HTS subheadings for spark-ignition internal combustion engines are typically differentiated by cylinder displacement (e.g., not exceeding 1,000 cc, 1,000–2,000 cc, over 2,000 cc), so the precise engine displacement must be documented on the commercial invoice and entry to ensure correct classification and duty assessment.

Under GRI 2(a), an incomplete or unassembled engine that has the essential character of a complete engine may be classified as a complete engine. Customs and Border Protection evaluates which components are present and whether the assembly constitutes the essential character of the finished article — a determination that can substantially change the applicable duty rate.