Marine compression-ignition engines are regulated under EPA 40 CFR Part 1042, which sets Tier emissions standards based on engine displacement and rated power. Importers must file an EPA Declaration Form at entry, and engines must be covered by a valid certificate of conformity unless a specific exemption applies (e.g., engines for export-only vessels or research use).

Large slow-speed two-stroke diesel engines installed in ocean-going vessels are typically entered as part of the vessel itself rather than as separate merchandise, unless imported as standalone units for installation or repair. When imported separately, they are classified under SITC 71333 and subject to standard entry procedures, EPA declaration requirements, and any applicable trade-remedy duties based on country of origin.