Thread gauge failure after plating
Why plated screws stop accepting the GO gauge: coating thickness lands in the thread allowance. How ISO 4042, ISO 965-1 and ISO 1502 handle it, and what to specify before production.
| Defect family | Dimensional |
|---|---|
| Related service | Zinc Plating Cr3+ |
| Reviewed |
Standards commonly cited
Grades this affects
What you see
Threads that gauged correctly before plating fail the GO gauge afterwards, or nuts no longer run onto their bolts. The parts are dimensionally sound in every other respect and the coating looks good. The effect is on pitch diameter: coating deposits on both flanks, so pitch diameter grows by roughly four times the coating thickness — 8 µm of zinc adds about 32 µm, which is most of the allowance on a fine pitch.
How to confirm it
Gauge the same pieces before and after plating to ISO 1502, and measure pitch diameter to ISO 965-1 so the growth is a number rather than an argument. Measure coating thickness by XRF to ISO 3497 on the thread flank, not on the shank. Then compare the pre-plating thread tolerance class against what the coating needs: ISO 4042 expects the coated thread to remain within the specified tolerance, which in practice means the thread was produced to a reduced allowance class before coating — for example 6g reduced to 6e or 6f for thicker deposits. If nobody specified that, the gauge failure was designed in long before the parts reached the plating tank.
Related calculator: Thread fit after plating — clearance checkRoot causes — most common first
- 1Thread produced to standard 6g and then plated with a thickness the allowance cannot absorb — the single most common cause.
- 2Coating thicker than specified, or thickness measured only on the shank where it is heaviest and never on the flank.
- 3Fine pitch or small diameter, where the allowance is small in absolute terms and a normal coating consumes it.
- 4Heavy build-up on ends, crests and the first threads from high current density at edges.
- 5Growth from heat treatment adding to coating growth on threads rolled at the top of the tolerance band.
- 6Gauge or class mismatch — plated parts checked with the class agreed for uncoated parts, or an inch/metric system difference.
Process controls that prevent it
- Specify the pre-plating thread class on the drawing against the coating thickness — that is the decision ISO 4042 expects, and it has to be made before production.
- Roll or cut threads to the middle of the band rather than the top, so heat-treatment growth plus coating still fit.
- Set thickness on significant surfaces and measure it on the thread flank, so a heavier deposit does not arrive unnoticed.
- For high-thickness corrosion systems, choose the coating with the fit in mind — a thinner electroplate plus sealer, or a zinc flake system, may meet the requirement inside the allowance.
- Agree the acceptance gauge and class for the plated condition in the order, checking nuts and bolts as a pair where fit matters.
- Gauge a plated first-off before the batch runs, not after it is packed.
What to send us for diagnosis
- Plated and unplated parts of the same part number from the same lot, so the growth can be measured directly.
- The drawing including the thread class, the pre-plating class if one was specified, and the coating call-out with thickness.
- The gauge you used and where it stopped — on the first threads or over the full length.
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Part types this shows up on
Questions engineers ask
Whose fault is a plated thread that will not gauge?+
Usually the specification’s, not the plater’s. If the thread was made to 6g and the drawing then asks for a coating thickness that a 6g allowance cannot hold, the parts cannot both be plated to thickness and pass a 6g GO gauge. ISO 4042 addresses this by requiring the pre-plating thread class to be chosen against the coating.
How much does pitch diameter grow?+
Approximately four times the local coating thickness on an external thread, because metal is added to both flanks. Use it as a planning figure and verify by measurement — real deposits are not perfectly uniform along a thread.
Can threads be re-cut or chased after plating?+
That removes the coating from exactly the surface that needs protection and exposes bare steel in the thread, so the corrosion requirement is lost. The correct fix is upstream: pre-plating thread class, coating thickness and gauge class agreed before production.
Guidance based on published standards and general fastener metallurgy. Values are typical ranges that depend on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. This page does not replace a qualified failure analysis of your own parts.