A coating is not merely color; it adds measurable thickness. Film h on each hole wall reduces diameter by about 2h; on an external shaft it increases size by about 2h. If both mating parts are coated, clearance may fall by about 4h. Powder also shows edge build/Faraday-cage effects, while electroplate and hot-dip zinc vary at threads, drains and rough areas.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
WORKED FIT
A 20.00 mm member enters a 20.30 mm hole, giving 0.30 mm diametral clearance. With 80 μm average powder on every mating face, the member becomes about 20.16 mm and the hole about 20.14 mm—a theoretical 0.02 mm interference. Film variation and particles worsen it. Design post-coat dimensions, mask one side, ream/tap after finishing or use uncoated guides.
THREAD STRATEGY
Internal threads are normally masked or chased after coating; external threads use caps/plugs. Hot-dip galvanized fasteners require a coordinated thread allowance system; do not force ordinary nuts onto coated bolts. See ISO 4042:2022 for electroplated fasteners. High-strength steel also requires hydrogen-embrittlement risk control and validated baking windows. Chasing must not leave unprotected corrosion starts.
PROJECT CHECKLIST
Coating/range; film min-max; pre/post-coat dimensions; masking map; thread strategy; drain points; gauge condition; repair.