Classification follows the type of engine the part is designed for. Parts identifiable as suitable solely or principally for spark-ignition engines go under 840991; those for diesel engines go under 840999. For parts that fit both engine types interchangeably, the tariff schedule's general rules of interpretation and CBP guidance apply — a binding ruling may be warranted for high-volume, dual-use components.

Turbochargers are generally classified under a separate heading covering turbo-compressors, not under 8409, because they function as air-compression devices rather than as parts of the combustion mechanism itself. Importers should verify the specific tariff line with a licensed customs broker, as the classification of turbochargers has been the subject of CBP rulings.