CBP's General Rules of Interpretation require first checking whether a more specific heading covers the article. If the part is dedicated to a single type of machine (e.g., a crankshaft bearing cap), it may classify with that machine's parts. Only when no specific provision applies does the NES parts heading become the correct classification. A binding ruling request to CBP is the most reliable way to resolve ambiguous cases.

Detailed technical specifications, OEM part numbers, and a clear description of the parent machine or assembly are essential. Importers should also retain engineering drawings and supplier declarations confirming the article's function, as CBP may request this documentation during a focused assessment or audit of mechanical parts entries.