipment in-service repair |
💡 Figure 1 (tapping depths) is the key reference for blind-hole installations: too shallow, and the insert cannot seat fully or even protrudes; too deep, and you waste machining and weaken the parent material.
3. Materials, Chemical Composition, and Mechanical Properties 🔬
Table 2 of the Standard specifies chemical composition of the insert wire. The mainstream product is 18-8 chromium-nickel austenitic stainless steel wire, offering corrosion resistance combined with strength and formability; bend testing follows ASTM E 290 and mechanical testing follows ASTM A 370. Where aluminum sheet/plate is specified, the material must conform to ASTM B 209M. All dimensions are given in millimeters (mm) and apply before plating unless stated otherwise.
| Element | Requirement (wt %) |
|---|---|
| Carbon C | 0.15 max |
| Manganese Mn | 2.00 max |
| Silicon Si | 1.00 max |
| Chromium Cr | 17.00 – 19.00 |
| Nickel Ni | 8.00 – 10.00 |
4. Free-Running vs Screw-Locking Types and Prevailing Torque Testing 🔩
Free-running inserts have a smooth continuous internal surface, letting a screw turn in without added resistance — suited to precise torque control and frequent assembly/disassembly. Screw-locking inserts incorporate a locking feature that generates a sustained prevailing torque on the screw, resisting loosening under vibration; acceptance values are given in Table 5. To make measurements repeatable across manufacturers, Figure 3 defines a torque test spacer (dimensions in Table 6) that must be used with the prevailing torque test procedure.
⚠️ Prevailing torque acceptance must distinguish “first assembly” from “reuse” states, and must be measured with the Standard’s torque test spacer and procedure — results obtained otherwise cannot be used to judge conformance.
5. Engineering Practice Essentials 🛠️
- 🔧 Installation sequence: drill → tap the oversize STI thread with a metric STI tap (Appendix B) → clear chips → wind in the insert with the installation tool and break off the driving tang → verify the finished internal thread with standard thread gages.
- 📐 Depth control: determine tapping depth per Figure 1; for through holes, likewise prevent the insert from protruding excessively.
- 🔍 Quality control: check the tapped hole with STI plug gages after tapping, then accept the assembled thread per the ASME B1.16M gaging system for metric M-profile threads.
- 🧰 Removal and replacement: extract the old insert with the tooling specified in the Appendices, clean the parent thread, and reinstall a new insert.
6. Frequently Asked Questions ❓
❓ Q: Why is there no ISO standard for metric helical coil inserts?
A: Per Clause 2, no ISO standard exists for metric helical coil screw threads and no ISO work is expected in the near term. This ASME metric standard therefore serves, together with the inch-series B18.29.1, as the principal international normative reference.
A: Per Clause 2, no ISO standard exists for metric helical coil screw threads and no ISO work is expected in the near term. This ASME metric standard therefore serves, together with the inch-series B18.29.1, as the principal international normative reference.
❓ Q: How is prevailing torque measured?
A: Using the torque test spacer of Figure 3 (Table 6) with a torque measurement device, against the values of Table 5, separately for first assembly and reuse conditions.
A: Using the torque test spacer of Figure 3 (Table 6) with a torque measurement device, against the values of Table 5, separately for first assembly and reuse conditions.
🔍 Q: Are metric inserts interchangeable with inch inserts?
A: No. Metric inserts are based on the M-profile thread and metric STI holes; inch inserts are based on the UNC/UNF form and inch STI holes. Thread systems, dimensions, and gages are incompatible — always select by the original design system.
A: No. Metric inserts are based on the M-profile thread and metric STI holes; inch inserts are based on the UNC/UNF form and inch STI holes. Thread systems, dimensions, and gages are incompatible — always select by the original design system.
❓ Q: Why does tapping depth matter so much?
A: Tapping depth determines how far the insert seats and the load-bearing length it provides. In blind holes, too little depth leaves the insert proud and blocks the screw; too much wastes machining and may weaken the base. Follow Figure 1 strictly.
A: Tapping depth determines how far the insert seats and the load-bearing length it provides. In blind holes, too little depth leaves the insert proud and blocks the screw; too much wastes machining and may weaken the base. Follow Figure 1 strictly.
