Selection and test data
Most thread failures are decided at the selection stage, not on the assembly line: the insert type does not match the wall thickness, the engagement length is shorter than the parent material needs, or the finish was chosen for colour rather than for the corrosion hours the joint actually sees. The four tables below put those decisions into numbers you can check against a drawing.
Which insert type fits which hole
The first decision is not the thread size but whether the hole can be tapped, how much wall is left around it, and whether the joint will be opened again. The wall figures are practical lower limits for aluminium and zinc alloys; below them the insert pulls out before the screw breaks.
| Type | Tapping needed | Min. wall around hole | Re-openable | Locking method | Parent material |
|---|---|---|---|---|---|
| CoilSert free-running | Yes — STI tap | 1.0–1.5 d | Yes | None; preload only | Aluminium, magnesium, cast iron, plastics |
| CoilSert screw-locking | Yes — STI tap | 1.0–1.5 d | Yes | Polygonal locking coil presses the screw flank | As above, where vibration is present |
| CoilSert tangless | Yes — STI tap | 1.0–1.5 d | Yes, removable with mandrel | None, or locking variant | Blind holes, chip-free assembly |
| Topsert heavy duty | No — drill plus key slots | 1.5–2.0 d | Yes | Keys driven down through the insert | Cast aluminium, zinc alloy, cast iron |
| Topsert thin-wall | No — drill plus key slots | 1.2–1.5 d | Yes | Keys driven down through the insert | Castings where the boss diameter is fixed |
| PreciSert heavy duty | No — drill plus key slots | 1.5–2.0 d | Yes | Keys, tighter dimensional envelope | Housings with a fixed position tolerance |
| SelfSert-SB / SelfSert-S | No — self-tapping | 2.0 d | Limited; the cut thread wears | None | Plastics, soft aluminium, zinc alloy |
| Inside/outside sleeve | No — external thread | 1.8–2.2 d | Yes, inner thread replaceable | None | Repair and thread-size conversion |
How much engagement each parent material needs
Engagement length is set by the shear strength of the parent material, not by the screw. The rule of thumb is simple: the softer the parent, the longer the engagement, because the first thread carries roughly half the load and yields first.
| Parent shear strength | Engagement length | What governs |
|---|---|---|
| Below 180 MPa — magnesium, soft aluminium | 2 d to 2.5 d | First threads yield; length spreads the load |
| 180–280 MPa — 6082-T6, 6061-T6, die-cast aluminium | 1.5 d to 2 d | Typical light-alloy structural housings |
| 280–420 MPa — cast iron, 7075-T73 | 1 d to 1.5 d | Parent stops being the weakest link |
| Above 420 MPa — steel, high-strength alloy | 1 d | Failure moves into the screw; check the screw instead |
Inspection items and the standard behind each
An inspection table is only useful if each row names a method. These are the items we report by default; a customer drawing can add pull-out force, torque-to-failure or a specific salt-spray duration.
