Is thicker plating better? Plating thickness and ISO 4042
Too thin rusts, too thick will not assemble. Pitch diameter and allowable thickness per the standard, thickness distribution, three measuring methods and gauge checks.
"The thicker the plating, the better" is a risky idea for fasteners. Plating that is too thin loses corrosion resistance and rusts; plating that is too thick builds up on the thread so the bolt and nut no longer fit — an assembly failure.
Allowable thickness has a standard behind it — ISO 4042
ISO 4042:2022, which covers electroplating of fasteners, limits the allowable coating thickness by the thread tolerance position (the g in 6g, etc.) and pitch. With the common 6H/6g combination the allowance shrinks sharply as the pitch gets smaller, so on small screws "a bit thicker to be safe" quickly becomes "a screw that will not go in".
5 μm of plating adds 20 μm to the pitch diameter
Because the metric thread angle is 60°, a coating of thickness t on the thread flanks increases the pitch diameter by about 4t (the geometric relationship in ISO 4042 Annex D). Five micrometres on the flanks therefore adds roughly 20 μm to the pitch diameter in theory.
A 6g external thread is made slightly under the basic size to leave room for this. The theoretical coating thickness that room can accept is as follows.
| Coarse thread | M2 | M3 | M4 | M5 | M6 | M8 |
|---|---|---|---|---|---|---|
| Pitch (mm) | 0.4 | 0.5 | 0.7 | 0.8 | 1.0 | 1.25 |
| Theoretical max. coating for 6g (μm) | 4 | 5 | 5 | 6 | 6 | 7 |
ISO 4042:2022 Annex D (tolerance position g). Equal to the 6g fundamental deviation divided by four.
These values are neither a recommended thickness nor a guaranteed maximum for a given product. Do not read them as "M4 may be plated up to 5 μm". Whether the part assembles depends on the thread size before plating, the thickness distribution, the thread gauge check after plating and the tolerance of the mating internal thread.
Plating does not build up evenly
Electroplating builds up more where current concentrates — the outer edge of the head and the thread end — and less in the middle of the shank or inside recesses (also covered in ISO 4042 Annex D). This is how "the thickness report passed, yet the thread end is tight" happens. Electroplating of high-strength steel fasteners also requires separate control of the hydrogen embrittlement risk (ISO 4042).
Three measuring methods compared
| Method | Principle | Destructive? | Use and limits |
|---|---|---|---|
| Magnetic | Converts the change in magnetic flux or reluctance into thickness | No | Non-magnetic coatings on a steel base (zinc, paint, etc.). Separating individual layers, such as chromate on zinc, with a general magnetic gauge alone is limited. This common method (ISO 2178 type) is hard to apply as it stands to electroplated nickel — use the nickel-specific magnetic method (ISO 2361) or XRF. Large errors on curved or rough surfaces |
| Coulometric | Dissolves the coating electrolytically and calculates from the voltage change when the base is exposed | Yes | Precise for multilayer and thick coatings. Damages the part, so used on samples and test pieces |
| XRF (X-ray fluorescence) | Compares the fluorescent X-ray intensity of the coating elements with a calibration curve | No | With suitable settings and calibration, can measure individual layers of multilayer coatings; range and accuracy depend on the coating/base combination and on the size and curvature of the measuring spot |
- Position — head, shank and thread differ in thickness. Fix the measuring position and repeat
- Calibration — accuracy depends on checking the zero and on calibration and verification with suitable standards. Small screw heads and threads differ in shape from flat standards, so confirm the measuring conditions suit the curvature, base material and surface condition of the spot
- Lot sampling — measure a set number of pieces at set positions per lot and judge against the specified criteria, such as minimum local thickness. Look at individual readings and their spread, not only the mean
Plating thickness inspection and the thread gauge check after plating are separate quality checks. ISO 4042 specifies a separate GO gauge check for metric threads after plating. Even when the thickness passes, a gauge is needed to confirm that the thread will assemble.
We control plating thickness lot by lot with a coating thickness gauge (HITACHI) and provide the measurement data on request. Where the plating specification (type and thickness) is not yet fixed, we will review it with you for the application.
FAQ
Why won't a thickly plated screw thread in?
A coating of thickness t on the flanks increases the pitch diameter by about 4t (5 μm on the flanks adds about 20 μm). ISO 4042 limits the allowable thickness by tolerance position and pitch; the theoretical limit for 6g runs from about 4 μm for M2 to 7 μm for M8. These are not guaranteed values; assembly must be confirmed with a thread gauge after plating.
Why can't a normal gauge measure nickel plating?
Common magnetic gauges assume a non-magnetic coating on a magnetic base. Electroplated nickel is itself magnetic, so that method is hard to apply as it stands. The nickel-specific magnetic method (ISO 2361) or XRF (X-ray fluorescence) is used instead.
How should plating thickness be inspected?
Fix the measuring position and repeat, calibrate on a standard first, and measure a set number of pieces per lot against the specified criteria such as minimum thickness. Look at individual readings and spread as well as the mean, and confirm assembly separately with a thread gauge after plating.
Why is the thread end tight when the thickness report passed?
Electroplating tends to build up at the head edge and thread end where current concentrates. Thickness inspection and thread gauging are separate checks, so assembly must be confirmed with a gauge after plating.
References
- ISO 4042:2022 Fasteners — Electroplated coating systems and Amd 1:2026 — allowable coating thickness by tolerance position and pitch, Annex D (pitch diameter about 4t, thickness distribution), gauging after plating, hydrogen embrittlement (Amd 1 adds a footnote on ZnNi passivation)
- ISO 2178:2016 — magnetic method (non-magnetic coatings on magnetic substrates)
- ISO 2177:2003 — coulometric method
- ISO 3497:2000 — X-ray spectrometric (XRF) methods
- ISO 2361:2025 — magnetic method for electrodeposited nickel
- ISO 965-1:2026 — fundamental deviation of 6g (allowance for coating)
Last revised 2026-10-09