Plating M1–M1.4: the tolerance sets nothing aside

A coating adds four times its thickness to pitch diameter

This is the step most often skipped. The coating sits on both flanks, and pitch diameter is measured across the thread, so the geometry gives:

The constraint is not band width. It is where the band sits.

It is tempting to compare coating growth against the tolerance band and conclude the band is too narrow. That is the wrong test, and at these sizes it gives the wrong answer. An M1.2×0.25-6h external thread has a pitch diameter band roughly 53 μm wide, so twenty microns does not consume it.

ISO 4042 Annex D: the theoretical maximum is thinner than expected

Annex D lists the theoretical maximum coating thickness by pitch and by the tolerance position before coating. Two examples close to our range:

4 μm is theoretical. Two things eat into it:

• Thickness distribution. Electroplating is not uniform, and rack and barrel plating distribute very differently. A specified average tells you little.
• The dog-bone effect. Current density is higher at edges and ends, so the coating runs thicker there — on the crest and the first few threads, which is exactly the part that has to enter the hole first.

Small parts are not less prone to hydrogen embrittlement

The intuition is that a small part has more surface per unit volume, so hydrogen escapes more easily and the bake can be shortened. ISO 4042 grants no such exception.

• Below M6, pickling is sometimes the only viable cleaning route — and pickling is a principal way hydrogen gets in.
• In a thin section the hardened layer occupies a larger fraction of the whole, which may increase susceptibility rather than reduce it.
• Shortening a bake requires sustained-load testing or process qualification, not reasoning from geometry.

ISO 4042 and ASTM B850 do not use the same classification and must not be mixed. ISO works from hardness (core above 390 HV is treated as susceptible) with typical conditions of 190–220 °C for 8–10 h, and states that 185–195 °C for four hours is generally insufficient — with the time counted from when the whole load reaches temperature, not from when it entered the oven. ASTM classifies by actual tensile strength instead.

What to put on the drawing

• Thread callout with tolerance position and class, not just “M1.2”
• Whether the tolerance applies before or after coating — the most disputed item of the lot
• For M1–M1.4 with plating, an explicit allowance: a pre-plating pitch diameter maximum, or a stated post-plating GO gauge requirement
• Acceptance as GO gauge after plating, not a thickness report alone
• For high-strength parts (core above 390 HV), the bake conditions and when the clock starts

None of this shows up at sample stage, which is why it bites late. Samples are often unplated, or plated in a small well-controlled batch. It surfaces on the first production lot.

References

• ISO 4042:2022 — Fasteners — Electroplated coating systems (clause 6.2, Annexes B and D). An Amendment 1:2026 exists; its technical content is not public and is not reflected here.
• ISO 965-2 — ISO general purpose metric screw threads — Tolerances — Part 2 (6H/6g from M1.6 to M100; 5H/6h for M1 to M1.4)
• ISO 965-1 — ISO general purpose metric screw threads — Tolerances — Part 1
• ASTM B850-98(2022) — Guide for Post-Coating Treatments of Steel for Reducing Hydrogen Embrittlement Risk

Standards are paid documents. Figures follow the clauses cited; the current edition governs. Purchase and acceptance should rest on the drawing agreement and the standard itself, not on this page.