| Item | Method / standard | Acceptance |
|---|---|---|
| Thread accuracy | ISO 965-1, 6H go/no-go gauges | Go gauge enters, no-go gauge does not |
| Material identity | Optical emission spectroscopy (PMI) | Matches the grade on the drawing |
| Hardness / strength class | ISO 898-1 for steel fasteners | Hardness window for 8.8 / 10.9 / 12.9 |
| Coating thickness | ISO 2178 magnetic method, or coulometric | Drawing value, or the TZ finish specification |
| Neutral salt spray | ISO 9227 NSS (equivalent to ASTM B117) | Hours per TZ specification; rating to ISO 10289 |
| Hydrogen embrittlement | ISO 15330 preload test, plated parts ≥ class 10.9 | No fracture within the specified hold time |
| Pull-out / push-out force | Customer drawing or agreed test protocol | Value stated on the drawing |
Salt-spray hours by finish code
| Finish Code | Finish | Salt Spray Standard | Appearance / Colour |
|---|---|---|---|
| TZ001 | Plating Blue-White Zinc (Trivalent Blue Zinc) | 48 h; higher grades available on request | Bright blue-white, uniform |
| TZ002 | Plating Colour Zinc (Iridescent / Trivalent Colour Zinc) | 72 h; higher grades available on request | Iridescent colour passivate |
| TZ003 | Plating (Black Passivated Zinc) | 48 h; higher grades available on request | Black, low reflectance |
| TZ004 | Geomet (Chrome-Free Zinc Flake, Eco Grade) | 500 h; higher grades available on request | Matt silver-grey |
| TZ005 | Zinc-Nickel Alloy Plating | 700 h and above | Silver-grey to light blue |
| TZ007 | Black Oxide (Blued) | 8-24 h after oil dipping; dry indoor service only | Black / blue-black |
| TZ008 | Manganese Phosphating (High-Temperature) | 24-72 h after oil dipping; dry indoor service | Dark grey to black crystalline |
| TZ010 | PTFE Coating | 500-1000 h; higher specification available on request | Matt black or dark grey (colour options on request) |
| TZ011 | DLC Diamond-Like Carbon Coating | 200-500 h; film system can be tuned on request | Dark grey to black |
| TZ012 | Stainless Steel Plain Passivated (304/316) | 304: 48-200 h; 316: 500 h and above; higher specification on request | Natural stainless finish |
| TZ012 | Black Nickel | 12-48 h; sealing treatment extends protection | Matt black |
| TZ013 | Chrome Plating (Bright Chrome) | 48-120 h; increases with nickel layer thickness | Mirror-bright chrome |
| TZ014 | PVD-TiN Titanium Nitride (Gold PVD) | 48-160 h; depends on substrate quality | Gold |
| TZ015 | PVD-CrN (PVD) | 80-200 h; improved by suitable pre-treatment | Silver-grey |
| TZ016 | PVD-TiAlN Titanium | 80-180 h; closely tied to substrate quality | Violet-grey to bronze |
| TZ017 | PVD- DLC Diamond-Like Carbon Coating | 200-500 h; film stack can be customised | Dark grey to black |
| TZ018 | PECVD-DLC (PECVD CVD DLC) | 200-500 h; strongly dependent on substrate pre-treatment | Dark grey to black |
| TZ019 | Plating () | 24-72 h; longer with increased plating thickness | Bright, mirror-like |
| TZ020 | Zinc plated | 240 h; higher specification available on request | Matt silver |
| TZ029 | LC (PLC Polymer Carbon Film) | 120-300 h with a metallic interlayer | Dark grey to black |
| TZ302 | CD Texture (Concentric Circles) Surface Finish | Not applicable (decorative texture) | Concentric-circle machined texture |
| TZ303 | CVD Hard Coating | Not applicable (functional coating) | Grey to black, matt |
| TZ304 | LC Coating (Organic Polymer Composite Dry Film) | 200 h and above | Dark grey |
| TZ305 | PVD Coating (PVD Hard Film) | Not applicable (functional coating) | Silver, gold, black or custom colours |
| TZ306 | Nano Coating (Nano-Ceramic Composite) | 480 h and above | Clear to light grey |
| TZ327 | Zinc-Aluminium Coating (Zinc Flake / Chrome-Free Series) | 500 h and above | Metallic silver-grey |
| TZ328 | Anodising (Aluminium Fastener Corrosion Series) | 336 h and above | Clear, black or dyed colours |
| TZ329 | Stainless Steel Fasteners (Self-Passivating, Coating-Free) | Per process standard (depends on grade) | Natural stainless finish |
Hours are neutral salt spray (NSS) results for the coating system itself. They change with substrate, pre-treatment and layer thickness, so treat the range as a selection band and confirm the number on the inspection report.
Table values are specification values taken from the standards named in each row. Actual test results are stated in the inspection report shipped with each batch; where a drawing is stricter than the table, the drawing governs.