How Does Powder Coating Change a Bore, Slot or Press-Fit Dimension?

Powder coating changes a feature by adding film normal to every coated surface. An outside diameter grows by approximately one local film thickness per side, an internal bore loses approximately twice the local thickness in diameter, and a slot narrows by the combined buildup on its opposing walls. A press-fit seat should normally be evaluated in the final coated or deliberately masked condition rather than relying on a nominal pre-coat size.

The calculation must use the approved maximum where reduced clearance creates the risk. If each wall of a coated slot can carry up to 0.12 mm of dry film, the opening can be about 0.24 mm smaller before considering substrate tolerance, edge buildup and measurement uncertainty. This is a dimensional principle, not a recommended film specification.

How to Calculate One-Sided and Two-Sided Buildup

Begin with the delivered function. Identify the smallest clearance, largest interference or required stack height. Mark every surface in that chain as coated, masked or controlled by another finish. Then combine the maximum substrate condition with maximum coating buildup in the direction that creates the tightest fit.

Do not subtract an average film value when the requirement controls every local point. Averages can hide one heavy wall and one light wall. Texture also affects contact: the first assembly contact may occur at coating peaks before a conventional caliper reports the expected mean size.

Why Press-Fit Seats Are Usually Masked

Press fits depend on controlled metal geometry, surface condition, elastic behavior and insertion force. Organic powder is not normally a precision bearing layer. It can compress, shear, chip or vary around the circumference, changing both retention and alignment. Masking a bushing or bearing seat keeps the engineered metal interface available for controlled post machining of cast parts.

A buyer may intentionally coat a loose clearance bore or noncritical locator, but that decision needs an assembly trial at the maximum film build. If corrosion protection is required inside a precision seat, the coating choice, tolerance and installation method must be jointly qualified rather than assumed.

What to Measure Before and After Coating

Pre-coat measurements establish whether the casting and machining process are centered correctly. Post-coat functional gauges prove the delivered condition. For a slot, a go/no-go gauge may be more repeatable than trying to measure powder on narrow walls. For a bore, a plug gauge or assembly master can confirm clearance, while the thickness instrument records accessible representative surfaces.

The measurement report should identify mask transitions, coating peaks, burrs and residue. A bore can fail even when no powder was intended if a plug leaked, adhesive remained or coating bridged the mask edge. Conversely, an isolated thickness reading on an exterior flat cannot prove an internal fit.

Inspection temperature and assembly method also matter when clearances are small. The buyer should state whether the interface is checked at room condition, after environmental conditioning or during a controlled insertion test. A forced assembly can scrape powder from a coated wall and appear to “make the part fit,” while leaving debris and an unprotected track that the dimensional report never records.

For repeat orders, retain the approved substrate size, mask method, film window and functional gauge identification together. Changing the powder texture, plug supplier or machining allowance can alter the interface even when the drawing revision does not change.

Hypothetical Fit-Chain Example

Consider a cover with a sliding tab passing through a coated slot. The slot has a machined minimum width, and the tab has a maximum width. The buyer adds the maximum approved powder build on both slot walls, then subtracts that total from the minimum opening. If the remaining clearance is below the assembly requirement, the options are to widen the substrate slot, mask one or both walls, change the finish boundary or redesign the interface.

This example does not set a universal clearance. The real decision depends on alignment, movement, temperature, debris, coating texture and whether assembly can damage the film. The powder coating specification and drawing must agree on the same maximum local build.

Buyers should release a coated bore, slot or fit only after the complete dimensional chain passes at production-intent film buildup. The most reliable answer may be a coating allowance, a mask, a different interface finish or a design change; it is not automatically a wider machining tolerance.