Helical thread inserts

KATO's 2TNC-02C-0129 free-running Tangless 2-56 coil uses 1.5D × 0.086 in = 0.129 in nominal length. That is the designation, separate from loose-coil dimensions and drill depth. The host receives an STI thread while the installed insert receives the screw. Tangless describes the installation drive. In Böllhoff's Tangfree Screwlock, polygonal turns instead provide screw locking. KATO nominal-length example and limitations, Böllhoff HELICOIL product types.

Identity and working principle

Böllhoff describes a helically wound wire with rhombic section. Its turns form the internal screw thread once installed. KATO CoilThread overview, Böllhoff HELICOIL family.

Specify the insert and host preparation for reinforcement or repair. Check usable parent material and the installed thread. Böllhoff application scope.

Receiving thread versus installed screw thread

The receiving thread is commonly called the STI thread, where STI means screw thread insert. It is the host interface, not the finished screw interface. KATO's STI taps specifically produce threads that accommodate its CoilThread inserts; its overview separately describes the resulting finished internal thread. Thus, a nominal screw size alone does not define the host-hole preparation, and an ordinary screw tap is not identified by that size alone as an STI tap. KATO STI taps, CoilThread interfaces.

KATO's inspection guidance checks the STI tapped hole before insertion. It warns that an ordinary thread gage may not freely enter an installed insert, and that the locking coil prevents ordinary gaging of a locking insert. Its STI GO/HI gages check defined thread features; the minor diameter needs a separate pin check. These are KATO's stated inspection boundaries, not a universal acceptance procedure for every insert or drawing. KATO STI gages.

Free-state geometry and nominal length

KATO explains that a loose insert has a larger outside diameter and shorter axial length than after installation. Compression into the receiving thread elongates the helix. Its identification method therefore uses free outside diameter and free coil count against the relevant insert specification, rather than treating loose axial length as the nominal installed designation. A locking coil can also distort an outside-diameter measurement. KATO, “Nominal Length”.

Length designations such as 1D or 1.5D are multiples of nominal screw diameter, not measurements of the loose coil. KATO's worked example is its 2TNC-02C-0129 Tangless free-running 2-56 insert: nominal diameter 0.086 in × 1.5 = 0.129 in nominal length. This arithmetic neither supplies the drill depth nor proves that a 0.129 in wall is adequate. KATO's separate minimum-thickness discussion has restrictive assumptions, including a through hole, no countersink and full supporting threads. KATO nominal-length example and limitations.

Installation drive and screw locking are separate choices

Tang presence describes how installation is driven. Free-running versus screw-locking describes interaction with the assembled screw. KATO's part-number system distinguishes tanged from Tangless inserts separately from locking and non-locking styles. KATO part-number definitions.

These distinctions come from Böllhoff's named product descriptions. Smart demonstrates why tang presence alone does not establish a break-off operation. Screwlock's clamping turns generate frictional locking; this does not provide a generic tightening torque, preload or permissible reuse count. Installation tooling and any tang handling remain specific to the selected family and assembly requirements. Böllhoff HELICOIL product types.

Material and finish identity

“Wire insert” does not specify an alloy or coating. KATO's part-number key separately identifies CRES 304 stainless steel, phosphor bronze, Nitronic 60 and Inconel X-750, with finish options including dry-film lubricant, silver plating and passivation. The page explicitly notes that not every material is available in every size. Its red identification dye for locking inserts is not an alloy designation or a corrosion coating specification. Temperature capability, galling behavior and mating-screw suitability cannot be assigned from the generic component name. KATO material, finish and identification key.

Boundaries with other insert types

A solid-wall insert has a sleeve body carrying its internal and external features; it is not a loose wire helix. KATO explicitly contrasts its wire architecture with thin-wall and solid inserts. This structural comparison does not establish interchangeability of host holes or tooling. KATO design comparison.

Heat-set and self-tapping identify different installation mechanisms. SPIROL's Heat / Ultrasonic inserts are installed after molding in thermoplastics, using softened plastic to fill the insert's external retention features. SPIROL Series 10 Self-Tapping inserts instead form a receiving thread in a molded or drilled plastic hole during installation and are intended for softer thermoplastics or thermosets. Neither mechanism describes a coil screwed into a previously STI-tapped hole. These SPIROL examples do not define every product sold under those generic names. SPIROL Heat / Ultrasonic inserts, SPIROL Series 10.

Evidence scope and remaining limits

The cited manufacturer HTML pages were retrieved and reviewed on 2026-10-30 PDT. No visible technical edition or publication date was identified in these pages; the review date is not their release date. Raw KATO responses carried HTTP Last-Modified dates of 2025-04-25 for nominal length, gages and part numbering, 2026-07-07 for the overview, and 2026-07-08 for taps. Those server metadata dates do not establish technical revisions. Search-index crawl dates likewise do not identify an edition.

This reference covers geometry and named-family distinctions. It does not establish drill/tap sizes, installation depths, removal methods, torque, temperature limits, fatigue performance, pull-out capacity or approval for any customer assembly. Full standards mentioned by manufacturers were not inspected; no compliance conclusion is drawn from those mentions. Product part number/revision, parent material and geometry, screw specification, environment and governing assembly or repair requirements remain application-specific. No PDF, paywalled standard, customer test or supplier qualification record is relied upon here.

For the broader process context, see Hardware and component references.